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    6713442 ceramic-arts-handbook 6713442 ceramic-arts-handbook Document Transcript

    • Ceramic Arts Handbook by Vince Pitelkawww.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society
    • Ta b l e o f C o n t e n t s Preface..........................................................................................   1 1 Clay and Claybodies. .............................................................   3 . The Nature of Clay.................................................................................  3 Claybodies ..............................................................................................  4 2 Handbuilding.........................................................................  5 . Wedging the Clay..................................................................................  6 Handbuilding: General Guidelines and Suggestions..................................................................................  6 Making Pinch Forms..............................................................................  9 Coil Construction................................................................................   11 Slab Construction. ..............................................................................   14 . Making Tiles........................................................................................   17 3 Throwing..............................................................................  19 Critical Points in Throwing.................................................................   19 Skill Development with Cylinders......................................................   21 Throwing Bowls..................................................................................   22 Drying Your Pots.................................................................................   26 Finishing the Bottoms of Your Pots...................................................   27 4 Surface Decoration on Greenware.....................................  29 . Decorative Effects during Forming....................................................   29 Impressed Decoration. .......................................................................   29 . Subtractive Methods. .........................................................................   31 . Additive Methods...............................................................................   32 . 5 Glazes and Glazing..............................................................  33 Introduction to Glazing......................................................................   33 Glaze-Firing Ranges............................................................................   34 Glazing Methods. ...............................................................................   34 . Glaze Faults.........................................................................................   36 6 Kilns and Firing....................................................................  38 General Kiln and Firing Practices.......................................................   38 Preparing and Loading Kilns..............................................................   39 Determining Appropriate Firing and Cooling Ramps. ................................................................................   41 . Kiln Firing Chart..................................................................................   43 7 Studio Safety........................................................................  44 Studio Safety Checklist.......................................................................   44 Toxic and Hazardous Materials..........................................................   45 Dust/Dirt Management......................................................................   45 Skin Care .............................................................................................   45 Equipment Safety................................................................................   46www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Preface This Ceramic Arts Handbook is an abridged version of Vince Pitelka’s best selling Clay: A Studio Handbook (The American Ceramic Society, 2001). His book has sold thousands of copies and is used both as a textbook and refer- ence resource in thousands of ceramics studios in art schools, community art centers, colleges, universities, and homes. While the book authoritatively explores each topic in great detail, we’ve extracted some of the basic essen- tials you need to get you started on your clay adventure. In his introduction, Vince writes “Through 30 years of experience as a studio potter, welder/fabricator/mechanic, and university educator, I have been collecting and disseminating information about ceramics. This book is a compilation of what I feel will be most valuable to ceramics students, studio artists, and educators, regardless of the particular direction of their ceramic work or teaching. I hope it will provide a convenient reference to help you through most of the steps in skill development, studio setup, and operations. It is my intent to offer answers and stimulate ideas.” Working in clay is one of the most rewarding activities. It’s suitable for all age groups and personalities and provides challenges for every skill level. We believe that this Handbook will provide you with a deeper understand- ing and appreciation of this medium, and hope that you’ll be able to spend many joyful hours ahead with your hands in clay. Bill Jones Ceramic Arts Book Manager Clay: A Studio Handbook by Vince Pitelka The American Ceramic Society, 2001www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Clay and Claybodies Chapter 1 The Nature of Clay We who work and play in clay have chosen well. Clay is among the most abundant and inexpensive materials on earth. The natural processes that weather and decay igneous rocks have been generous in providing us with extensive clay deposits in a variety of forms. Clay is abundantly available almost everywhere on earth, awaiting our need, often requiring little processing. Clay is a remarkable material for so many reasons. There is no other art or craft ­material that has the versatility and possibility of clay. We can cast it, throw it, extrude it, model it, roll it, pinch it, press it, slump it, stamp it, pull it, and push it. We can use it to create any form or shape, tiny or monumental, organic or rectilinear, thin and ­fragile, or thick and heavy. It is the most malleable and forgiving of art materials. It asks little of us, but with commitment and respect on our part, it rewards us generously. When subjected to a simple firing process, clay is transformed to hard, impermeable stone, and what was once so malleable and impermanent might now remain stable and unchanged for millennia. As if the mere workability and fired permanence of clay were not enough, we can also apply an unending variety of mineral coatings that fuse into glassy glaze surfaces of unlimited color and texture. When you place a lump of clay in anyone’s hand, the response is automatic. The hand closes and squeezes the clay, and a unique sculptural form is produced, subtly different from any other before. Few of us stop at that point, for the clay encourages us to apply different forces, responding to every push and pull. Until the clay begins to stiffen, there are no rules, and no externally imposed finality. We can undo what we have done, and we can immediately return any form or shape to a simple lump and begin anew. We do not know what we can do until we find out what we cannot do, and in order to fully discover the possibilities, we must take chances and experience lots of failure and mistakes. When in doubt, make something. Never allow frustration or failure to drive you from this medium. Do not ever stop experimenting and exploring. Do not be satisfied with a single direction in your work. Do not become smug with any aspect of the medium, no matter how well you think you know it. The clay will catch you off guard and will throw you for a loop every time. But as long as you maintain a spirit of discovery and curiosity, the clay will reward you frequently and generously.www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Clay and Claybodies • Chapter 1What Is Clay? and bisque strength, and yet it mini- Porcelain claybodies include Clay results from the natural mizes the water content and result- gritless high-fire bodies that firedecomposition of certain igneous ing shrinkage. close to pure white. Under cer-rocks—primarily granite and feld- tain circumstances, fired porce-spar. The end result of the decom- Claybodies lain can be translucent. True boneposition of granite and feldspar pro- Clay­bodies are mixtures of clay china (traditional translucent por-duces microscopic flat clay crystals and other materials designed to celain) is so-titled due to thecalled platelets. accomplish specific goals like addition of bone ash (calcium When microscopic clay platelets plasticity in throwing, stability in phosphate). Bone china bodies areare wet, they tend to stick together large-scale work, thermal shock very prone to warpage unless firedand slide smoothly against one resistance, dry and fired strength, on flat shelves with no hot spotsanother. The most plastic clays are or vitrification and density. in the firing. Actually, any reason-those with the smallest particle size. Earthenware claybodies remain ably well-fluxed cone 10 porcelain Different clays behave differently porous at low-fire, and yet at thrown very thin will give somedepending on the range and dis- higher temperatures will likely translucence without the disad-tribution of particle size and the deform and bloat before vitrifica- vantages of bone china.p­ resence of non-clay contaminants, tion. Traditional earthenware bod- Stoneware claybodies use naturalprimarily organic materials and ies are usually red or buff, a blend stoneware clay and/or fireclay asnonplastic minerals. of iron-rich surface clay plus sand a base, with additions of ball clay, or grog to give structure and often kao­lin, flint, fluxes, and/or grog orHow Does Particle Size with fireclay or stoneware clay to sand. Whiteness is rarely an issue,Affect Drying and Firing? increase firing temperature and so the materials are selected for The size and shape of clay par- reduce the chances of deformation desirable performance in form-ticles help determine plasticity, but and bloating. Modern low-fire ing and firing, regardless of color.they also have profound effects in bodies are often white, composed Natural stone­ware clays and plasticdry­­ing and firing the clay. The evap- of 50-50 ball clay and talc, and are fireclays with the addition of balloration of the water layer existing actually very similar to ones used clay produce an extremely plasticbe­tween each particle in the plastic by the Egyptians 5000 years ago. throwing body. Ad­ding sand orstate is what causes drying shrink- A low-temperature firing pro- grog gives tooth or structure in theage. The finer the particle size, the cess does not necessarily mean plastic state and reduces slumpingmore water layers are present, and an earthenware or whiteware during throwing or handbuilding,there­fore the greater the water con- clay—the raku and bonfire pro- allows thinner, taller wares withtent, and the greater the drying cesses often use highly refractory greater horizontal extension, andshrinkage. But at the same time, stoneware bodies that are simply reduces drying shrinkage. •the finer the particle size, the morecontact points between particles in underfired at low-fire tempera-the dry state, which gives greater tures and are therefore very porousdry strength in greenware and more and open, giving high thermalbonding surfaces in the early stages shock resistance. Low firing isof the firing. The ideal condition is especially appropriate for largeto have a mixture of sizes of clay par- sculptural work, as there is littleticles. This creates as much contact or no shrinkage in low firing, andsurface as possible between particles, common problems with crackinggiving good plasticity, dry strength, and warpage are minimized. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Handbuilding Chapter 2 With handbuilding, the full scope of sculptural form and expression is available to the potter and the sculptor. It offers innumerable possibilities in structure and surface that are impossible or impractical on the wheel. With handbuilding processes, one can construct almost any imaginable shape. These processes are divided into three main categories: pinch, coil, and slab construction. Pinch and coil construction are the prevalent forming meth- ods in all ancient and tribal cultures. Slab construction was widely used in Meso-American pre-Columbian cultures, but otherwise is rarely found until modern times. Pinch construction is by far the simplest of these pro- cesses and provides the ideal introduction to all other ceramics construction methods. Pinch forms are usually confined to less than six inches in diam- eter, although there are exceptions to this. Pinch forms can be made either very thin and delicate or thick and substantial. There is something quite remarkable about an eggshell-thin pinched form, sanded and burnished, and blackware bonfired to a satin black luster. Coil constructed forms can take almost any shape imaginable. Coiling is the only method where novices can quickly learn to make very large ves- sels or sculpture, and the experience can rapidly increase their confidence in clayworking. Coiling is especially appropriate for organic forms, large volumetric vessels, and figurative sculpture. Clay slabs can be worked in either the soft-slab or stiff-slab method, indicating the condition of the slab while it is being worked. The soft- slab method is appropriate for slumped, draped, or soft-formed vessels, for clay masks, or for draping onto forms to make components to be stiffened and assembled. It is also ideal for making a wide variety of curvilinear and organic forms. The stiff slab technique is more appropriate for rectilinear and architectural forms, although the two approaches can be combined. Even for hard-edged geometric forms, many sculptors and vessel makers slump-mold curved components and assemble them leather-hard along with flat, stiff-slab components.www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Handbuilding • Chapter 2Wedging the Clay homogenizing any irregularities in Forming Method: Use the The wedging process is important composition or moisture content, method appropriate for the formsin all ceramic construction tech- and more importantly, eliminating you wish to make—for organicniques, as poorly wedged clay sim- air bubbles. Remember not to push shapes use coil, pinch, or soft slab,ply does not respond well. Wedging down from above, flattening the for geometric shapes use coil ormixes and homogenizes the clay and lump, and be sure not to push so stiff slab.eliminates air bubbles. Do not ever hard on each stroke that you smear Awareness of Surface Effects:settle for clay that is too wet or too the ball out too flat, because when With an appropriate forming methoddry—it simply isn’t worth it. With you stand it up and push again it and sensitivity to the surface createdclay that is too wet it is a simple will fold over on itself, trapping air during initial con­struction, subse-matter to wedge it on a dry plaster instead of eliminating it. Each push quent surface finishing may not beslab, or to slice it, stand the slices should just smear the bottom of the necessary. The forming or assemblyvertically and allow them to stiffen lump over a little, without ever trap- process itself can often create patternsup. Any clay that is too dry can be ping more air in the clay. or marks that are very pleasing.sliced, wetted down, and left in a Generally a minute or so of wedg- Concentration of Pressurebag or barrel overnight to absorb ing is enough, but, if you are blend- Points: All stages of manipulatingthe moisture. If you stiffen or soften ing different clays or working dry the clay depend on concentrationyour clay by these methods, be sure material into overly wet clay, it may of pressure points, giving you muchto wedge it very thoroughly before take considerably more wedging. If greater control than with a broadusing it. the clay lump begins to elongate to application of pressure over a large When first learning to wedge, either side, slap your palms against it area. EVERY part of your hand iscylinder wedging is usually the easi- to narrow it. useful. Remember that for everyest approach to master. Start with a action there is a reaction. Keep inball of clay that you can comfortably Handbuilding: mind the consequences of everystretch your fingers around (like a General Guidelines type of pressure that you apply tolarge orange). Set the ball on a flat and Suggestions the clay.surface, preferably a canvas-covered The following guidelines and sug- Wall Thickness: Make pieces onlytable. Hold both hands as you would gestions apply to all clayworking, as thick as is necessary. Extra thick-to shake hands with someone, and but especially to handbuilding. If ness does not necessarily increasegrab the ball firmly with your hands you familiarize yourself thoroughly structural integrity. In tall pieces youwrapped around either side, as indi- with these guidelines, the learning may wish to make the lower wallscated in Fig. 2.1. Applying pressure curve will be accelerated. slightly thicker, but otherwise main-downwards against the table, push Commanding Approach: Clay tain uniform thickness. For mostthe ball away from you slightly, mov- appreciates a vigorous, command- sculpture and vessels, even a verying it two or three inches so that the ing approach with an economy of large piece should not be more thanbottom smears forcibly against the motion. Whatever you do, do it for I" to 1" thick anywhere.table. Roll the ball back up towards a specific reason. With an aggres- Base Support and Bottom:you (standing it up on the smeared sive, adventurous approach, you will Always construct large forms on a“extension”), grab again on either learn much faster than with a weak, sturdy movable board, with severalside as before, and push down and tentative approach. layers of newspaper under the clayaway as before. Repeat this at least Clay Consistency: Always start to prevent sticking and to allow for20 times for each lump of clay. with well-wedged clay of an appro- contraction of the piece during dry-This process creates a spiral twist- priate stiffness or softness for the ing shrinkage. On coil or slab work,ing and stretching within the lump, desired project. always build on top of a bottom slab www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Handbuilding • Chapter 2 of clay. Whenever possible, haveFigure 2.1 Cylinder Wedging a single continuous base that supports all parts of a piece. For example, if you are building an animal form with delicate legs, incorporate a base as part of the piece, thereby supporting and protecting the legs. Closed Spaces: Never create completely closed spaces, as they will trap steam pressure and may explode in the kiln. Always make breather holes. Tiny pinholes are adequate, but make several in casePlace hands on either side of the lump in Lean into the lump. one becomes clogged. Wheneverhandshaking position. possible, ALWAYS leave larger breather holes. Never apply addi- tions or appliques in such a way that air is trapped beneath them. Joining: Always join clay aggressively and firmly in a way that is appropriate for the con- sistency of the clay. NEVER press clay straight together with- out proper preparation, as this will result in the alignment of particles in a fracture plane. Soft clay may be joined with-Continue leaning into the lump, smear- Roll the top of the lump directly uping the lower portion against the wedg- towards you and grasp the sides. out scoring or slurry, especiallying table. in coil construction, as long as the pieces are smeared thor- oughly together, blending the seams inside and out to ensure thorough interlocking of the particles. In all other circum- stances, when joining parts or adding soft clay or appliques to a surface, score thoroughly with a fork or toothed rib, be sure to use enough slurry to force out all air bubbles, and press the parts in place firmly to eject allLean into the lump again, roll it back C  ontinue until the lump is well blended excess slurry. With leather-hardtowards you, and repeat. and shows this appearance. clay, always fit the pieces well, www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Handbuilding • Chapter 2score thoroughly, use a generous metal tool, the surface feels scratchy, the clay’s ability to support itself.amount of slurry, and immediately and the displaced material falls away With large pieces, cover the surfacesjoin the pieces with firm pressure. freely as small particles. Many peo- where more parts are to be joined,In joining component parts or slabs, ple like to carve the clay at this and allow the rest of the piece towhenever possible score the inside stage, although a point between stiffen before continuing (or accel-of the joint and press in a small coil medium and hard leather-hard is erate stiffening with a hair dryer orof soft clay along the seam. Ideally, usually more desirable. This stage is propane torch).pieces to be joined together should appropriate for thin surface paint- Structure: At all stages, considerbe the same consistency and stiffness. ing, but not for thick slip techniques the structural integrity of the clayHowever, this is not always possible, like slip-trailing. Once the clay has (in both wet and dry stages) and itsand when joining pieces of dissimilar begun to bleach (lighten in color ability to support the upper por-moisture content, cover the assem- from drying) it has passed beyond tions of the piece. With large sculp-bled form and allow the moisture the leather-hard stages. tural forms, cross brace the interior.content to equalize before allowing Assembly: Regardless of forming Always support horizontal protru-the form to dry. Do not attempt to method, it is often advantageous sions or extensions with a tempo-join pieces that have dried beyond to build large pieces in sections to rary prop until they are firm enoughmedium leather-hard, except when be joined when soft or medium to support themselves. Do not over-using paper clay techniques. leather-hard or to be fired separately. look the possibility of using waddedStages of Leather-Hard: After the fire, they may be stacked newspaper as an interior support to Soft leather-hard is the stage loose, assembled on an armature, or maintain the shape of closed formswhere you can easily pick up a small cemented together with epoxy or or to support relief forms such asor medium-size piece without dis- silicone adhesive. masks. It may be left in place andtorting it, but the surface is still a bit Avoid Using Water: Do not use will burn away in the firing.tacky. When trimmed with a trim- water to smooth or finish a piece as Controlled Drying: Always care-ming tool, the trimmings tend to you work on it—it will just soften fully control the rate and degreeball up and stick under the tool, and the clay, removing structural integ- of drying. When time constraintsthe form is easily distorted or dam- rity and making it harder to work. demand it, large pieces may be stiff-aged. This is an ideal stage for joining Do not use water in place of slurry ened with a heat gun, hair dryer,parts, as long as they can be handled when joining pieces, except perhaps or propane torch before continu-without serious damage. with very soft clay. Other­wise, it ing with construction, but this can Medium leather-hard is the stage may just lubricate the clay surface result in uneven stresses on the clay.when all surface tackiness is gone, and encourage the formation of When necessary during construc-and plastic clay will not stick easily fracture planes. You may, of course, tion, pieces may be moistened withwhen pressed against the clay. When use water (applied with a sponge, a spray bottle to retard drying. Anythe surface is incised with a model- brush, or spray bottle) to slow down large pieces to be left covered withing tool, the displaced clay rises in the drying process when­ever it is plastic should first be draped withattached ridges along the groove. proceeding too ­rapidly. a large cloth (old towels or bedWhen trimmed, the trimmings fall Working in Stages: Each part sheet) to absorb and dissipate mois-freely away, and will not stick to the of the process should be done at ture. Otherwise, condensation undersurface or the trimming tool. the appropriate stage of softness or the plastic can run back onto the Hard leather-hard is the stage hardness. When constructing large piece in concentrated areas, possiblywhere the color is still “damp,” but shapes consider the clay’s ability causing collapse. Large complicatedthe clay is too stiff for easy trimming. to support higher sections. Do not pieces should be kept covered andWhen incised with a wooden or attempt to keep working beyond allowed to dry very slowly. Small www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Handbuilding • Chapter 2parts that protrude from a form thick­ness and the strength and resil- and thinning only the clay in themust be protected from quick dry- iency of the clay. Through pinch- lower portion of the pot, withouting, especially if they attach in two ing, we become extremely sensitive expanding the opening and theor more separate places. The easiest to the use of touch rather than upper rim.solution to this problem is to coat sight in determining and control- When you have made one passthose parts with wax resist. This will ling ceramic form. With practice, around the bottom, or when thecause the moisture to wick into the pinch construction can become area you are pinching has reachedbody of the piece, so that everything a viable method of creating small a suitable thickness (no more thandries at the same rate. In general, be vessels very quickly without the G"), begin moving up the wall,sure to complete each part of the imposed mechanical precision of pinching in a very gradual spiral,process at the appropriate stage of the potter’s wheel. thinning the wall as you go. Thedryness. Do not attempt to bring a As you work through these outside surface may develop shallowpiece that is too dry back to a work- instructions, refer to the accom- surface cracks, but these are not aable moisture content. panying series of images in Fig. problem as long as the clay doesn’t Finish the Bottom: This is one 2.2.To begin, wedge a softball-size get too dry while you are still work-of the most frequently neglected lump of clay thoroughly. Divide ing it. You can smooth these cracksareas in ceramics. Whether a non- the clay into 2"-diameter balls, and with your fingernail or a rib, but, iffunctional sculptural form or a loosely wrap all but one in plas- left, these shallow cracks can formfunctional vessel, a sloppily fin- tic to protect from drying. Place a very attractive surface texture notished bottom or lower edge can a ball in your cupped hand and unlike aged leather. Deep cracksruin the appearance and feel of with the thumb of that hand, begin are another story, especially if theyotherwise good work. In general, penetrating the center of the ball. are forming in the rim. As you area sharp edge is a mistake, as the With a rhythmic series of motions, expanding the pot and workingform seems to blend into or grow alternate between pressing your your way up the walls, if the sur-out of the surface on which it sits. thumb into the clay and rotating f ­ace begins to crack badly or if theUnless this is your intention, cre- the ball around your thumb with rim begins to crack at all, you mustate a slight undercut at the base the fingers of the other hand, keep- tend to it right away. Have a smallto create that all-important line of ing the ball resting in your cupped container of slurry handy, and atshadow that sets the piece off from hand the whole time. Keep this up the first indication of a bad surfacethe surface beneath. until you can feel the pressure of crack or a rim crack, put a very small your thumb through the bottom amount of slurry on the crack andMaking Pinch Forms of the ball, but stop before it actu- work it back together with your fin- No matter what one’s intended ally breaks through the surface. You ger or a modeling tool. Other­wisegoals or present skill level in ceram- have now established what will be do not use any water on the surfaceics, the pinch construction method is the center of the bottom of the pot. of the pot while it is underway, asan excellent exercise and an exciting At this point, instead of pressing water will soften the clay and inter-way to make small vessels. Whether straight down into the lump, begin fere with your progress.you are handbuilding or working on pressing the tip of your thumb to As you work your way up thethe wheel, the actions of the hand in the side to widen the bottom of the walls of the pot, the rim may inevi-squeezing and manipulating the clay pot, again alternating with rotating tably expand more than you want.and the relationship between the the pot (press, rotate, press, rotate, If this happens, cup your handinside and outside of the vessel are press, rotate, etc.). With your thumb over the rim and gently squeeze,of critical importance. Pinch con- bent, pressing only with the tip of rotate, squeeze, rotate until you havestruction teaches sensitivity to wall the thumb, you will be spreading reduced the rim to the desired www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 
    • Handbuilding • Chapter 2Figure 2.2Pinch Construction a.  egin pinch construction with a 2" B b.  radling and rotating the lump C diameter ball of clay. in one hand, penetrate it with your thumb.c.  otate continuously as you R d.  top when your thumb pressure S e.  inch to the side to begin P penetrate the lump. forms a dimple on the bottom. forming the walls.f.  otate continuously, working up the walls. R g.  inch to the final thickness and P h.  ith a finger inside, smear the clay W resolve the surface. inwards to reduce the opening. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 10
    • Handbuilding • Chapter 2diameter. Another good method for been the method known as coil and the personality of the makerreducing the rim diameter is to construction. This is still the stan- throughout its form. Altogether tooreverse the position of your fingers, dard method in all tribal pottery often, these qualities are erased onwith a finger on the inside under traditions, such as those currently wheel-thrown forms.the rim and your thumb on the active in Africa and the Americas. In general, a smooth-turningoutside smearing the clay inwards We must avoid any assumption that banding wheel offers tremendoustowards the center of the opening. coil construction is in any way advantages in most coil construc-Rotate the pot continuously as you inferior or primitive. The form and tion. If a kick wheel is available, it isdo this, and with practice you can surface of a coil pot is controlled by great to be able to rotate the formclose the rim to a very small open- the potter from start to finish with- with your feet while both hands areing, or even close it completely to out the influence of any mechani- busy adding coils and smearing themcreate an enclosed form. cal devices, and the expressive and together or in modifying the form. Finish the rim however you wish. imaginative possibilities are endless. Coil vessels are constructed by anSome pinch-potters like to leave a Even the most uniform and sym- additive process of building up thethicker rim, whereas others like to metrical coiled pot expresses the walls with long ropelike coils of claypinch the rim to a sharp edge. Some beautiful imperfection of humanity (or other repeating modules, suchpinchers like to keep a symmetrical,even pot and/or rim; others pre-fer an asymmetrical pot and/or anuneven rim. Each to his or her own. Figure 2.3 Rolling Coils for ConstructionIf you want to have an even, circularopening, let the pot get leather-hard, and then trim the rim. If youare going to sand the pot when dry,wait until then to resolve the shapeof the opening. When the pot is completed, youof course have numerous optionssuch as the application of tripodfeet, a pedestal base, small hanginglugs, handles, or other appendages. Ifyou wish to make spherical shapes,it is possible with a single lump, butyou also may wish to pinch twohemispheres and join them whenleather-hard. Pinch construction isonly practical for vessels up to fouror five inches in diameter, so theaddition of ornamental or func-tional elements can greatly increasethe visual impact.Coil Construction Throughout history the prevalenthandbuilding approach has always www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 11
    • Handbuilding • Chapter 2as small pancakes). Always start off the coil, bringing both hands back and press a single course of coils inwith a good supply of well-wedged to the center, moving then outwards place, working it well to force outclay—keep your clay supply cov- again, and repeating the process until excess slurry. As long as you areered with plastic so it doesn’t dry you achieve the desired length and using coils of soft clay, you need notout while you are working. This is diameter of coil. Do not attempt to score and slurry further as long ascritical, as the coils should be very roll coils with stiff clay. Use soft clay, you proceed with adding coils. Iftacky when joined. You must make and make sure the rolling surface you leave the form for a period ofthe choice of hand-rolled coils or, if is well dampened. If the coil starts time and the walls begin to stiffen atthe equipment is available, extruded to go oval, purposefully restore it all, then you should score and slurrycoils. You can roll long, uniform to round before continuing with before adding the next coil.coils very quickly by hand, without your rolling. With a little practice, When adding each successive coil,the mechanical precision and shape you will be able to roll uniform drape the coil from one hand so thatimposed by the extruder (Fig.2.3). coils of whatever size you wish as the coil end lays on the vessel wall on If you wish to make a ­ flat-bot- quickly as they can be produced with the far side of the form. With yourtomed form, select an appropriate an extruder. thumb on the inside and fingers onwooden board or bat and cover it For a small coil pot (8"–12" diam- the outside, aggressively smear thewith several thicknesses of newspa- eter) the coils should be K" to 1" in coil downwards on the inside withper. Make a flat slab of clay for the diameter, depending on how aggres- the thumb and upwards on the out-base—this need not be precisely sively you thin out the walls as you side with your fingers, as shown inrolled out—you can simply slap a join the coils. For a larger coil pot, Fig. 2.4e. Rotate the vessel (or movelump of clay between your hands the coils can be I" to 1H" or more around it) and slowly lower the coilor pound it out on a flat surface to in diameter. There are several con- onto the wall, smearing it in place asform a slab of an appropriate thick- siderations here. Once you become you go. Some coilers prefer to applyness for the intended form. Usually accustomed to coil construction, you single-level courses of coils one-by-it’s best to start with a slab larger will be able to anticipate how much one, whereas others tend to coil in athan the desired base and trim it you thin the coils during the join- continuous slow spiral. It will greatlyback after the lower portion of the ing process. Some tribal potters use help in this process if your coil potpot is complete. This also ensures coils that are several inches thick, but is sitting on a banding wheel orthat you always begin coiling the during the joining process the wall a kick-wheel. Tribal potters oftenwalls on top of the base, rather than is thinned to less than K". Also, once build their coil pots on the groundby laying the first coils next to it. the basic form is roughed out, you or on a low pedestal and walkThis will give a much stronger joint can come back and thin and form around the pot backwards as theybetween the slab base and the coiled it with the paddle-and-anvil or rib- add the coils and work the form. Ifwalls (Fig. 2.5). and-hand methods described below. you want the form to expand in size A very thick cylindrical coil form as the walls rise, you can reverse theCoiling the Walls may be thinned and shaped to a broad smearing movements, smearing up Roll out (or extrude) a supply of volumetric jar. It is essential that you on the inside and down on the out-coils. For the best coils, roll them anticipate these things in determining side on the side closest to you. Withon a well-moistened porous or the size of coils to be used. practice you can cause the diametercanvas-covered table. When rolling Don’t roll out too many coils to increase or decrease at will bycoils, spread your fingers apart (see ahead of time, because if they stiffen altering these movements. Do notFig. 2.4) and, as you roll forwards up at all they will not join satisfac- worry about cosmetic surface dur-and backwards, quickly move your torily. Score and slurry the desired ing this process. If you are smearinghands outwards towards the ends of attachment point on your base slab, aggressively, you will leave a rather www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 12
    • Handbuilding • Chapter 2Figure 2.4Coil Construction a.  esuming coil construction on an in-prog- R b. Apply a coil and gently paddle in place.  ress vessel: Score and slurry the edge.c.  mear the soft coil downwards inside S d.  s the next coil is lowered into place, A e.  dd additional coils. A and out over the stiffened edge. smear it aggressively onto the previ- ous one.f.  fter every three or four courses, A g.  mear additional coils inwards to S h.  o further close the rim T smear the coils together with diago- reduce the opening. opening, smear inwards nal strokes. with a rib. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 13
    • Handbuilding • Chapter 2ragged surface appearance, but this Meso-American, and South­west convolutions and formed or assem-is evidence of well-joined coils. It is Native American cultures all feature bled immediately. With the stiff-slaba simple matter to stop periodically extraordinary design and technique method, the slabs are allowed to dryand smear the surface smooth with in coil construction. to leather-hard and are then cut toyour fingers or a rib. size and joined together. Be careful when increasing or Closing the Mouthdecreasing the diameter of your coil of a Coil Form Rolled Slabs andform to any radical degree, because When you wish to narrow the “Memory”the clay must have the structural neck or mouth of a coil vessel you When clay is formed by applica-integrity to support itself. Any time can simply apply the coils to the tion of pressure, the clay mass isa clay wall veers away from the verti- inner surface of the rim and smear compressed and the platelets arecal, gravity will make it tend to lean inwards on the outside of the vessel. pushed around in currents, depend-or collapse. If you are contemplat- As a refinement of this, support the ing on the type and direction ofing a radical expansion or reduction inside of the rim with your fingers pressure applied. The clay retains ain diameter, you must either allow and smear inwards over the top sur- memory of the compression and thethe clay to harden sufficiently as face of the rim with a rib, rotating currents, and it will shrink accord-you proceed, or you must provide a the form continuously. As long as ingly during the drying and firing.physical support for the clay. there is sufficient thickness in the This is true with all forming meth- If you leave your vessel for any rim, you can close the opening as ods, but it is rarely a problem. In slabtime, always cover the top coil with much as you want by this method. If construction, however, it must bedamp paper towels to keep it moist the rim is not thick enough to allow taken seriously. When rolling slabsand wrap the whole pot in plastic. this, stop and add another coil, and with a slabroller or rolling pin, if youWhen you resume work, if the then proceed with this method. roll only in one direction, you aretop coil has stiffened at all, always setting up a grain structure similar toscore and slurry before proceeding, Slab Construction a wooden board. Both a board andand paddle the first coil in place to Of the primary ceramic form- a unidirectionally rolled slab haveensure a good joint (see Fig. 2.4b). ing techniques, slab construction is greater strength along the lengthIf there seems to be a lot of con- the most modern. The technique than across the width, but will shrinkdensation inside the plastic in your of constructing with clay slabs is more across the width than alongstudio environment, drape a cloth or suitable for a wide variety of forms the length. If you assemble a largesheets of newspaper over the form and is the ideal method for achiev- slab form from these slabs, with thebefore covering it with plastic. This ing flat-sided geometric and archi- grain structures intersecting at angles,diffuses any moisture, which evapo- tectural forms. Historically, such the piece will likely pull itself apartrates and recondenses where the forms were generally built by the because of differential shrinkage inplastic touches the form. Without coil method, and except for the the drying and/or firing, especiallythis precaution, the condensation remarkable ­ slab-constructions of with high-­shrinkage claybodies, andcan concentrate and cause the clay pre-Columbian Meso-America, in high-fired work. This is the mostform to collapse. there is little evidence worldwide common problem area in slab con- The possibilities of coil construc- of slab con­struction before the struction. There are several solu-tion are unlimited, so almost any twentieth century. tions. If your slab­roller has a wideobject or form can serve as inspira- There are two primary kinds of enough bed, initially roll the slabtion. The ancient and tribal tradi- slab construction.With soft-slab con- thicker than you want, turn the slabtions of East Asian, Middle Eastern, struction, the slabs are manipulated 90 degrees, reset for a thinner rollerMediterranean, African, Peruvian, while still soft to form curves and height, and roll the slab again. With www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 14
    • Handbuilding • Chapter 2hand-rolled slabs, simply change the laying across the table pointing away degrees, and reroll until you get therolling direction frequently. In either from you, with the clay between the size, shape, and thickness you want.case, this will equalize the compres- two slats, and with the ends of the For greatest accuracy, measure thesion and currents, minimizing subse- rolling pin resting on the two slats, thickness in several places with aquent problems. you can roll slabs to an exact thick- needle tool, just as you would mea- All slabs are subject to this ness. However, with practice you sure the bottom of a pot.p­ roblem, whether you are mak- can roll slabs very uniformly with-ing slumped dinnerware, geometric out using the slats, and you are then Soft-Slab Constructionboxes, or tiles. In all cases, memory able to make use of the full width Soft-slab construction is the pre-of improper rolling can ruin the of the rolling pin. In either case, be ferred method for a wide variety offinished product. sure to turn the slab 90 degrees sev- vessel and sculptural approaches. It is eral times while rolling to equalize ideal for any form that may be cre-Rolling Out Slabs the compression. ated by wrapping or draping slabs on Commercial slabrollers are a Pound your clay into a rough slab, slump or drape molds, or by simplywonderful innovation, and if your place it on an appropriate sheet of manipulating a soft slab by hand. Thework calls for quantities of large canvas (depending on the size you following is a range of very simpleuniform slabs, you should consider want) on a sturdy table, and start projects that will serve as a goodthis major investment. If you do not rolling it, changing directions fre- introduction to soft-slab construc-have a commercial slabroller at your quently.Very soon the clay will stick tion and should provide a foundationdisposal you can easily roll out your to the canvas and won’t expand any for more ambitious soft-slab work.slabs by hand with a rolling pin. As more. Lay another sheet of canvaslong as you don’t need really huge on top, grab both sheets and the Soft-Slab Cylindersslabs, this is not a disadvantage, as slab, using the broad surface of your A wide variety of cylindrical orwith practice you will be able to fingers to minimize distortion of cone-shaped vessels may be maderoll out slabs by hand almost as the slab, and flip the whole work free-form out of soft slabs. Cups orfast and just as uniform as with a over. Pull off the top (formerly bot- mugs provide an excellent soft-slabslab­roller. Even if you are using a tom) sheet, pull the wrinkles out of project, and for these you should useslab­roller, keep in mind also that the the bottom sheet, turn the slab 90 slabs no more than G" thick. Cut aroller tends to stretch slabs only in degrees, and roll some more. Roll strip of slab as wide as the desiredthe direction of travel. In order to out from the center, and minimize height of the cup and as long as theavoid irregular shrinkage or serious pressure as you approach the outer desired circumference. Score andwarping, these slabs should really edges, because it is very easy to get slurry the ends of the strip and joinbe aggressively hand-rolled on both the slab very thin at this point. Roll them together to form a cylinder.sides at 90 degrees to the original along the edges as well, but when You can either butt-join them (enddirection of travel. doing so, apply much more pressure to end) or you can overlap the ends. against the end of the roller that is Set the cylinder on a flat piece ofRolling Slabs by Hand on the slab than that hanging over slab and, with your needle tool, Using an extra-long rolling pin the edge. With practice, you will mark lightly around the bottomwith bearing-mounted handles, you find that by rolling outwards from circumference. Remove the cylin-can quickly roll out very large uni- the center and along the edges in der and score and slurry inside thisform slabs. If you need absolutely specific directions, you will be able circle, then press the cylinder gentlyuniform thickness, make a series of to control the way the slab expands into place. Cut away the excess basepairs of long wooden slats in gradu- and thus control the finished shape. slab, turn the cup over, and gentlyated thicknesses. With a set of slats Continue to flip the slab, turn it 90 tap the bottom slab around the www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 15
    • Handbuilding • Chapter 2edges to join it more firmly to the This is an excellent technique to molds made from plywood or foamcylinder. Finish the cup however use if consistency and uniformity of insulating board have become veryyou wish. An alternate approach is size and shape are desired, because popular. When these are set on ato use bisque stamps or a sheet of all the vessels made from one mold flat table surface, clay slabs may betextured material to create a pattern can be very similar in size and shape. slumped into these forms to createor texture in the flat slab before Professional potters make their flat-­bottom plates and trays. Similarly,making it into a cup. molds out of plaster, bisque-fired simple hump forms of plaster, wood, If you want uniform, evenly clay, plywood, or foam insulating or foam board provide excellentmatching cups, you may wish to board, but almost anything of the hump molds, and when slabs areform your cups around a removable desired shape will work as a mold. slumped facedown over such molds,core such as an empty soda can. Cut See Figs. 2.5 and 2.6. the edge may be trimmed and a foota strip of G"-thick slab and wrap it You can make bowls and plates ring added immediately, minimizingaround the soda can to determine by simply slumping slabs inside an later finishing.the length of strip needed, then lay existing bowl or plate. Whenever For improvised slump-molds, shal-the strip back down on the table and the “mold” is a nonporous material, low forms work best, where a singlecut it to length. Wrap an appropriate be sure to use several layers of news- slab can usually be gently pressedwidth and length of dry newspaper paper strips beneath the slumped or into place. If you use a deep bowlaround the can (or other cylindrical draped clay to keep it from stick- as a mold you will have to apply theform). Don’t try to do this without ing to the mold. With slump molds slab in several pieces, scoring, slur-the layer of dry newspaper, as you made of plaster, bisque-fired clay, rying, and vigorously connectingwill be unable to slide the form out plywood, or foam board, no news- the joints. After the bowl stiffens upof the cylinder. Score and slurry paper layer is required. Open-center a bit, invert it on a board, removethe ends of the slab-strip, wrap itaround the newspaper-covered can,and join the ends together. Whilethe clay strip is still wrapped aroundthe can, if you wish you may press Figure 2.5 Foam Slump Moldsbisque stamps or textured materi-als into the surface for decoration. After you have joined the ends ofthe slab strip and added any addi-tional stamped decoration, hold thecylinder carefully cradled in onehand, push out the can, and removethe newspaper. Proceed with addingthe bottom as explained above.Slump-Molds In the current world of functionaland decorative ceramics, slump- ordrape-molded bowls and platters a.  sing a foam slump mold: Drape U b.  ently drop the mold repeatedly Ghave become extremely popular. to settle the slab into the mold. a slab over the mold.This method simply involves slump-ing a soft slab inside or over a mold. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 16
    • Handbuilding • Chapter 2the mold or form, and use a wooden the added advantage of allowing you to the subject. There are a number ofrib or modeling tool to smear together to add a raised foot-ring to the base if ways to make tiles. The simplest is toany wrinkles or gaps on the outside you wish. However, if you try to slump roll out uniform slabs with a slabrollersurface. Or, as a decorative option, a deep-dished form on the outside sur- or by using a large rolling pin withmake sure to smear the inside connec- face of a rigid mold, you must remove a set of wooden slats as previouslytions very well and leave the outside it while still very damp, as any drying described. Let the slab stiffen to softseams visible. shrinkage could cause it to crack. leather-hard, and then carefully cut As mentioned, you can slump inside your tiles using a razor knife and aor outside of a mold or form. If you Making Tiles straightedge, taking into consider-slump a very shallow platter or bowl There are some excellent books on ation drying and firing shrinkage.over a convex form, you may be able the market about tile making, and this Be sure to consult the section onto allow it to dry in place, and this has will be only a very short introduction “Rolled Slabs and Memory.” If you Figure 2.10  Foam Hump Molds a.  sing a foam hump mold: Place U b.  ently drop the board repeatedly G the mold on a board and drape to settle the slab over the mold. the slab over it. c.  core and slurry the back to accept S d.  lace a coiled foot ring and gently P e.  lean off excess slurry with a C a foot ring. paddle it level. damp sponge. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 17
    • Handbuilding • Chapter 2make tiles from improperly made slabs, then this method will not work. Inthe clay memory will cause them to this case, the best solution is to simplywarp during drying and/or firing. make sure that the tiles are mobile Some production tile makers extrude (they are not stuck to the surface) andtheir tiles from a pugmill, using a spe- ensure that they dry slowly and evenlycial extrusion die with a thin horizontal under plastic or in a controlled humid-opening, producing a long continuous ity environment like a damp/dry box.ribbon of clay that can be cut to any Whatever method you use for mak-desired length. For small quantities of ing tiles, if your intention is to glazetiles, a similar extrusion die on a stan- them, all your efforts will be wasteddard clay extruder works quite well. unless you use a glaze that is a perfect Drying tiles is always a challenge, in match to the claybody. This is espe-order to produce perfectly flat tiles. cially true in high-fired tiles, where theOne good approach is to sandwich the clay becomes very pyroplastic. If thetiles between 24" squares of 1⁄ 2" dry- firing shrinkage of the glaze is greaterwall (sheetrock) until completely dry. than the clay, the tile will becomeFor economical use of space, you can concave during the glaze firing. If thestack up to six layers of drywall and firing shrinkage of the glaze is less thantiles. Obviously, this method works the clay, the tile will become convex.only for tiles that are of absolutely You can do simple tests ahead of timeuniform thickness, are completely to determine which glazes are appro-flat, or have only shallow incised or priate for your tiles. When necessary,impressed decoration. you can adjust the formula of a glaze If the upper surface of the tile is not to correct thermal expansion and makeflat, or if the thicknesses are irregular, it fit your claybody. •www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 18
    • Throwing Chapter 3 Of the various ceramic construction methods, throwing on the potter’s wheel is the most common in Western, European-based culture. For producing large quantities of individually handmade functional wares it is the most efficient method. A traditional approach to throwing involves completion of the pot upon the wheel so that very little needs to be done afterwards to finish the form.This is especially true of certain jar, vase, bottle, and bowl forms. Even if a trimmed foot or a knob is needed, we often use the wheel for those tasks as well. It is a joy to use the wheel in this way, but consider also that the wheel is an extremely versatile tool for making com- ponent parts to be assembled off-wheel into vessels or sculpture. The wheel excels for making hemispheres, spheres, ovoids, flat disks, cylinders, cones, tubes, and myriad other shapes. All of these components can be assembled in an unlimited number of ways. If you do not explore these possibilities, you are missing one of the best things that the wheel can do. Critical Points in Throwing The following review is designed to help beginning clayworkers become familiar with the most important points in throwing. There are lots of steps to keep track of, and it will help greatly to go over this list frequently and commit it to memory. Prepare Clay Properly. Wedge the clay thoroughly before using. This is especially important for wheel-thrown work. Don’t waste time on clay that is too wet or too dry. If clay is too wet, let it sit out until stiffened adequately and rewedge, or wedge it on a dry plaster surface, or wedge in some dry material (although this decreases plasticity). Don’t try to reuse clay from a previous pot that didn’t work out—let it stiffen up and rewedge it before using again. Prepare Plenty of Clay. With only one or two balls of clay in reserve, it is easy to develop an attitude of preciousness about each pot and a despera- tion to make each one work. Always prepare at least a dozen balls, and if a pot is not working, remove it immediately and start another one.www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 19
    • Throwing • Chapter 3 Clean and Dampen the for any shaping task. If you want and finishing small vessels. Use slowWheelhead. The ideal surface for to smooth or flatten a broad area, speed for large vessels.receiving the clay is slightly damp use a rib. Action→Reaction. For everybut not wet. Use a rubber rib to Centering: Full Body Con­ action there is a reaction. Keep insqueegee off all remaining slurry trol. Centering and wheel wedg- mind the consequences of everyfrom the previous pot, and if any ing depend on full body control. type of pressure that you apply tosurface moisture remains, remove it The work is not done by your arm the clay, and, when lifting or shap-with an old towel (Fig. 3.1). muscles, but rather by the full force ing a vessel, apply corresponding Slap Center. Before applying of your upper torso when you roll pressure both inside and outside thewater, slap center a ball of prop- your hips forward on the chair or form. Avoid expanding the diametererly prepared clay on the wheel- bench (Fig. 3.3). of a form without pressure fromhead or bat, and seal down the Work on the Right-Hand both sides, unless you are very sureedges thoroughly (Fig. 3.2). Side of the Wheel. After center- of the anticipated outcome. Avoid Touching the Clay ing, always work at the right-hand Lubrication. While throwing,When the Wheel Isn’t Turning. side of the wheel where the clay keep the clay lubricated adequatelyExcept for slap centering, always is moving away from your hands at all times. Excess friction is yourstart the wheel before applying (assuming you are throwing counter­ enemy. But remember that water ispressure, and always remove pressure clockwise). continuously absorbed into the clay,before stopping the wheel. Work at the Correct Speed. decreasing structural stability. As you Aggressive Commanding Use full speed for centering, wheel practice throwing, try to work fast.Approach. Clay appreciates a wedging, and penetrating the lump Remove Excess Water. Removevigorous commanding approach of clay. Use medium speed for wid- excess water whenever possible andwith an economy of motion. Be ening the bottom, lifting the walls, as soon as possible. If you workclear about the specific reason forwhat- ever you do, and rememberthat with an aggressive, adventur-ous approach you will learn very Figure 3.1 Throwing Positionquickly. Take risks. Concentration of PressurePoints. All stages of manipulatingthe clay depend on concentrationof pressure points. Concentratingforce on a small area of the claywill give much greater controlthan broad application of pres-sure over a large area. Centeringprimarily involves pressure withthe base of the right hand. Liftingthe walls involves pressure withslightly bent fingertips rather thanthe flats of the fingers or the hand.Once the clay has taken the formof a basic vessel, you should never Proper seating position when throwing.use the broad surface of your hand www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 20
    • Throwing • Chapter 3slowly and a piece begins to soften, inwards before cutting the pot off in throwing is to keep the outsideyou may remove excess water and the wheel. When cutting off pots, profile and inside profile as similar asslurry with sponges and ribs. When hold the cutoff wire very taut. possible. In other words, if the wallsyou reach an approximation of the When cutting off large flat shapes, are of uniform thickness, then thedesired shape you may remove all always do so with the wheel turn- outside and inside will be very closeexcess water and do the final shap- ing to keep the cutoff wire from to the same contour, whereas if theing with fingers, damp sponges, climbing up into the base of the pot. lower wall is very thick, the outsidethrowing stick (jug finger), or rub- Trim or otherwise finish the bottom and inside will be very different inber, metal, or wooden ribs. carefully, and make sure that there shape. Start off with a series of six When Throwing, Use the is a line of shadow around the base, or eight small cylinders, workingBest Tool for the Job. Usually separating it from the surface upon up to about 4" or 5" in height. Trywe throw with our fingers, but which it sits. to achieve walls that are no morethat is not always the best choice. than K" thick at the bottom and G"When throwing porcelain, it often Skill Development just below the rim, with a smoothlyworks very well to throw with a with Cylinders tapering wall in between. The bot-sponge on one side and a rubber or One of the best ways to develop tom thickness is not so crucial, butmetal rib on the other, or even with your ability on the wheel is to throw remember that if you wish to trimtwo rubber or metal ribs, especially lots of cylinders (Fig. 3.4). As you the bottom it should be K" to H" orwhen making broad, voluminous complete each one, cut it in half more thick (this will give you ampleforms. When making a tall form vertically with your cut-off wire thickness for trimming a raised foot-with a narrow rim, it makes sense to to observe the cross-section pro- ring), and make sure that the bot-neck in the rim immediately upon file. Don’t try to do this with other tom is flat and level, with a distinctachieving the desired height, and shapes, because they will collapse as rounded corner where the bottomthen use a jug-finger to broaden the soon as you cut them, whereas half ends and the wall begins. Later youbody of the vessel. of a cylinder will still stand up. When may wish to throw forms with a Attention to Rims. When lift­ you view the cut cylinder there curved bottom, but for skill develop-ing walls, always stop just short of are several important things to look ment it really helps to become famil-the rim. Compress rim regularly, and for. First of all, a primary objective iar with flat-bottom cylinders.on cylinder-based forms keep rimdiameter smaller than base diameteruntil wall height is established. Synchronize Movements. Figure 3.2 Slap CenteringWhen lifting or wheel wedging,synchronize the movement of yourhands with the speed of the wheelso as not to leave deep spiral groovesor marks. Much of the problemsin learning to throw are caused bylifting too fast in proportion to thespeed of the wheel. Cutting off the Pot and Fin­ishing the Bottom. These can Seal the lump down with the lower edge of your hands while the wheel ismake or break an otherwise good turning slowly.pot. Always bevel the lower edge www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 21
    • Throwing • Chapter 3 When you cut each cylinder in mentioned earlier, centrifugal force not attempt to repair it. If there ishalf remember to carefully observe tends to force the clay outward from sufficient clay below the crack youthe uniformity of the walls, the the center, and in making bowls may cut away the damaged portionsmoothness and thickness of the you can take advantage of this force and proceed with what is left.bottom, and the uniformity and rather than having to counteract As a general rule, in the early stagethickness of the rim. There should it. When lifting the walls, simply of throwing any form it is wise tobe very little variation in thickness lift outward and allow the form avoid radical horizontal orientationof the walls from bottom to top, to expand. Be especially careful to of the clay. In other words, in lift-and the lower inside corner should compress the rim with every lift, ing and widening a bowl form dobe quite square, as indicated in Fig. because the rapid expansion of the not widen the bottom too much3.5g. With each successive cylinder, diameter can easily cause the rim to initially so that it hangs over thetry to respond to the problems or crack. If the rim does crack badly do foot. Instead, raise the walls so thatfaults in the previous ones, so thatyou are continuously experimentingand developing your technique. Figure 3.6 Throwing a Bowl If you find yourself frustrated withthe ordinary cylinder, try doingtimed cylinders. Sit down at thewheel with 20 or 30 small balls ofclay, and have someone time you.This is a great exercise to do in aclassroom situation. Start with five-minute cylinders, with 30 secondsin between to scrape off the previ-ous one and put a new ball of clayon the wheel. When time is called,immediately stop the cylinder youare working on and scrape it off the a.  hrowing a bowl: Lift outwards. T b.  ift repeatedly, maintaining Lwheel. After two or three of these, straight walls.reduce the time to four minutes,then three, two, and finally one-min-ute cylinders. You will be absolutelyamazed at the progress you can makethrough several of these sessions.Throwing Bowls Generally, most bowls are thrownon bats. In making bowls, penetratethe lump and widen the bottomjust as in forming cylindrical forms,but there is no need to undercutwhen widening to keep the opening c.  rim away excess clay from base. T d.  esolve shape with rib, fingers, or Rnarrow. This will result in a wider, sponge.lower doughnut-stage (Fig. 3.6). As www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 22
    • Throwing • Chapter 3Figure 3.3 Centeringa.  entering and wheel wedging: C b.  rofile of properly centered lump. P c.  ontinue the side pressure while C Initially center the clay with firm hinging hands together, causing side pressure. clay to rise.d.  ross section showing intermedi- C e.  aise the lump to a rounded peak. R f.  ross section showing maximum C ate stage of wheel-wedging. rise, with heel of thumb poised to press clay back down.g.  ith continuing side pressure to W h.  ontinue pressing downwards as C i.  inalize centered form after F prevent mushrooming, press clay lump widens. repeated wheel-wedging. back down. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 23
    • Throwing • Chapter 3Figure 3.4 Throwinga.  asic throwing: After centering B b.  enetrate the lump to create a P c.  iden the bottom using the “claw” W and wheel-wedging, form a dim- V-shaped opening. motion, and compress the bottom. ple and apply water.d.  egin lifting the walls. B e.  aintain firm, even pressure and M f.  top lifting just shy of the rim. S steady upwards movement.g.  ompress the rim. C h.  egin second lift, pressing into B i.  ontinue lifting slowly and steadily C the clay at the base. up to the rim.j.  epeat with the third lift. R k.  egin shaping the vessel as you B l.  hink about the shape as you fol- T lift. low through. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 24
    • Throwing • Chapter 3Figure 3.5 Throwing Cross Sectionsa.  ross sections of basic thrown C b.  enetrating the centered lump. P c.  idening the bottom. W form: Creating the dimple.d.  he initial lift. T e.  top just shy of the rim. S f.  econd lift, pressing into the base. Sg.  roper position during lifting, P h.  inal lift. F i.  esolving the shape. R with inside fingers slightly ahead of outside fingers. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 25
    • Throwing • Chapter 3they are straight or slightly convex shape you desire. For small bowls the open, depending on tempera-(like the flare of a trumpet) rather it is generally best to finish the ture, humidity, and air movement. Ifthan concave (like a hemisphere), shape with your fingers, but for you won’t be able to get back to itand make sure that initially the larger bowls it usually works best until the next day or later, you mustwalls flare out at no more than a to use a sponge or a curved rib to cover it with plastic sheeting. The45º angle from the bottom. This is establish the inside profile. With best material to use for this is a dry-an extremely stable form that will the wheel rotating at low-medium cleaner bag or trash bag cut open toallow you to achieve the desired speed (medium for a small bowl, form a rectangular sheet. Wrap theheight, diameter, and wall thickness. low for a large one) work the fin- plastic under the edges of the bat Before proceeding further, always gers, sponge, or rib up from the or ware board to prevent air cur-trim away excess clay from the out- bottom of the bowl, curving the rents from drying the pot. In veryside base, (Fig. 3.7) because it will be clay outward a little with each pass dry weather, especially in the wintervery difficult to get access to this area from bottom to top. With your right when a forced-air heater is running,when the curvature of the bowl is hand, always follow the position of a pot may dry out in a day or twocomplete. If you are planning to trim the rib with gentle sponge or finger even when well wrapped. Checkthe bowl, then you need only trim pressure on the outside of the bowl. your pots frequently, and if they areaway excess clay and create a slight Repeat these passes until the bowl drying too much, spray them lightlyundercut to facilitate cutoff. If you takes the desired shape. When using with a household spray bottle filledare not planning to trim the bottom, a rib to shape a bowl be sure to with water. If you anticipate havingyou can undercut the edge with a incline it so that it slides smoothly to leave them for several days andwooden rib, applying gentle pressure over the clay rather than scraping or are concerned about excessive dry-with a sponge to the outside base, cutting into it. ing, you can drape dampened paperand create a very pleasing “foot.” towels or rags over your pots or sim- For small bowls you may use your Drying Your Pots ply place several wads of wet paperfingers, a sponge, or a curved rib to It is possible to throw some shapes towels on the ware board next to,bring the walls out to the desired so that they have a perfectly accept- but not touching, the pots and wrapcurvature. On large bowls it is a able base straight off the wheel, with them securely in plastic. Even ingood idea to remove all slurry and very little finish work (Fig. 3.7). In very dry weather, this should keepwater from the inside and outside most cases, however, you will need them at the leather-hard stage for atsurfaces before final shaping. You to trim the bottoms of your pots. least a week.can accomplish this with a sharp- After finishing the pots and plac- Pay very close attention to theedged rib, like the standard stain- ing them on ware boards you must drying process. It is much easierless steel kidney-shaped rib. You anticipate when you will be able to trim a pot or add handles withcan also squeeze all water out of to work on them next and pro- good results when you do it at thetwo sponges and, with the wheel tect them appropriately from exces- proper soft-to-medium leather-hardat slow-medium speed, use them to sive drying. All trimming, attaching stage. Checking pots frequently toremove water and slurry from the of handles, slip painting, appliquesurface. Either of these techniques work, and most decorative carvingwill in effect “wring” water out of must be completed at the leather-the clay, firming it up structurally so hard stage, when the clay is stiffit is less likely to distort or collapse enough to handle but still damp.as you proceed. Upon completing a pot, if you Final shaping can be done with a plan to trim it later that same dayvariety of tools, depending on the you may be able to leave it out in www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 26
    • Throwing • Chapter 3monitor drying is simply part of the forth, until the four lumps are equally surface and to the trimming tool. Ifprocess. If a pot dries beyond proper spaced around the pot. Maintain pres- the clay is too dry, it will feel verytrimming stage before you trim it, sure with your hand against the center hard and scratchy when trimming.don’t waste time on it—recycle it. of the pot throughout this process, Do not continue with trimming if so that it doesn’t shift on the wheel- the clay is too wet or too dry.Finishing the Bottoms head. For a low bowl, plate, or cup After the bottom is completely shape, very small lumps are adequate. level, begin trimming off all excessof Your Pots For taller mug and jar shapes, larger clay from the outer edges of the Place the pot upside down in the lumps will be necessary. The lumps base. If the lower walls are thickcenter of the clean wheelhead. Start should never be pressed hard against you may trim some clay off them,the wheel at slow speed, and hold the clay form, but they always must but try not to rely on this any moreone finger near the edge of the pot. be smeared down thoroughly against than necessary. As much as possible,Slowly move it in towards the pot the wheelhead. trim the excess clay off the loweruntil your finger touches the edge When selecting trimming tools, walls while the pot is still wet onof the pot once on each rotation. you will have the greatest control the wheel.This is the point where the pot with a very small trimming sur- When trimming a leather-hard pot,is farthest off center. On the next face. The corners of a square-ended remember that the objective is torotation when the pot touches your band-loop trimming tool give you make the outside profile (except forfinger, stop the wheel right at that the best option. Hold the handle of the foot ring) as much as possible likepoint. Move the pot slightly towards the tool in your right or left hand the inside profile, and trimming excessthe center of the wheel and repeat just as you would a pen or pencil, wall thickness is perfectly acceptable.the process until the pot seems on with the square end pointing down. Use the flat cutting edge to refinecenter. When you get good at this, Brace it firmly near the tip with the trimmed area and to blend it intoyou may wish to try the tap-center the fingers of your other hand and the untrimmed part of the wall. Usemethod, where you gauge whether with the cutting edge of one corner the curved end of a trimming tool tothe pot is on center as describedabove, but then, without stopping begin trimming the pot—remember trim any convex areas.the wheel, tap the form to move concentration of pressure points. If the bottom of the pot was quiteit further on center. Many profes- Bring the wheel up to medium thin to begin with, you may notsional potters use this method and speed, start at the center, and trim a be able to trim a raised foot ring,can center a pot almost instantly. If slow overlapping spiral all the way in which case the trimming pro-you are not accustomed to it, it may to the outer edge. If the bottom is at cess is finished at this point. If youseem all but impossible. all irregular in height, then you will really want to find out how good When the pot is centered, form need to take repeated passes to trim a job you’re doing, you may havefour small lumps of clay, each the size it down to level. Hold the tool very to sacrifice a few trimmed pots byof your thumb. While gently applying firmly close to the tip with both cutting or breaking them in half todownwards pressure against the center hands during this process to over- observe the cross-section profile ofof the bottom with one hand, press come any irregularity in the bottom. the trimmed bottom.one lump in place, gently nestling If the clay is the proper medium If there is enough thickness tothe lump against the pot and apply- leather-hard, then it will trim very trim a raised foot ring, after you haveing primary force to smear the lump easily and evenly, and the trimmings leveled the bottom and trimmed offdown against the wheelhead. Avoid can be easily brushed off the surface any excess clay from outer edgesapplying excessive pressure against the with a soft brush or with your fin- and lower walls, examine the profilepot. Rotate the wheel and pot 90º, gers. If the clay is too wet, the trim- of the pot and decide what kind ofpress another lump in place, and so mings will ball up and stick to the foot ring you want. In some cases www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 27
    • Throwing • Chapter 3the foot ring might be right up alternative, you can throw a cookie a small metal disk, like a jar lid oragainst the base of the wall, but more of clay on the wheelhead, remove a bottle cap, in the center of theoften it is recessed back beneath the all moisture with your metal rib, pot for the finger to rest against tobase of the wall, as on Japanese tea and place the pots upside down hold the pot down to the wheel.bowls. On the one hand, a wide on this surface to trim them. In With practice, this system can befoot ring gives more stability, which either case, some potters like to put extremely quick and efficient. •may be desirable in a coffee cup orpitcher. On the other hand, a nar-row foot ring, set back beneath thelower edge, elevates the pot from Figure 3.7 Trimming Excess Claythe table surface, which can beextremely important in the overalldesign. If you decide on a recessedfoot ring, trim the lower cornerof the pot inwards to where youwant the foot ring to start, andtry to get the outer shape as closeas possible to the inner shape, sothat the wall is a uniform thicknessdown the side and inwards towardsthe foot ring. After the outer edgeof the foot ring is established, takeanother pass starting from the cen-ter of the bottom, stopping in timeto leave a K"-wide foot ring. Tap a.  rimming excess clay away from T b.  ribble water along the knife to Don the center with your fingertip. base of finished pot: Slice into the feed it into the cut.If it makes a dull click, then there clay with a wooden knife.is still plenty of thickness, and youmay take several more passes acrossthe bottom inside the foot ring. Tapthe center of the bottom betweeneach pass, and as soon as you beginto detect a hollow, drumlike sound,stop trimming, as this indicates thatthe bottom is quite thin. Be sureto trim any sharp corners from thefoot ring. Some potters trim without theuse of clay lumps to hold the potin place. If you wet the wheelheadslightly, the pot will stick to it, andwith slight pressure against the c.  lice horizontally under the waste S d.  top the wheel, cut through the Sbottom center of the pot with one flange with the wooden knife. waste flange, and remove it fromfinger, you can trim very effectively the pot.without attachment wads. As an www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 28
    • Surface Decoration on Greenware Chapter 4 There are a multitude of surface decorating options available at many stages during the ceramic process. This chapter explores a wide range of surface decoration possibilities on damp or dry greenware. The first category covers methods where clay is moved around on the surface. These include impressed, additive, and subtractive decoration. Decorative Effects During Forming It is frequently possible to create interesting pattern and surface effects during the process of building and shaping a vessel or sculpture. As you work the clay, constantly be aware of the marks your fingers and tools are making. In learning to draw we explore “marks on the ground” in order to discover all the mark-making possibilities in each medium. It is to your advantage to do the same with clay whenever possible. Interesting coil patterns may be achieved during the coil construction process by applying coils, balls, or wads in any combination or orientation to give decorative surface variation. Join the coils well and smear together thoroughly on the inside. Pinch/smear patterns may result from the way you manipulate the clay during the construction process, as when working coils together in coil construction. Paddled facets may be formed in the clay by gently beating with a wooden paddle. Colored clays may be laminated onto or pressed into the surface of the clay on any sort of vessel or sculpture. Impressed Decoration There is a range of techniques that involves pressing fingers, stamps, found materials, or tools into the clay, displacing clay without actually removing it from the surface. Fingertips, fingernails, or knuckles may be used to impress simple grooves or patterns into the soft clay. In thrown or soft-slab forms, a fluted effect may be created by drawing fingers upwards inside and outside the form simultaneously, with a finger on the inside corresponding with the space between two fingers on the outside. Repeat this motion around the entire form.www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 29
    • Surface Decoration on Greenware • Chapter 4 Template ribs have a cut or plaster contamination in the clay.The allow all surface moisture to evapo-carved profile that may be dragged best stamps are made from clay and rate before using stamps on the sur-across a wet clay surface to cre- are bisque-fired, creating a porous face. Make sure to provide supportate effects very much like wooden surface that will not stick to damp behind the surface where you aremolding, as shown in Fig. 4.1. They clay. Stamps may be made by lift- using a stamp or roulette. If you usemay also be used to create these ing a negative impression off of any a bisque stamp repeatedly on a veryeffects on wheel-thrown pots, using raised-relief patterned or textured wet clay surface it will soon absorbplenty of water to lubricate the rib. surface with a piece of soft clay or so much water that it will stick to Modeling tools may be used to may be carved in hard leather-hard the clay. When this happens, washimpress decorative marks or patterns or bone-dry clay. When stamps are all the clay off the stamp and dry itwithout removing any clay. bone-dry or bisque-fired you may thoroughly before using again. Combed effects may be accom- also take a second impression off of Roulettes or coggles are patternplished by dragging a toothed tool the stamp with another piece of clay, rollers used on wet or soft leather-across the surface (Fig. 4.2). Comb- returning to the positive impression hard clay forms, either static or turn-ing is usually done in wet clay, of the surface you started with, giv- ing on the potter’s wheel. Roulettesespecially on pots on the wheel. ing you both positive and negative may be made from any hard mate-Normal combing tools include a stamps. However you make your rial, but the most effective rollers arefork, toothed rib, notched modeling stamps, attach a convenient handle made of clay and are bisque-fired.tool, or hair comb. to them before or after firing. A circular disk of soft clay may be Stamps for impressing patterns or Bisque stamps work best on rolled on a patterned surface, or asymbols into soft clay may be made freshly worked clay that has had all leather-hard or bone-dry disk mayfrom any rigid material, but durable surface slurry and moisture scraped be carved around the circumference.porous materials work best. Plaster away with a metal rib. Or, you may Rollers made of other material mayworks, but there is always the risk of wish simply to set the forms aside to be wrapped with string, rope, orFigure 4.1 Using Template Rib Figure 4.2 Combing Surface Using a template rib on a well-lubri- Combing the surface with a fork. cated thrown form. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 30
    • Surface Decoration on Greenware • Chapter 4other textured materials. Coggles on layer of small pebbles or wood of dryness of the clay. The quality ofshould be made with a hole accu- chips. In some cases you may wish mark and surface will also depend onrately formed through the center to simply paddle the surface of a the kind of carving tool used. Theseand after firing may be mounted on form with no backup support, while might include a pointed razor knife,a wire or wooden axle and handle in other situations you may wish trimming tool, coring tool, fettlingthat allows them to turn smoothly. to use a rounded stone or a clay or knife, or cheap wood-­carving tools. Like bisque stamps, roulettes work wood “anvil” on the opposite side of Although carving tends to be mostbest on soft clay, but you must the wall you are paddling. satisfying when the clay is medium toscrape away all slurry or mois- Patterned or textured materials hard leather-hard, gestural “smeared”ture, or let the form dry slightly may be pressed into the clay—bur- effects are possible with very soft clay,before using the roulette (Fig. 4.3). lap, lace, string, cord, cheesecloth, and very interesting stonelike effectsRoulettes work especially well on wood grain, and plant materials such can be attained by carving grittythrown forms on the wheel. Be sure as leaves, ferns, twigs, or bark, or a claybodies when bone-dry. In theto provide backup pressure on the piece of clay may be pressed against classic Chinese and Korean carvedopposite side of the wall where you a patterned or textured material celadons, the carving was usuallyare using the roulette. and then fired, making a permanent done in the very hard leather-hard or Textured paddles may be made reusable stamp (see stamps). bone-dry stages.from wood or bisque-fired clay. Or,a bisque-fired textured tile may be Subtractive Methods Fluting can be accomplished by aglued to a wooden paddle. A wood This category includes decorating number of means. One is describedor clay paddle may be incised with a methods that involve removing clay previously in the section on creat-pattern of grooves or holes. A wood from the surface of a piece. ing decorative surface effects withpaddle may be wrapped with fabric, Carving may be done at any stage the fingers. For the most dramaticrope, cord, or wire. It may be cov- from soft to bone-dry. You will find effects, clay is carved from the surfaceered with coarse sandpaper or other that very different surface effects are of a leather-hard pot, using a trim-patterned material or with a glued- produced depending on the degree ming tool or a fluting tool (Fig. 4.4). Figure 4.3 Using a Roulette a.  sing a bisque roulette on a plate. U b.  sing a bisque roulette on a rotat- U c.  sing a wooden roulette. U ing pot. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 31
    • Surface Decoration on Greenware • Chapter 4 Faceting may be done by pad- Piercing openings into a clay form balls of clay onto the surface. A thindling, as described previously. More may be done for a variety of reasons. coat of slip (without scoring) is ade-often, facets are cut into the surface The effect can be purely decorative quate when applying sprigging, butof a thicker-than-usual piece using on sculptural forms, or can be func- be sure to apply the slip a patch at aa cutoff wire when the piece is still tional on incense-­burners, candle time so that it is still wet when thevery soft, or when slightly stiffer lanterns, etc. Tools used in pierc- sprigging is pressed into place. Won-using a fettling knife or cheese ing include hole punchers, razor derful patterns like fish scales or furslicer, or when leather-hard using a knives, or fettling knives. Piercing may be created by sprigging on smallSurform file or potato peeler. is normally done at the medium pads or rolls of clay. Consider doing Scraping can give a very inter- leather-hard stage, but may also be sprigging with a contrasting coloredesting surface, especially on a accomplished at the hard leather- clay or with a layer of contrastinggritty claybody. For a good stone- hard or bone-dry stage with an colored slip under the sprigging.like effect, scrape the surface at ordinary drill bit. Modeled decoration includes athe hard leather-hard stage with Incising usually refers to shallow variety of methods where clay isa sharp metal rib. Other scraping carved lines or patterns. As a general added to the surface and then mod-tools include wood and rubber ribs, rule, always do incising with a fine- eled with fingers or tools to createspoons, seashells, household scrap- pointed carving tool, a dull-pointed decorative elements. Coils may beers, gourd-ribs, and corncobs. dowel, or a dull pencil. Avoid incis- added on and then smoothed into the ing with a very sharp pointed tool, surface to create raised ridges. Wads, because each incised line is a score- strips, or pads of clay may be applied mark waiting to crack. Incised lines and formed with fingers or tools. Figure 4.4 may be carved through a contrasting For large molded sculptural add- Using a Fluting slip coating as in the sgraffito tech- ons, instead of modeling the form Tool nique, or may be filled with slip as and then attaching it to the piece, in the mishima technique. considering attaching a large mass of clay to the piece, and then doing the Additive Methods modeling with both hands. When This category includes all meth- leather-hard, you can always carve ods where clay is added to the through from the inside to remove existing surface of a clay form. In extra clay. • general, you should score and slurry all add-ons, except in sprigging. If the add-on is soft plastic clay, score and slurry only the attachment point on the form. If the add-on is leather- hard, score and slurry both surfaces. Appliqué generally refers to pre- formed flat pressed or carved clay decorations added onto the surface of a piece. Appliqués are often press molded ahead of time in plaster or bisqued-clay molds. Using a fluting tool on a soft leather-hard pot. Sprigged decoration (sprigging) is done by pressing small coils and/or www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 32
    • Glazes and Glazing Chapter 5 Introduction to Glazing As you begin each ceramic piece, and as you proceed through the stages of formation and decoration, consider all the possibilities of surface decoration, and make sure that each surface effect you employ enhances what is already there. Some people choose to carefully plan each piece completely from start to finish, whereas others prefer to allow the form and surface to evolve as the piece is constructed. Either approach is fine, as long as the results are satisfactory. As you resolve each piece, carefully consider what the final finish should be. If you are making functional vessels, a glazed surface is expected (at least inside). If you are making sculpture, glaze is just one option to be considered. In some cases, the unornamented clay or a chalky slip surface may be just what you want. Glaze Color Glaze color alone in­volves a range of variations including hue, value, intensity, and saturation. Hue is the actual color, such as red, blue, or orange. Value is the quality of light or dark. Intensity is the brightness of the hue— the degree to which it seems to jump off the surface. Saturation is the depth and richness of color, resulting from the concentration of pigment. Glaze Transparency and Surface Glazes are glass with ­ various modifiers added to affect their behavior and appearance. Depending on the glaze materials and modifiers present, the glaze can be gloss, semigloss, semimatt, or matt. These are all qualities of reflectivity and visual/actual texture on the glaze ­ surface, but variables in the glaze appearance also involve qualities within the glaze, includ- ing transparency/opacity, translucency, iridescence, thickness, and mobil- ity (viscosity at maturation temperature). The most transparent glazes are invariably glossy, because anything other than a high-gloss surface interferes with transparency. However, high-gloss glazes can be a bit garish and tendwww.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 33
    • Glazes and Glazing • Chapter 5to show every surface flaw. Potters active glaze–claybody interface, and Many experienced potters applyare often much happier with semi- other physical phenomena, which wax only to the bottom of the piece,gloss or semimatt glazes. Transparent produce glaze strength and surface and, after glazing, remove any glazesemigloss glazes are very appropriate characteristic of natural stone, thus residue from the waxed surface withover slip-­decorated surfaces, show- the term stone­ware. a broad flat sponge. With moderateing the variations of slip color while pressure, this will also remove a bitmaintaining a softer surface quality. Glazing Methods of glaze from the lower walls. Wax resist and other resist com- For application of wax emulsionGlaze-Firing Ranges pounds such as liquid latex are a to the foot of a piece, some forms Glaze-firing temperatures are tra- very important accessory to suc- can simply be dipped into a veryditionally measured with pyrometric cessful glazing and surface decora- shallow pool of resist in a widecones, and we often refer to glazes tion. There are a variety of water- pan. Another option is to place aaccording to the pyrometric cone at e ­ mulsion wax resists available flat sponge in a shallow pan of waxwhich they mature. commercially. Some potters prefer emulsion so that the surface of the Low-Fire: Throughout history hot paraffin or candle wax heated in sponge sticks up slightly above theworldwide, most ceramics have been an electric frying pan. All of them liquid level, allowing the wax tolow-fired, in the range from cone work, but some are better than oth- soak into the sponge. The foot may012 (1623°F) to cone 02 (2048°F). ers. Experiment. Commercial wax then be rotated against this spongeAlthough the clay remains porous resist can be applied with a brush or to give an adequate coat of waxat low-fire temperatures, many sponge (the small triangular makeup on the bottom and slightly up theceramic colorants that volatilize or sponges work well). For sponge wall. The softness and “tooth” of thebreak down at higher temperatures application the consistency may be sponge will determine the degreeare stable in low-fire, and available appropriate right from the jar, but to which the lower wall of the formglazes offer virtually every color of generally a little water is added to is coated.the rainbow. give a good brushing consistency. It is important to realize that wax Mid-Range: Mid-range, extend- Commercial wax resist is usually resist works only to restrict adher-ing from cone 4 (2167°F) to cone colorless and may be hard to see ence of glaze, but it is never com-7 (2264°F), has become an increas- once it is applied to a surface. To pletely effective, so you must alwaysingly popular firing range. Mid- remedy this, add a few drops of food go back and wipe any glaze accu-range firing can give a truly vitreous coloring to the wax emulsion. mulation off the waxed surface withbody, with a range of color almost Apply wax to any surface that will a damp sponge or clean damp brush.as extensive as low-fire. For bright be within J" of the kiln shelf. With Also, it is difficult or impossiblecontemporary effects, mid-range a trimmed foot, there should be at to remove wax resist once applied,gives the ideal combination of good least J" vertical clearance to allow except by rebisque-firing the piece.durability and impermeability and the bottom to be glazed inside the When applying wax resist with abroad color possibility. foot. On any piece that sits directly brush, never approach your work High-Fire: High-fire, extending on the kiln shelf without a raised with an overloaded brush. Alwaysfrom cone 8 (2305°F) to cone 12 foot, there should also be a narrow drag the freshly dipped brush on the(2419°F), produces the most vitre- band of unglazed clay at the base of edge of the wax container to lightenous and durable wares. Accelerated the walls. Of course, this all depends the load a bit.glaze mobility in the superheated on the performance of the glaze atmelt encourages mineral disper- maturation temperature, and once Contamination of Glazession, desirable crystal formation, you get to know your glaze you In any studio, especially in groupagglomeration of colloidal particles, may be able to reduce the clearance. studios, glaze contamination can www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 34
    • Glazes and Glazing • Chapter 5be a severe problem and must be Many clear high-gloss glazes must must be done very cautiously. Whenconsciously avoided by all involved. be applied very thinly or they will using multiple overlays of glazes, care-Never dip a still-wet glazed piece turn milky. Because clear glazes fully consider possible shifting andinto another glaze. Never use a are often applied over polychrome running of the glaze accumulation.whisk, stir-stick, drill-mixer, or dip- slip decoration, an excessively thickper in another glaze without wash- glaze coating would be a disaster. Brushing Glazesing it off thoroughly. Always label With any unfamiliar clear glaze, test The practicality of brushing glazescontainer lids, and put the appro- various thicknesses of application onto the surface depends on thepriate lid back on each glaze. When before using it on work you really firing temperature and the viscosityyou dip out a portion of glaze, be care about. Matt glazes often tend of the maturing glaze. Any “tight”sure to return it to the appropriate to be less runny, whereas gloss glazes glaze that resists flowing may showcontainer as soon as you are done are usually more mobile in the melt. all brushmarks, and therefore maywith it. It is often impossible to These are not absolutes, however, and not be suitable for brush application.identify a liquid glaze from appear- you must observe the performance In the case of overglaze decora-ance, and stray containers must then of every glaze through previous tion on top of a glaze that has beenbe thrown away. No one can afford examples or appropriate tests. Firing dipped, poured, or sprayed, brush-such waste. temperature is, of course, critical, as marks are often accepted as part of a tight glaze can often become much the process.Glaze Consistency and more mobile with only a single cone Most gloss raku glazes are fluidThickness of Application increase in temperature. enough in the melt for all brush- There are no hard and fast rules The wall-thickness of a piece will marks to heal. Most satin or matthere. The most common fault in have a major effect on the thickness raku glazes are usually texturalglazing is excessive glaze thick- of glaze coating it accepts. Very thin enough that brushmarks are not aness on the wares. Some glazes do pieces become saturated with water concern, and in fact often add to therequire thick application, but many very quickly and will accept only a surface quality.perform far better when applied thin coat of glaze. Remember, it is In general, brushes used for glazethinly. If applied thickly, any glaze the water soaking into the surface application should have long, softthat is quite liquid at maturing tem- that causes the glaze to deposit bristles, which give good reservoirperature will run off the piece onto properly on the surface. With very capacity. Bisqued clay or a dry-the kiln shelf and other wares. In thin pieces, it is usually a good idea glaze coating absorbs a great deal ofthis case, the piece is usually ruined, to glaze the inside, allow the piece water, and a brush with small reser-and the process of grinding glaze to dry completely, and then glaze voir capacity will not work unlessoff the shelf is not a pleasant task. the outside. streaky dry-brush effects are desired.With a new glaze, always run tests to When applied with a brush, glazedetermine how the glaze performs Using Multiple Glazes thickness and resulting visual effectsat various thicknesses. Whenever using multiple glazes on tend to respond to the movement For an accurate gauge of proper one piece, remember that any overlap of the brush along with the surfaceglaze consistency, take a piece of may produce a logical blend of the contour and texture of the pot.scrap bisque, dip it into the glaze, two glazes or it may produce effects To minimize brushmarks, soak theshake it off, and as soon as the glaze is completely unlike either of the two brush in water beforehand, shakedry, scratch through the coating with glazes. In such cases it is best to do lots out all water, load it completelya needle tool.You can then gauge the of experimentation on test pieces. with glaze, and flow the glaze evenlythickness of the coating, and thin the Multiple layers of different glazes can onto the surface with slow strokes.liquid glaze accordingly. give very interesting effects, but this With a little practice, it is possible www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 35
    • Glazes and Glazing • Chapter 5to get a fairly smooth glaze coating wok works extremely well for glaz- On a vertical surface, until youwith minimal brushmarks. ing plates and bowls. Special glazing know your glazes very well, it is wise tongs can be constructed to assist in to limit double dipping to the upperDipping Glazes this task. half of the piece and triple dipping Dipping and pouring are the most With many pieces, the simplest to the rim.common glaze-application methods means of getting an overall coatingfor functional pottery. It is essential of a single glaze is to dip half of the Pouring Glazesthat glazes be mixed to the right piece and when dry, grasp that half Pouring is very closely related toconsistency for dipping, as discussed to dip the other half. This works well dipping and requires similar glazepreviously. Only you can determine even for functional work, depending consistency. Pouring may be used tothe correct dipping consistency for on the glaze. With some glazes the glaze the inside of any vessel, to glazeyour glazes. It is critically impor- overlap will be very obvious, and you the outside of a vessel too large fortant to think about the amount must place the overlap with overall dipping, or to apply decoration overof time an object is immersed in design in mind. If you dip quickly another glaze.glaze. Usually a very quick dip and and shake excess glaze off quickly, When planning to glaze the in-a quick, firm shake-off is adequate you can usually minimize any visible side of a vessel by pouring, andand will give an even glaze coating overlap mark. the outside by dipping or pouring,with minimal runs and drips. This Interesting glaze results may often always glaze the inside first. If youis a matter of practice and con- be achieved with overlapping coats spill glaze over the unglazed outsidescious intent. Some glazes perform of different glazes, although this tech- it is easy to sponge off, whereas ifbest with a slightly thicker coating, nique requires considerable experi- the outside is already glazed youwhich may be achieved by hold- mentation to avoid serious problems will have to live with it or clean theing the piece in the glaze slightly with running and crawling. Some- entire outside and start again. Whenlonger. Counting seconds will give times an overlapping combination of glazing the inside, pour a cup or twoyou consistent results. Before you two glazes displays qualities of both of glaze (depending on the size ofdip a piece, be sure you are holding glazes, whereas on other occasions the vessel) in and quickly tilt vesselit firmly enough to dip it and shake the result is completely different around to coat all surfaces, then pourit off. Avoid holding forms by thin, from either glaze. This technique is excess glaze back into the bucket.weak sections. Also, keep in mind especially advantageous when work- With a little practice, you can pourwhere the heaviest accumulation of ing with a limited number of glazes, the glaze in, slosh it around, windglaze ends up, depending on how because it offers many more possibili- up your hand and arm, and pour theyou shake off the excess glaze. ties.There are some simple guidelines glaze out while unwinding so as to For small objects, a set of dipping to consider. Any time you double dip glaze the entire inside and inner rimtongs usually works well for overall you are, of course, increasing glaze in one smooth movement.glaze coating. The pointed prongs thickness, and therefore increasinggrip the piece firmly for dipping, risk of glaze problems such as run- Glaze Faultsbut leave only very small marks in ning and crawling. If the first glaze No matter how careful and con-the glaze coating, which usually heal has a fairly tenuous, powdery con- sistent we are in buying and storingcompletely in the glaze melt. nection with the clay surface, dip- materials, in mixing and applying For overall glazing on plates and ping in a second glaze will often glazes, and in loading, firing, andbowls, grasp the piece with several cause both glazes to peel and flake cooling kilns, periodic glaze prob-fingers on opposite sides of the off the piece. When double dipping, lems are inevitable. Glaze materialsrim, and dip the whole piece into always dip the second glaze after the change from one batch to the next,a wide tub or bowl of glaze. A large first has dried completely. and we are all capable of human www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 36
    • Glazes and Glazing • Chapter 5 error. It is important that we learn may be caused by a number of fac- to recognize glaze faults and to deal tors. If localized, especially if the with them in an appropriate manner glaze is quite thick, the cause is most to correct the problem. likely a residue of grease or dust on Pitting and pinholing usually the bisque surface before glazing, result from air escaping from the which can interrupt the glaze–clay porous clay during the application of interface. The solution is to keep the glaze, and occasionally from out- unwanted grease, oil, or wax off gassing of volatiles during the glaze the bisquewares and wash or brush firing. In many cases, pits and pin- off any dust or powdery residue of holes are already present in the raw, grinding and sanding. When apply- unfired glaze surface, and if left alone ing wax resist, be especially careful they may not heal in the firing. When to avoid getting waxy fingerprints you see pinholes in the dry glaze you on the wares. If you use a skin pro- can seal them over by rubbing gently tectant before glazing, do so at least with your fingertip. You can usually a half hour ahead of time, and then eliminate serious pinholing problems wash your hands with cold water by bisque-firing slightly higher, by and mild hand soap immediately applying the glaze(s) slightly thinner, before glazing. or by dipping wares in water 10 min- Some coloring oxides, including utes before‑glazing. iron oxide, cobalt carbonate, and If the pinholes are not already vis- rutile, are especially troublesome if ible in the dry glaze coating, they are used underglaze and will almost usually due to outgassing of volatiles certainly result in bad crawling on in the latter stages of the firing. A flat areas. glaze that tends to pinhole during fir- Glazes applied excessively thick ing can often be cured with a simple frequently crawl in the firing, espe- oxidation soak at the end of the fir- cially if there are visible cracks in the ing, which stabilizes outgassing and raw glaze surface after it is dry. This allows the glaze to heal. If none of is especially common in improper these measures cure the problem, it double- and triple-dipping of glazes is likely due to excessive viscosity at (see the section on glaze thickness the maturation point, in which case earlier in this chapter). a slight increase in flux content or Any glaze that is excessively pow- a substitution of more powerful flux dery will tend to develop a very might help. weak, dry bond with the clay surface Crawling is one of the most and may peel in the melt. In this destructive and troublesome glaze case, you can add 2% bentonite or flaws and is characterized by the liq- substitute ball clay for kaolin in the uid glaze peeling or receding, leaving recipe or reformulate the recipe to areas of dry clay surface. Crawling increase clay content. •www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 37
    • Kilns and Firing Chapter 6 All of us have in common the use of clay, usually the use of glazes, almost always the use of kilns. Ideally, our relationship with the kiln is that of friend and collaborator, with each firing bringing a sense of thrill and discovery. With proper knowledge and training, any good kiln becomes a benign collaborator. General Kiln and Firing Practices Electric Kilns Electric kilns come in many sizes and configurations and are the most inexpensive commercially made kilns available. The installation of an elec- tric kiln is very simple and inexpensive, making them very attractive for first studios and home studios. A broad range of ceramic processes and fir- ing temperatures are suitable for the electric kiln, including ultra-low-fire luster and enamel firing, low-fire oxidation and raku, and mid-range and high-fire oxidation stoneware and porcelain. For those wanting an earthy, smoky effect, wares can be bisque-fired in an electric kiln and then bon- fired, raku-fired, or sawdust-smoked. There are also limited possibilities for sagger firing and reduction firing in electric kilns. An electric kiln is simply a refractory box containing electric heating ele- ments. The smallest electric test kilns and top-loaders operate on 110 volts, whereas most of the studio top-loaders, oval kilns, and smaller front-load- ers require 220 volts. Larger commercial electric kilns generally operate on 208-volt three-phase power. Because of the specialized features of the wiring and control systems and the nature of the element installation, it is normally impractical to consider building electric kilns. Firing Logs Keep complete written logs for all firings, including notations of any anomalies in kiln performance and firing outcomes. You will never regret this practice. For electric top-loaders the firing log may consist of only a few lines written in a notebook, but that is enough to tell you a great deal about the performance of your kiln over time. Firing logs should includewww.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 38
    • Kilns and Firing • Chapter 6the time, date, intended maturation Opening Hot Kilns in the element grooves, and do suit-point (cone number), notation of Never begin opening a hot kiln able repairs.which cones are in the kiln, a com- while there is any red heat vis- In electric kilns equipped withplete firing schedule including all ible, and never open the kiln any the Dawson Kiln Sitter, make surechanges and adjustments, comments appreciable amount without doing that the cone-support prongs areon kiln condition and performance, the newspaper-char-test to ensure in good shape and free of crustyand summary of firing outcomes. that the temperature is lower than oxidation or the residue of pre- 451°F. Insert a newspaper twist vious cones. If not, install newVentilation in an upper peephole. If it chars prongs. They are very inexpensive, In general, all firing processes pro- or burns, the kiln is still above and they might save you the lossduce toxic and/or corrosive fumes, 451°F. If not, it is safe to open the of a kiln load of wares and possiblywhich must be efficiently exhausted kiln, although one should always even the kiln itself. If you ever mustfrom the area. For any firing, make be cautious in opening a kiln too use a set of prongs that seem cor-sure that the appropriate ventilation abruptly, especially if the firing roded, sand them off well with finesystems are fully operational during contains large vessels or sculpture. sandpaper and/or give them a coatthe firing. Even the simplest electric of kiln wash.kiln gives off toxic fumes during fir- Care of Refractorying. Unless such a kiln is installed in Surfaces Kiln Shelves and Furniturea separate shed with no one present Softbrick interiors are crumbly Use appropriate kiln shelvesduring firing, an appropriate ventila- and can be easily damaged by any rated for the particular type andtion system is mandatory. abrasion. When lifting wares and temperature of firing. Top-loader shelves into or out of a kiln, brace electric kilns usually use L"-thickDon’t Burn Yourself! yourself well and avoid any contact (or up to 1"-thick, depending on Kilns are often much hotter than with the refractory surfaces. Attend the size of the shelf and the firingthey look, and even the outside sur- to any degenerating refractory sur- temperature) mullite or cordieriteface can severely burn you. Never faces promptly. shelves, which are appropriate onlyplace hands or face close to an open for low and mid-range firing, andpeephole on a hot kiln. Even if there Preparing and will warp if repeatedly subjected toare no flames or visible fumes, the Loading Kilns high-fire temperatures.emerging gases may be hot enough Always examine any kiln thor-to burn you. Use common sense oughly before loading wares, and tend Cleaning Shelves andwhen unloading a kiln. Think about to any cleanup or small repairs. Make Applying Shelf Washwhere the wares may have remained sure that all shelves, furniture, and As you are loading any kiln,insulated from cool air. Heat rises, refractory surfaces are in good shape. examine the shelves and furnitureso the wares at the bottom usually carefully. Loose, flaky shelf washcool first. Also, wares in the center Electric Kiln Preparations may be scraped off with a sturdyof a tight set can remain extremely Make sure that element grooves metal scraper or a heavy wire brush.hot when wares at the edges are are free of any clay or glaze residue. Always wear safety glasses or a facequite cool. Wear gloves, and throw Molten glaze is acidic and can erode shield and a good dust mask whenaway any gloves with holes. Avoid through an electric element very scraping, brushing, or grindingwet gloves, because moisture can quickly. Use a vacuum cleaner to shelves, and if possible do this workconvert to steam instantaneously on remove any residue, and if necessary outdoors. If there is a serious accu-contact with a hot surface, causing grind or chisel away any accumula- mulation of glaze residue it must beserious burns. tions of glaze stuck to the refractory chipped or ground off before the www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 39
    • Kilns and Firing • Chapter 6shelf or furniture is used. Minor challenge, use wadding on at least In kiln sets with multiple shelvesglaze drips may be chipped off with one post so that the shelf sits level side by side, whenever possible placea hammer and chisel. More seri- with no rocking. the shelves at the same height andous glaze runs must be ground off. In many kilns the bottom shelf use common posting. This simplyNever chip or grind shelves while is left in place from one firing to means that where two shelves meet,they are resting on a concrete floor the next, unless it needs cleaning single posts are used to support bothor any other hard surface—always and rewashing. Normally the bot- shelves. As mentioned, each shelfplace them on a cushioning bed of tom shelf should be raised up at must have three support points, butcloth or foam rubber. In a pinch, a least 1" off the floor of the kiln in wherever possible each post maybag of sand makes a great support. order to allow even circulation of support two adjacent shelves. ByWhen chipping glaze accumulations heat and atmosphere. If the bottom this method you can support twowith a chisel, never hold the chisel shelf is already in place, make sure shelves with four posts. When con-vertically against the shelf. Always you know where the supports are sidering the number of posts in asharpen the chisel so that only one located beneath the shelf, so that large kiln set, common posting willedge is beveled, and hold the flat you can properly align successive save a lot of posts. All commercialedge against the kiln shelf, so that tiers of posts. kiln posts are capable of supportingthe force is parallel to the shelf, To start out, always decide what enormous amounts of weight, andagainst the glaze accumulation. height of wares (or piled combina- as long as the posts and shelves are tions of greenware in bisque) you kept in good shape so that there isLoading Kilns are going to load in the first layer, no wobbling, common posting is In general, kilns fire best with a and select kiln posts of the appro- never a problem.fairly tight set. In electric kilns with priate height. Have a good assort- Always make sure that the surfacea kiln sitter, be sure to place shelves ment of kiln posts on hand. When of the shelf is smooth and leveland wares so that they will not properly cared for, they last forever, where the posts are to be placed, andinterfere with the cone and cone and you will be able to stack the set make sure the ends of the posts areholder on the inside of the kiln. On much more efficiently. If you have smooth and free of glaze residue. Ifother kilns, don’t place any shelf so any scrap kiln shelf pieces around, needed, see the previous section onthat it interferes with the placement consider cutting them into small cleaning kiln shelves and furniture.of cones in front of the peepholes, blocks with a hammer and sharp On each subsequent course ofand don’t ever allow wares or fur- brick-chisel. They make excellent shelves, place the posts in exactly theniture to touch any thermocouple shims for use with standard kiln same locations, so that loads are car-probes protruding into the kiln. posts. Coat all posts and shims with ried in a straight vertical line from shelf wash on both contact surfaces. top to bottom. If the bottom shelvesSelecting and Placing Kiln posts come in sizes from 1" to have not been removed from theKiln Furniture 12" in 1" increments, and having an kiln, make sure that the stilts beneath As a general rule, kiln shelves assortment of shims cut from shelves them are in the same position as theshould always rest on three posts, so will give you considerably more ones you place above. Deviationsthey sit firmly in place with no wob- flexibility in determining the height from this practice can be disastrous.bling. Fewer than three supports is of each layer. Whenever possible, use Very tall posts tend to be unstable,obviously too precarious. With four single post sections to achieve the and their use should be avoidedposts, the shelf never sits squarely on needed height, and when this is not whenever possible. Always try toall four points. If you occasionally possible, use the tallest post available place the tallest wares on the topfind it necessary to use four posts with the shortest possible extension layer of shelves, eliminating thebecause of a particular kiln-loading post or shim on top of it. need for such tall posting. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 40
    • Kilns and Firing • Chapter 6 To those new to kiln loading, the Loading a Glaze-Firing Bisque-Firing Rampsarrangement of shelves, posts, and When loading normal electric The rate of temperature risewares in a loaded kiln may seem oxidation glaze-firings, always leave through the early stages of the firingprecarious, but if properly done at least J" ­clearance between wares, is obviously a much greater concernit is actually a very stable system. from kiln walls, and from the shelf in bisque firing. Even if your wareAs long as you follow the above overhead. When glazing your wares, is bone dry, it still contains atmo-instructions and use quality shelves avoid having any glaze where it will spheric humidity and chemicallyand posts, you should not hesitate be within J" of the kiln shelf or on combined water, and must be heatedto stack a dozen or more tiers of places where multiple parts touch, slowly in the initial stages. Usuallyshelves when necessary. as with lids on covered jars or boxes. it is safest to simply warm the kiln If you become very sure of your overnight to drive off excess mois-Loading a Bisque-Firing glazes, you can reduce this clear- ture. On a top-loader electric kiln When loading a bisque fire, the ance. Lids should always be fired in this involves leaving the lid ajar withwares can gently touch each other place, because if fired separately they the peepholes out and the bottomand may even rest inside and/or could warp and no longer fit, but element on low.on top of each other, but use com- there must be no trace of glaze on It is important to point out that ifmon sense—bone-dry greenware is this contact surface. the wares are thin and quite dry, thevery fragile. Use space as efficiently When loading a glaze-firing in top- preheat period may be reduced toas possible, by placing small forms loader electrics, avoid placing the top as little as two to four hours, depen-within large forms, and by stacking shelf within six inches of the lid. Oth- erwise, the lid will draw off so much dent on ­atmospheric humidity.pieces carefully. Don’t pile things so heat that the wares on the uppermost After an adequate preheat in athat too much pressure is applied shelf will not reach temperature. bisque-firing containing averageagainst any one small area of a single Remember that a full kiln fires well. size and thickness of wares, the ini-piece, and do not pile up more than If you must fire only a partial load, tial heating ramp should not exceedtwo or three bowls or plates on topof one another unless you are very use taller kiln furniture than necessary, 200°F per hour for the first twosure of yourself. Place bowls or so that the set fills the kiln. This will hours, and 300°F per hour for theplates inside or on top of each other give a more efficient, even firing. If next two hours. If the firing con-only if the bottom of one bowl or you ever need to glaze-fire only a few tains large work, this ramp shouldplate rests squarely on the bottom of things in an otherwise empty kiln, be extended to six hours. In eitherthe one below, without any pressure place three or four high-duty hard- case, at that time red heat willagainst the walls or rim. In other brick around the wares. These will have been achieved (approximatelywords, don’t ever place one piece absorb a tremendous amount of heat, 1000°F) and the heat may be turnedinside another if the walls of the slowing down both the firing and up fairly rapidly to bring the kiln up the cooling ramps, so that the glaze to bisque temperature. Dependingupper piece are wedging inside the matures properly, and pinholes and on the individual kiln, the ramp raterim of the lower one. bubbles have a chance to heal. at this stage is usually from 300°F to Bowls or cups with matching rimsmay be stacked rim to rim, and this 400°F per hour. Keep in mind thatmay be continued to considerable Determining the hotter the kiln, the more energyheight. Or, a large diameter base Appropriate Firing required to raise the temperaturemay be placed on a matching rim and Cooling Ramps further. Thus even if you turn upbeneath. Or, one pot can be placed The rates at which a kiln is heated the heat a great deal, the kiln mayrim-down, with another above it and cooled are referred to as the fir- not climb more than 300 to 400right side up. ing ramp and cooling ramp. degrees per hour. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 41
    • Kilns and Firing • Chapter 6 For the average top-loader elec- raku kiln—allow at least 24 hours If it does not char, it is safe to begintric, after an overnight preheat with after glazing before placing wares in opening the door. A bisque-firingthe bottom element on low, normal a preheated raku-firing). Bisque-fired containing average wares can beprotocol is to close the lid but leave wares are extremely thermal shock opened all the way at this time. Athe peepholes open, and turn all resistant and will withstand a very glaze-firing, or any firing contain-switches to low. After two hours, steep heating ramp. Just to make ing large work, should be openedturn all switches to medium, and sure that residual moisture is driven slowly over a period of an hour orafter another two hours, turn all off, one should normally maintain a two, as such pieces can crack fromswitches to high. Depending on the gentle ramp for the first hour, but abrupt thermal shock even at suchage of the elements, and the condi- then the heat may be turned up low temperatures.tion of the wiring and the power quite quickly. On the standard top-supply, the kiln should shut off from loader electric, for a normal glaze- The Degreesthree to eight hours after you turn firing ramp, we would set the kiln of Kiln Firingit to high. on low for two hours, medium for Firing converts ceramic work At temperatures below red heat two hours, and then turn to high. In from weak greenware into a strong,it becomes difficult to measure the an electric kiln with good elements, durable form. As the temperatureramp unless the kiln is equipped it is easy to fire much too quickly, in a kiln rises, many changes takewith a pyrometer. In most kilns, which gives poor glaze results, and place in the clay; and, if all goeswith a little care and common you must watch for this. A low-fire well, you end up with a permanentsense we can control the heating glaze-firing ramp should take at result ready for using, showing orramp, but it is an excellent idea to least five or six hours, with at least giving. Understanding what hap-fit any kiln with a pyrometer. It will another two or three hours to reach pens during the firing can helpprovide you with peace of mind mid-range or high-fire. Problems you avoid problems that may ruinwhen monitoring the heating and often arise in firing just a few pieces hours of effort on your part. Thecooling ramps. in an otherwise empty kiln. In this chart on the next page provides It is critically important to avoid event, a good practice is to place highlights of what happens whenheating a bisque-firing too quickly, three or four hardbrick in with the firing your work. •especially during the water- wares. They will soak up a tremen-s­ moking period, as this can seri- dous amount of heat, slowing downously limit outgassing, trapping both the heating and the coolingcarbon and other volatiles in the and allowing the glaze to mature andclaybody, increasing chances of car- heal properly.bon and sulfur coring and laterbloating and blistering. Excessively Cooling Rampsfast bisque-firing can also aggravate In general practice, a kiln canglaze problems such as crawling, be cooled at least as fast as it ispeeling, pinholing, and blistering. heated. Excessively long cooling ramps accomplish nothing. When allGlaze-Firing Ramps red heat is gone you can determine When glaze-firing previously when it is safe to open the door bybisque-fired wares, there is little carefully inserting a newspaper twistdanger of wares exploding even if into the top peephole. If the news-they are recently glazed (except in paper chars or burns, the kiln is stillrepeated loads in a previously heated above 451°F and cannot be opened. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 42
    • Kilns and Firing • Chapter 6 Kiln Firing ChartTemperature Color Cone EventC° F° (approx.) 1400 2552 Brilliant white 14 End of porcelain range 13 121300 2372 White 11 End of stoneware range 9 Yellow-white 71200 2192 5H End of stoneware range Yellow 4 2 Between 1100-1200°C, mullite and cristobalite (two types of silica) form when clay starts converting1100 2012 Yellow-orange 1 to glass. Clay and ceramic particles start to melt together and form crystals. These changes make the material shrink as it becomes more dense. 04 Soaking (holding the end temperature) increases the amount of fused matter and the amount of Orange 05 chemical action between the fluxes and the more refractory materials.1000 1832 06 07 Red-orange 08900 1652 010 Between 800-900°C sintering begins. This is the stage where clay particles begin to cement them- 012 selves together to create a hard material called bisque. Cherry red 013800 1472 015 016 Between 300-800°C, the temperature must be raised steadily and ample air must be present to permit the complete burning of carbonaceous Dull red 017 materials (impurities in the clay along with paper, wax, etc.). After 800°C, the clay surface will start to700 1292 018 seal off, trapping unburned carbonaceous materi- als and sulfides, which could cause bloating and 019 black coring. Dark red 020 Quartz inversion occurs at 573°C. When clay is refired for a glaze firing, quartz crystals change600 1112 021 from an alpha (a) crystal structure to a beta (b) crystal structure. The inversion is reversed on cool- ing. This conversion creates stresses in the clay so 022 temperature increase and decrease must be slow to avoid cracking the work. Dull red glow Between 480-700°C chemical water (referred to as500 932 Black “water smoke”) is driven off.400 752 Upon cooling, cristobalite, a crystalline form of300 572 silica found in all clay bodies, shrinks suddenly at 220°C. Fast cooling at this temperature will cause200 392 ware to crack. Water boils and converts to steam. Trapped water100 212 will cause clay to explode so all water should be evaporated below 100°C. Begin a firing by keeping the kiln below 100°C until all water has evaporated. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 43
    • Studio Safety Chapter 7 Studio Safety Checklist The following items should be of concern in all ceramics studios. Commercial and academic studios will always require other items and con- siderations according to local codes and practices. ✓ First aid kit containing general supplies ✓ Fire extinguisher mounted close to outside door, away from loca- tion of kilns ✓ Dust masks approved for ultrafine particulates ✓ Disposable rubber gloves for use while handling toxic materials ✓ Appropriate skin protectant/hand lotion for combatting dry skin ✓ Approved eye/face protection for use while grinding pot bottoms, cleaning kiln shelves, etc. ✓ Approved tinted safety glasses or face shield (shade 1.7 to 3.0) for looking into hot kilns ✓ Insulated heat-resistant gloves for checking/unloading kilns ✓ Appropriate ventilation for all kiln fumes and any other toxic fumes ✓ Approved clearance and heat protection around all kilns ✓ Good lighting in all work areaswww.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 44
    • Studio Safety • Chapter 7Toxic and Hazardous well marked and should be kept in free of dust and any clay scraps thatMaterials locked cabinets or storerooms out will create dust when walked on or of reach of children, pets, or any other­wise disturbed. Don’t leave clay It is essential that you are aware unauthorized personnel. Any food scraps on the floor. Don’t leave wareof all the possible toxicity hazards containers recycled for clay or glaze boards where they might fall overin the materials you use. For each use should have all food labels com- and raise a cloud of dust. Confineindividual ceramic material and pletely removed. all dust-producing processes to anrefractory in your studio, you cancontact the appropriate supplier and appropriate ventilated area or sprayrequest the MSDS (materials safety Dust/Dirt booth, or take the work outside.data sheet). Suppliers are required Managementby law to provide the MSDS upon Dust control is perhaps the most Floor and Surface Cleaning pervasive problem in the ceramics In general, it is critically impor-request for the ceramic materi- studio, and yet it is really a simple tant never to sweep without usingals they sell you; these sheets will thing to address with proper and water or sweeping compound, nogive detailed information about consistent precautions. The amount matter how efficient your dust mask,toxicity hazards. In addition, sev- of dust created in different cir- because as mentioned, when youeral good books about studio safety cumstances is in proportion to the raise dust into the air, it settles onare available, and excellent articles amount of dry powdered mate- everything and is easily disturbed byaddressing toxic materials can be rials present, the particle size of any studio activity.found in contemporary ceramics the materials, the way the materi- For the very cleanest floors, aftermagazines. Also, you will find a als are handled, and the amount sweeping, spray down the floor verygreat deal of information on stu- of air movement in the vicinity. lightly with a hose or a gardendio safety and toxic materials on The smaller the particle, the greater sprayer, and use an industrial squee-Internet discussion groups such as potential for airborne dust. And of gee to localize the dirty water,Clay­art. It is important to review course different materials have dif- making it a simple matter to pickthe most current information. ferent levels of toxicity. The material it up with a mop or a large sponge. With almost all ceramic raw most likely to be present as airborne Long-handled squeegees in varyingmaterials there is very little danger dust is clay, and although the clay widths are available very inexpen-of toxicity in skin contact. Somematerials can cause some minor or platelets themselves are not par- sively from janitorial suppliers.serious problems as a result of skin ticularly toxic or hazardous, in mostcontact and these materials should claybodies we find very hazardous Skin Carebe handled with rubber gloves. It materials, including silica and talc. For the most part, the materialsis important for all ceramic artists/ As a good general rule, you should we use in ceramics are relativelyartisans to realize that almost ALL avoid breathing dust of any kind in benign in contact with the skin,ceramic raw materials are consid- any circumstance. In other words, and the greatest problem is dry-ered highly toxic when inhaled, and any situation that raises dust into ness. Problems with skin drynessmany when ingested. This is not the air should be avoided or should are almost inevitable when workinga matter to be taken lightly, and it be approached only with appropri- with wet clay, and minor rashes areis essential that proper precautions ate dust protection. This is especially common. If you experience thesebe taken to ensure safe use of these true indoors, because dust will settle problems, use a good skin protec-materials, especially in mixing clay- on all surfaces and is easily raised tant before working with clay andbodies and glazes. into the air later on by any air move- a good skin moisturizer after. Many In all studio situations, toxic or ment. In normal studio practice, potters like the effects of plain aloehazardous materials should be very floors and tables should be kept vera gel, available in most drug- www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 45
    • Studio Safety • Chapter 7stores and health food stores. Glazing can be Safety with Kilns and Firingespecially hard on the skin, and a good skin It is normal to be curious about a kiln thatprotectant will help, but be sure to apply it is firing, and, with reasonable precautions,well in advance of handling any bisque-ware, this is not a problem. The following guide-and wipe your hands well directly before lines should always apply to everyone in theglazing, as any oily residue can interfere vicinity of a kiln being fired.with glaze adhesion. Some people’s skin is All wiring for any electric kiln should beespecially sensitive to glaze materials. If so, done strictly by local building and safety codes.keep a package of disposable rubber gloves Make sure that all ventilation and safetyon hand. equipment is in place and operating properly. See the previous section on kiln ventilation.Equipment Safety Make absolutely sure that there is adequate All new ceramics equipment comes with clearance between any kiln and adjacentsafety guidelines, and many pieces of equip- flammable surfaces. Keep in mind that woodment come with safety guards or specific surfaces repeatedly exposed to heat maysafety fittings and switches. Always observe become progressively desiccated over time,the manufacturer’s safety guide-lines, and increasing the chances of a fire. Always adherenever tamper with safety guards and safety to local fire codes and, when in doubt, installfittings or switches. additional insulation or heat guards. Never assume that a kiln is cold just becauseVentilation for Kilns it is not on. Air convection over the surface Make sure that all kilns are properly venti- may reduce radiated heat, and yet the surfacelated, and, when firing, ensure that appropri- may still be hot enough to burn you.ate exhaust fans are turned on. All kiln firings Whenever you must look into a hot kilnproduce toxic fumes that must be vented to to check the atmosphere or cones, alwaysthe outside. No unvented kiln should ever wear tinted safety glasses or a tinted facebe located in a work space or anywhere in a shield. Make sure that the shade of the faceresidence. A by-­product of all bisque-firings shield or goggles is appropriate (shade 1.7 toand (to a lesser degree) glaze-firings is sulfur 3.0) to protect your eyes from the extremedioxide, which is toxic and corrosive. Many brightness. Goggles for gas welding aremetallic oxides release toxic fumes at high appropriate, whereas those for arc weldingtemperatures. Wax resist, although benign at are too extreme. If you see spots before yourlower temperatures, produces harmful fumes eyes after turning away from the kiln, yourwhen it burns off the wares in the early eye protection is inadequate. •stages of a glaze-firing. The organic vehicleused in commercial lusters and china paintsreleases toxic fumes during firing. www.ceramicartshandbook.com | Copyright © 2001, 2007 The American Ceramic Society | 46