Today’s objectives-glass and clay products and processing What are standard glass additives, and how do they effect structure and properties? How do specific volume and viscosity vary with temperature? How are glass sheets and containers prepared? Why are annealing and tempering important for glass? What are the steps in processing clays? What is slip, and what is hydroplasticity? Describe casting Describe extruding Describe drying Describe firing Primary glass resource: Corning Museum of Glass (www.cmog.org)
History of Glass
The history of the origin of glass can be categorized by periods according to the methods of the manufacturing process as follows:
The First Period: 1700 BC through 100 AD
Primitive method of making glass using molds.
The Second Period: 100 AD through 400-500 AD
Glassblowing technique discovered, and glass manufacture becomes a more practical process.
The Third Period: 4-500 AD ~ 1200 AD
Middle Ages, characterized by Byzantine glass.
The Fourth Period: 1200 AD ~ 1900 AD
Venetian glass, foundation for modern glass making is set.
The Fifth Period: 1900 AD ~ present
Glass objects used as everyday goods; large scale manufacturing.
As early as 3,300 years ago, secret "instructions" for furnace building and glassmaking in Mesopotamia were written on clay tablets in a cuneiform script. Typical instructions include:
When you set up the foundation of a good furnace to make glass, first search in a favorable month for a day of good omen …
You regularly perform libation offerings (drink of honey and liquid butter honoring a deity).
On the day when you plan to make (glass), you make a sheep sacrifice .
Place juniper incense on the incense burner
Pour out a libation ; only then can you make the fire in the hearth of the furnace and place the glass in the furnace.
The wood that you burn in the hearth of the furnace should be thick, peeled poplar wood , which has no knots , bound together with leather straps , cut in the month of the Abu (Jul. or Aug.).
If you want to produce zagindurû-colored (blue) glass, you finely grind separately:
10 minas (about one pound) of immanakku-stone (quartz)
15 minas of naga-plant ashes
1 2/3 minas of 'white plant.'
Mix these together.
Place the mixture into a cold furnace that has four openings,
As soon as the mixture glows yellow, you pour it on a kiln fired brick and this is called zukû-glass....
Glasses and glass ceramics
Now commonly applied for:
Chemically, glass is comprised of noncrystalline silica plus additives:
Na 2 O
K 2 O
Al 2 O 3
GLASS STRUCTURE • Basic Unit: • Glass is amorphous • Amorphous structure occurs by adding impurities (Na + ,Mg 2+ ,Ca 2+ , Al 3+ ) • Impurities: interfere with formation of crystalline structure. • Quartz is crystalline SiO 2 : (soda glass) Adapted from Fig. 12.11, Callister, 6e .
Glass composition determines properties
Thermal expansion coefficient of pyrex is 1/3 that of standard silica glass. 10 13 B 2 O 3 >99.5% Fused silica 16 5 CaO 4 MgO 15 55 fiberglass 4 MgO 1 16 74 Container (soda-lime) 2.5 3.5 81 Pyrex (borosilicate) other Al 2 O 3 Na 2 O SiO 2 glass
GLASS PROPERTIES • Specific volume (1 ) vs Temperature (T): • Glasses : --do not crystallize --spec. vol. varies smoothly with T -- Glass transition temp , T g • Crystalline materials : --crystallize at melting temp, T m --have abrupt change in spec. vol. at T m Adapted from Fig. 13.5, Callister, 6e .
Amorphous materials (glasses)
For non-crystalline ceramics, deformation occurs by viscous flow.
The deformation rate is proportional to the applied stress.
Common units are N*s/m 2 = Pa*s (or Poisies=0.1Pa*s)
Viscosity unit, giving the inverse of how well the material flows.
Water at 25 ° C is 0.001 Pa*s.
Motor oil (SAE10W…) is 0.132 at 25 ° C.
Glass is 10 14 Pa*s = 10 15 Poisies (a crystal is infinite).
GLASS VISCOSITY VS T AND IMPURITIES • Viscosity decreases with T - (sample flows easier) • Impurities lower T deform • Viscosity : --relates shear stress & velocity gradient: --has units of (Pa-s) Adapted from Fig. 13.6, Callister, 6e . (Fig. 13.6 is from E.B. Shand, Engineering Glass , Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.) Tmelt
Originally, glass plates made one at a time using the Pittsburgh process (vertically draw a continuous sheet of glass of a consistent width from the tank).
Pilkington discovered how to make glass continuously.
Float Glass, Pilkington, 1950.
Originally able to make only 6mm thick glass, now made as thin as 0.4mm and as thick as 25mm.
There are around 260 float plants worldwide with a combined output of about 800,000 tonnes of glass a week.
A float plant, which operates non-stop for between 11-15 years, makes around 6000 kilometres of glass a year.
Molten glass, at approximately 1000ºC, is poured continuously from a furnace onto a shallow bath of molten tin.
It floats on the tin, spreads out and forms a level surface.
Thickness is controlled by the speed at which solidifying glass ribbon is drawn off from the bath.
After annealing (controlled cooling) the glass emerges as a 'fire' polished product with virtually parallel surfaces.
HEAT TREATING GLASS • Annealing : --removes internal stress caused by uneven cooling. Chemical tempering is also possible. • Tempering : --puts surface of glass part into compression --suppresses growth of cracks from surface scratches. --sequence:
Fabricating Glass in other shapes • Pressing: GLASS FORMING • Blowing: Adapted from Fig. 13.7, Callister, 6e . (Fig. 13.7 is adapted from C.J. Phillips, Glass: The Miracle Maker , Pittman Publishing Ltd., London.)
Prolongued exposure to chemical attack can lead to local microcracks, ‘Crizzling.’
Eventually leads to failure.
In 221 BC during the Qing Dynasty, Qin Shi Huang was the Emperor in reign. The Terra Cotta Army was built as a way of creating an illusion of strength and manpower.
Discovered in 1974.
Most soldiers standing in formation, many with their horses ready for battle. en.wikipedia.org/wiki/Terra cotta _Army
7000 ‘soldiers’ thus far discovered
Clay is an abundant raw material
When mixed with water it forms a plastic mass
Structural clay products
Only whiten after high temperature firing
Plumbing “sanitary ware” = toilets
How porcelain came to Europe
1701, August der Starke, King of Saxony and Poland, hires Johann Friedrich Böttger into his castle workshop as an alchemist to make gold.
The alchemist cannot make gold (surprise…)
Instead, in 1709, he is the first Westerner to discover how to make porcelain.
The castle gets bigger (near Dresden).
Symbolized the mysterious Far East (but made in China since 7 th century).
Chinese porcelain began reaching Europe around the 13 th /14 th century.
Europeans tried for years to make their own, leading to faience, majolica and bone china. But not porcelain.
The secret lies in firing mineral quartz, feldspar, and kaolin at such a high temperature that they actually melt and fuse.
Meissen recognized the need to protect their porcelain against imitations, so they began painting the famous "Crossed Swords" under the glaze on all Meissen porcelain as a distinguishing mark.
This is the oldest commercial trademark in continuous use.
Ingredients Kaolin: is a clay mineral with the chemical composition Al 2 Si 2 O 5 (OH) 4 . It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra. Feldspar: is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust. Feldspars crystallize from magma in both intrusive and extrusive rocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock. Quartz: belongs to the hexagonal rhombohedral crystal system, and is made up of silica (SiO 2 ) tetrahedra.
Under painting (monotone, more durable)
Over painting (colors, delicate work, more vivid)
What are basic ceramics made of? • Clay (inexpensive) • Milling and screening: achieve desired particle size. • Adding water to the clay particles produces a "slip:" --layered structure --allows material to shear easily along weak van der Waals bonds Hydroplastic --enables extrusion --enables slip casting --enables pottery wheel (Ghost) • Structure of Kaolinite Clay Adapted from Fig. 12.14, Callister 6e . (Fig. 12.14 is adapted from W.E. Hauth, "Crystal Chemistry of Ceramics", American Ceramic Society Bulletin , Vol. 30 (4), 1951, p. 140.)
Pour slip into a mold (e.g. plaster of paris).
The mold is extremely water absorbing.
As water is drawn from the slip into the mold, the remaining part has some structural integrity (hydroplasticity at work).
Remove the mold.
Place in the oven to drive out the rest of the water (‘green state’).
Fire to harden
Add glaze, fire again to vitrify it.
Start with the same slip.
Extrude, taking advantage of hydroplastic characteristic.
Dry and fire the same as for molded structures.
Adapted from Fig. 11.7, Callister 6e .
Drying • Drying : layer size and spacing decrease. Adapted from Fig. 13.11, Callister 6e . (Fig. 13.11 is from W.D. Kingery, Introduction to Ceramics , John Wiley and Sons, Inc., 1960.)
Thick ceramic wares are likely to crack, since water is easily drawn out of the surface but not the center of a ceramic part.
Thus, there is shrinkage (densification) on the outside but not on the inside.
This is not an issue for large particles, but as surface area increases (smaller particles) more water can be removed faster, and thus there are increased drying stresses.
Increasing vitrification leads to shrinkage as more liquid will be present at the firing temperature.
Strength increases with vitrification since glass fills in the pores where the water was before drying, thus fewer bad flaws for stress concentration, Weibull, etc.
Conductivity increases with vitrification as glass has a higher conductivity than (air filled) pores that it filled.
Corrosion resistance improves.
--T raised to (900-1400 C, much higher than drying temperatures) -- vitrification : glass forms from clay and flows between SiO 2 particles.
Vitrification is a function of:
Time at temperature
Microstructure development during firing
Quartz grains (large dark particles)
Surrounded by glass
Pores (dark holes with white borders)
Cracks in the quartz that weren’t there at the start (different thermal coefficients build up stress upon cooling, leading to cracks)
Glass forming and properties
Clay forming and properties
History of each
Reading for next class Powder, refractory, abrasives, and advanced ceramics and processing Chapter sections 13.4-7, 13.10-11