CONDENSATION AND FIRING IN PORCEALIN

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy....
Definition
Condensation is the process of
packing the porcelain powder particles
together and of removing the liquid
binde...
Objectives of Condensation
1.

Improve contact between the metal framework and porcelain
-

Bond Strength

-

Interfacial ...
4. Cracking & distortion prevented through reduced firing shrinkage.
5.

Breaking of the built up structure prevented by i...
Strength:
• Generally strength of porcelain material depend on

(a) Composition
(b) Internal Structure
(c) Space Between p...
ACCORDING TO SKINNER AND PHILLIPS (1967)
Method of
Condensation

Firing
Shrinkage
Volumetric (%)

Apparent
Specific
Gravit...
The condensation procedures do have a significant effect on the coefficient of
rupture i.e., a stronger porcelain structur...
• Load was applied at incisal portion of porcelain on the lingual aspect at an
angle of 45’ against longitudinal axis of t...


Gunmetal used for die



Shofu ceramic Gold used for metal ceramic structure.



Casting – vaccum pressuring casting ...
RESULTS OF AVERAGE VALUE AND LIMITS OF RELIABILITY (95%) OF
BREAKING STRENGTH FOR EACH CATEGORY OF CONDENSATION.

Lower Li...
Inference: •Fracture or exfoliation of a ceramo metallic restoration in the mouth is
not caused by insufficient condensati...
• With ultrasonic condensation density of porcelain particles varies in a
smooth transition, increasing from the inside to...
Firing Shrinkage :
• Firing Shrinkage of porcelain usually reaches an approximate volume
of 40%.
• Most current dental por...
• The resulting space will cause shrinkage during firing at a ratio
corresponding to the volume that had existed before fi...
• According to Hodson (1959), porosity of porcelain mass if 45% with a
mixture of single diameter 25% with a mixture of tw...
Skinners
Condensation Techniques

1. Vibration

2. Spatulation

3. Spattering

4. No condensation

Firing Shrinkage
(Volum...
• If porcelain is built upto a layer of 2mm on a framework, it contracts
to 1.75mm during firing after condensation. While...
Condensation – Adding water to porcelain.

+
Vibration applied
Cross-linked structure broken
Small particles move into vac...
Explained by skinner taking a brush made from camel’s hair as an example.
An intentional increase in the number of applica...
• Attributed to incorrect or obscurely demarcated layer construction of
erroneous porcelain, reduction in content of color...
• Porcelain slurry should be scooped to avoid strongly pressing the
spatula and causing a crevice in the slurry. A crevice...
• For this reason porcelain should be built up quickly with water being
supplied constantly.
• Covering with most paper ti...
•

Once an independent void has formed, air will no longer be

evacuated from the void even under strongly reduced pressur...
CONDENSATION TECHNIQUES:

1.

Spattering

2.

Fixation by adding water

3.

Tapping

4.

Spatulation

5.

Vibration

www.i...
From a technical viewpoint, following requirements are
important:1. Contour of porcelain structure should be retained as i...
4. Condensation should be performed without causing separation of
particles into groups of different particles size which ...
Indirect (Model)
Vibration technique
Direct (Crown/ bridge)
 Use of hammer (or) the serrated end of Lecron carver.

 Spa...
 Pressure applications moves the porcelain, not only altering the
correctly formed layer construction but also producing ...
Various vibration techniques:1. Impact given by striking the model on the bench or tapping it with a
hammer (hammer techni...
 A technique, which causes relatively weak vibration continuously
and requires some time before breaking the contour of a...
Vibrator technique : Continuously vibration amplitude as small as
about 30 mm

Masakaetal, advantages of less bubble forma...


“Grouping effect“ -

Very fine particles float up together with
excess water rising to the surface if
condensation is a...
 Ratio of the amplitude of the particles Xp to the amplitude of the
medium Xg is given by
Xp
1
Xg =
r p = Density of part...
Advantages

(i) Less grouping effect
(ii) proper layer construction
(iii) Without causing irregular distribution of colour...
 Pressure in cavities are very low, its often regarded as a vaccum
state. This is helpful in removing small bubbles conta...
FIRING
 This is a the process of porcelain fusion, in dentistry, specifically to
produce porcelain restorations (GPT-6).
...
As the furnace temperature is raised to the manufacturer’s
recommended maturing temperature, the porosity in the porcelain...
Types of binders :1.
2.
3.
4.
5.

Distilled waters – dentine + enamel porcelain
Propylene glycol – alumina core buildup
Al...
 Always break vaccum whilst the work is in the not zone of furnace.
The dense surface skin of porcelain will then hydraul...
Classification of stage in maturity: 
Low Bisque :
 
•  Surface of porcelain very porous
•  Will easily absorb a water sol...
High bisque: 
•   Surface of porcelain should be completely sealed.
•   Smooth surface with a slight shine
•   Shrinkage c...
•  Prolonged firing of alumina porcelain in normal atmosphere (air firing) 
improve strength of ceramic.
 
•  Air firing a...
DEVITRIFICATION :   
•  Vitrification in ceramic terms is the development of a liquid phase, by 
reaction  or  melting  wh...
Thermal shock:
 
  Caused by uneven or rapid heating or cooling of the fired crown.
    Cracking  of  enamel  veneers  o...
Lighting:
•    Northern  day  –  light  is  the  best  light  for  seeing  colour  in  porcelain 
crown.
•  Artificial day...
Furnace
  
There are 2 types of furnace available today 
 
1.    Horizontal Muffle
2.    Vertical muffle
 
The requirement...
5. Firing programme should be able to be discontinued during the firing 
cycle if required.
 
6.  Vaccum  pump  should  be...
 
 

Types of Furnaces

(i) Vita- Vaccumat “S”:
Horizontal Muffle.
1.    Semi-automatic furnace with a horizontal muffle w...
(ii) DeTrey Biodent Systomat:
Vertical Muffle:
 
1. The muffles on this furnace are mounted vertically above a moving plat...
(iii) Unitek Ultra – Mat Furnace
 
 Horizontal muffle furnace – fully automatic. 
 Single muffle with a firing table of ...
(iv) Rapid cycle Furnace (Doxc Euromat): 
 
1. The term “rapid cycle: does not mean quick firing.
2. In  this  furnace  th...
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Condensation and firing in porcealin /certified fixed orthodontic courses by Indian dental academy

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Condensation and firing in porcealin /certified fixed orthodontic courses by Indian dental academy

  1. 1. CONDENSATION AND FIRING IN PORCEALIN INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com www.indiandentalacademy.com
  2. 2. Definition Condensation is the process of packing the porcelain powder particles together and of removing the liquid binder. The term also include any process by which an unfired dental porcelain paste is compacted. www.indiandentalacademy.com
  3. 3. Objectives of Condensation 1. Improve contact between the metal framework and porcelain - Bond Strength - Interfacial Bubbles 2. To decrease bubbles in the porcelain strength of fired porcelain 3. translucency, esthetics, and Distance between porcelain particles porosity of the entire mass Strength of porcelain ( density) www.indiandentalacademy.com
  4. 4. 4. Cracking & distortion prevented through reduced firing shrinkage. 5. Breaking of the built up structure prevented by increased strength after drying. Effects of condensation on (i) Strength (ii) Firing Shrinkage (iii) Shade www.indiandentalacademy.com
  5. 5. Strength: • Generally strength of porcelain material depend on (a) Composition (b) Internal Structure (c) Space Between porcelain particles (d) Presence of bubble. (e) Method + performance of condensation (f) Firing technique (atmosphere / Vaccum) (g) Temperature of firing (h) Rate of cooling. www.indiandentalacademy.com
  6. 6. ACCORDING TO SKINNER AND PHILLIPS (1967) Method of Condensation Firing Shrinkage Volumetric (%) Apparent Specific Gravity Modulus of rupture Kg/cm2 1. Vibration 38.1 2.35 490 2. Spatulation 38.4 2.34 400 3. Brush application 40.5 2.36 370 4. No Condensation 41.5 2.36 340 www.indiandentalacademy.com
  7. 7. The condensation procedures do have a significant effect on the coefficient of rupture i.e., a stronger porcelain structure can be obtained if condensation is performed through vibration or with a spatula. An experiment performed to determine the effect of condensation on strength of ceramometallic crowns. • Each test performed by preparing a metal die inform of real abutment simulating a maxillary central incisor, fusing porcelain to the metal die and attaching the ceramo-metallic crown to a test bar with bonding cement. www.indiandentalacademy.com
  8. 8. • Load was applied at incisal portion of porcelain on the lingual aspect at an angle of 45’ against longitudinal axis of tooth at a rate of 1 mm/mts to simulate the patients incisal occlusion •Load was recorded automatically by means of a shimazu universal Testing machine Autograph IS 200. • Each specimen was provided with an indentation with diameter of 1mm and a depth of 0.5mm at incisal portion to subject the specimen to a more severe condition and to prevent tip of the loading apparatus from slipping off. www.indiandentalacademy.com
  9. 9.  Gunmetal used for die  Shofu ceramic Gold used for metal ceramic structure.  Casting – vaccum pressuring casting machine  Procelain was crystar kit A2.  To provide better oxide films for surface to be bended every test was made with a fresh casting. www.indiandentalacademy.com
  10. 10. RESULTS OF AVERAGE VALUE AND LIMITS OF RELIABILITY (95%) OF BREAKING STRENGTH FOR EACH CATEGORY OF CONDENSATION. Lower Limit of reliable range Average Upper Limit of reliable range 1.No Condensation 75.25 77.27 79.29 2.Thorough manual condensation 71.99 75.51 79.004 3.Conventional condensation 71.5126 73.14 74.717 79.30 83.1 86.90 4.Ultrasonic Condensation (Shofuceramo-sonic condenser ) www.indiandentalacademy.com
  11. 11. Inference: •Fracture or exfoliation of a ceramo metallic restoration in the mouth is not caused by insufficient condensation in porcelain or the technique used during its fabrication in most instances. •Failure attributed to improper location of finish lines. Inadequate occlusal equilibration or low bond strength or porcelain due to improper laboratory manipulation or distortion resulting from incorrect framework design. •Average value in the category of “thorough condensation with ultrasonic vibration” is considerably higher - so slightly higher breaking strength than a crown made with other techniques. •Breaking strength is more influenced by the state of porcelain after condensation than by the degree of condensation. www.indiandentalacademy.com
  12. 12. • With ultrasonic condensation density of porcelain particles varies in a smooth transition, increasing from the inside to the outside. • Difficult to achieve homogeneous condensation if viberation is applied manually with serrated end of a Lecron carver or by tapping with a hammer. • Insufficient condensation, particularly in the region near the underlying opaque porcelain layer which has been fired. • Low strength ever after firing, hence stress concentration occur when a load is applied. www.indiandentalacademy.com
  13. 13. Firing Shrinkage : • Firing Shrinkage of porcelain usually reaches an approximate volume of 40%. • Most current dental porcelain are manufactured through a process of fritting. • In the laboratory. It is only necessary to heat and melt the surface of fritted particles to fuse them together. • As these particles are fused to each other before melting during firing and the unmolten portions are also pulled toward the center or into vacant space by the surface tension of melting porcelain, water, air and organic binders which have been included in the built structure before firing are lost. www.indiandentalacademy.com
  14. 14. • The resulting space will cause shrinkage during firing at a ratio corresponding to the volume that had existed before firing. • Densely condensed porcelain built-up structure undergoes less firing shrinkage. • Firing shrinkage of porcelain depends on the total volume of vacant space existing prior to firing in a built-up structure. • Condensation in pre firing built up structure is significantly influenced by distribution or particles size in a mass of fritted powders. • 47.6% is the volumetric percentage of the vacant space if spherical particles of equal size are most loosely packed. • 25.95% if they are most densely packed. www.indiandentalacademy.com
  15. 15. • According to Hodson (1959), porosity of porcelain mass if 45% with a mixture of single diameter 25% with a mixture of two different diameter and 22% with a mixture of more than two different diameter particles. • Generally particles bridge over each other during condensation. • The resulting cross linking produces large vacant spaces and actual porosity usually is more than expected. • Condensation is the application of vibration and pressure to the aggregate of cross-linked particles to break these bridges and to obtain a high density built-up structure with low porosity www.indiandentalacademy.com
  16. 16. Skinners Condensation Techniques 1. Vibration 2. Spatulation 3. Spattering 4. No condensation Firing Shrinkage (Volumetric contraction in %) Firing Shrinkage (linear contraction %) 38.01 14.8 38.4 15.0 40.5 15.9 41.5 16.4 www.indiandentalacademy.com
  17. 17. • If porcelain is built upto a layer of 2mm on a framework, it contracts to 1.75mm during firing after condensation. While 1.72mm without condensation. • Porcelain powder is usually kneaded with water. The porcelain mass containing water becomes a paste like aggregate due to binding force of surface tension of water. • Surface tension is a force acting to reduce volume. Water serves as a force in reducing porosity in porcelain. www.indiandentalacademy.com
  18. 18. Condensation – Adding water to porcelain. + Vibration applied Cross-linked structure broken Small particles move into vacant space between large particles because of surface tension. Vacant space is reduced + Water existing is expelled floating up to surface of porcelain structure as excess Floating water absorbed (dry paper tissue / gauze) Pressure between porcelain reduced (Bernoulli’s Theorem) particles More densely interlocked porcelain particles www.indiandentalacademy.com is
  19. 19. Explained by skinner taking a brush made from camel’s hair as an example. An intentional increase in the number of applications of vibration is clinically insignificant with respect to degree of firing shrinkage and strength. Shade: • The result of shade variation is because of translucency which in turn depends on presence of bubbles in porcelain material. • Effect of condensation on the shade of porcelain is clinically insignificant. www.indiandentalacademy.com
  20. 20. • Attributed to incorrect or obscurely demarcated layer construction of erroneous porcelain, reduction in content of coloring particles from erroneous condensation operations and inclusion of air bubbles during kneading or building of porcelain. • Small bubbles have an effect on translucency and on shade. • Care must be taken so to avoid inclusion of small bubbles during buildup rather than trying to eliminate them by through condensation. • Spatulation and vibration should be done carefully to avoid such inclusions during porcelain mixing process. • A quantity of porcelain which approximately corresponds to the volume of built up body for a single tooth should be built up at one time with a spatula. www.indiandentalacademy.com
  21. 21. • Porcelain slurry should be scooped to avoid strongly pressing the spatula and causing a crevice in the slurry. A crevice may include air, which will be a cause of bubbles in the slurry. • When building porcelain with a brush, it should be scooped so as to put a ball of porcelain slurry on the fine tip of the brush whose hairs must be always finely arranged. • Irregularly arranged tip may easily include air bubbles in a built up structure. • Porcelain should be kept properly moist always, as it is once dried, air bubbles will be include when water is added. • Added water invade from one direction causing secondary bubbles to remain in the porcelain structure. www.indiandentalacademy.com
  22. 22. • For this reason porcelain should be built up quickly with water being supplied constantly. • Covering with most paper tissue / gauze/ placing in humidified box if it’s a long span bridge. • Translucency decreased with decreased in pressure reduction as more small bubbles remain in fired porcelain because of the difficulty of reducing dimensions of voids. • Selection of the time at which reduction of pressure starts also is important. www.indiandentalacademy.com
  23. 23. • Once an independent void has formed, air will no longer be evacuated from the void even under strongly reduced pressure. • If the timing of pressure reduction is delayed translucency will reduced due to increased in number of small air bubbles remaining in the porcelain structure after firing. • Instruments used for kneading porcelain (metal spatula used metal powder is mixed in this way, fired porcelain will have a shade more graying than usual). • In clinical striations, it is more important to control build-up and firing carefully, rather than condensation itself. www.indiandentalacademy.com
  24. 24. CONDENSATION TECHNIQUES: 1. Spattering 2. Fixation by adding water 3. Tapping 4. Spatulation 5. Vibration www.indiandentalacademy.com
  25. 25. From a technical viewpoint, following requirements are important:1. Contour of porcelain structure should be retained as it has been built up, without deformation. 2. Porosity of the porcelain structure should be reduced by bringing porcelain particles in close contact with each other and with metal as well. 3. Condensation should be performed without changing the location of each layer (dentin, enamel, special colour and transparent) the layer should be kept clearly demarcated and regularly arranged to obtain desirable shade. www.indiandentalacademy.com
  26. 26. 4. Condensation should be performed without causing separation of particles into groups of different particles size which have been originally mixed randomly to reduce porosity. 5. If vibration and absorption of water are repeated unnecessarily, contour or layer construction or both – may be modified and separation of particles into groups of different particles sizes occur.  Spattering and fixation by adding water are not effective.  Tapping techniques is performed by tapping and patting the surface of a built-up porcelain structure with a dry brush to absorb water rising to the surface. Not adequately effective used a secondary procedure.  Primary procedure involves vibration and spatulation technique. www.indiandentalacademy.com
  27. 27. Indirect (Model) Vibration technique Direct (Crown/ bridge)  Use of hammer (or) the serrated end of Lecron carver.  Spatulation technique currently is often abused and misused.  Spatulation is accomplished by patting and tapping and surface of built-up porcelain gently with the flat surface of a porcelain carver to form the correct coronal contour and to absorb water rising to its surface.  Variation of vibration technique.  Misuse occurs when porcelain powder is condensed tightly by applying pressure with the spatula. www.indiandentalacademy.com
  28. 28.  Pressure applications moves the porcelain, not only altering the correctly formed layer construction but also producing a number of fine cracks in the built-up structure which has already lost much water through absorption.  When most porcelain is pressured with a spatula, the surrounding area appears dry because of retreating water. This may give an illusion that porcelain has been tightly condensed. If a mass of powder has been condensed, excess water must rise to the surface because of reduction in porosity. This phenomenon is known as “Dilatancy”.  Vibration technique causes vibration of the porcelain crown / model while spatualtion accomplishes vibration of the porcelain structure itself more directly. www.indiandentalacademy.com
  29. 29. Various vibration techniques:1. Impact given by striking the model on the bench or tapping it with a hammer (hammer technique)., 2. The model or articulator is vibrated stroking with the serrated end of a lecron carver (Lecron technique). 3. Mechanical vibration (50-60 Hz) is applied by means of an electromagnetic vibrator (vibrator techniques). 4. Ultrasonic vibration (above 20,000 Hz) is applied (ultrasonic techniques). www.indiandentalacademy.com
  30. 30.  A technique, which causes relatively weak vibration continuously and requires some time before breaking the contour of a structure is easy to control and unlikely to cause destruction of the contour.  Vibration with small amplitude is recommended for condensation of porcelain to minimize the chance of dislocations between different layers of porcelain material as well as separation into groups of different particles size.  Oscillographic wave patterns indicated that hammer technique and lecron technique produce apparently intermittent impacts strokes upto 80-100 mm in amplitude so that the entire crown will be shaken strongly. • Contour broken easily • Layer construction modified • Mutual relationship between different layers & between particles change easily. www.indiandentalacademy.com
  31. 31. Vibrator technique : Continuously vibration amplitude as small as about 30 mm Masakaetal, advantages of less bubble formation at the interface between porcelain and metal and between porcelain particles & high translucency obtained. But oscillographic wave pattern similar to Lecron’s technique. Each of these three types of condensation technique cause vibration with long strokes and intermittent impact which causes separation of particles easily into groups of different size in such a way that large and heavy particles are apt to sink while small and light particles are apt to float up. Most coloring materials for porcelain are very small particles and may be separated through this tendency for aggregation leading to irregular colour distribution particularly in opaque layer (increased conc. of colouring materials). www.indiandentalacademy.com
  32. 32.  “Grouping effect“ - Very fine particles float up together with excess water rising to the surface if condensation is applied by intermittent impacts  Change in porosity & firing shrinkage, crack formation.  Ultrasonic vibrations – homogeneous, Continuous vibrations with strokes limited to 10 as displayed in oscillographs.  Acoustic effect of sound waves. www.indiandentalacademy.com
  33. 33.  Ratio of the amplitude of the particles Xp to the amplitude of the medium Xg is given by Xp 1 Xg = r p = Density of particles 1 + (p r p d2f)2 1/2 d = Diameter 9 mm f = Frequency m = vis costy of the medium  Xp approaches Xg if d, f, r p decreases and viscosity increases.  Effective range of condensation 0.2 < . 8 in which particles move with various (Xp/Xg ) amplitudes.  <.2 and >.8 effective condensation does not occur owing to insufficient vibration (or) displaced particles by excessive agitation.  Recommended frequency as to achieve 0.5 = Xp/Xg.  Acoustic pressure + hydro mechanical effects bring about condensation. www.indiandentalacademy.com
  34. 34. Advantages (i) Less grouping effect (ii) proper layer construction (iii) Without causing irregular distribution of colours. (iv)No deformation of layer during condensation. (v) Greatest effect with small amplitude as vibrations are continues and quite even  Small cavities produced when an ultrasonic waver is emitted into water (cavitations).  Ultrasonic wave is a compression wave - over pressure and negative pressure are caused in water. The elasticity of water cannot respond to ultrasonic vibration because its cycle is very small & rapid. This leads to be pressure which tears water and produce cavities throughout. www.indiandentalacademy.com
  35. 35.  Pressure in cavities are very low, its often regarded as a vaccum state. This is helpful in removing small bubbles contained in porcelain and minute bubbles attached to porcelain particles together with air in depressions on metal surface are eliminated  The framework is held by a locking tweezer and being in contact with an ultrasonic applicator instead of a Lecron carver.  Tweezer must be held on place where vibrations are present, only then is audible sound heard & sufficient condensation can take place. www.indiandentalacademy.com
  36. 36. FIRING  This is a the process of porcelain fusion, in dentistry, specifically to produce porcelain restorations (GPT-6).  After condensation and building of a crown it is fired to high density and correct form.  Initially the unfired or “Green” Porcelain is placed on a sagger and introduced into either a drying chamber or the entrance of a furnace muffle.  The liquid binder drives off and the porcelain becomes brittle and chalky.  At this stage green porcelain is introduced into hot zone of furnace and firing process starts.  During firing, glass particles soften at their contact areas (grain boundaries) and fuse together.  The partial fusion of a compact of glass is often referred to as sintering. www.indiandentalacademy.com
  37. 37. As the furnace temperature is raised to the manufacturer’s recommended maturing temperature, the porosity in the porcelain powder escapes in a the grain boundaries of the glass powder by action of surface tension.   The Porcelain will shrink and become denser.  In air fired porcelain, flow of the glass grains around the air spaces traps air remaining in the porcelain and it cannot escape. On cooling, spherical bubbles are left in the porcelain.  In vaccum firing, the air/atmosphere is removed from the interstitial spaces before sealing of the surface occurs and hence a dense porcelain mass obtained.  The “Green” Crown must be dried slowly to eliminate all binder / water vapour before porcelain enters the hot zone of the furnace. www.indiandentalacademy.com
  38. 38. Types of binders :1. 2. 3. 4. 5. Distilled waters – dentine + enamel porcelain Propylene glycol – alumina core buildup Alcohol / formaldehyde based liquids – opaque or core build-up. Proprietary formaldehyde based liquids – opaque or core built up. Paint – liquids for stain application. • Do not use rapid cycle. Internal pores can be trapped if the surface skin seals off the interior too rapidly. • Do not prolong vaccum firing at the manufacturer’s recommended maturing temperature, surface blistering occurs as the residual air bubbles try to rise to the surface through molten porcelain . 1. Do not fire at temperatures in excess of those recommended by manufacturer. The ceramic may “bloat” or swell (decrease in viscosity) www.indiandentalacademy.com
  39. 39.  Always break vaccum whilst the work is in the not zone of furnace. The dense surface skin of porcelain will then hydraulically compress residual air bubbles left in interior of denser ceramic results.  Vaccum firing will not remove large air bubbles left by faulty condensation.  Always glaze in normal atmosphere. Repeated vaccum firing cause blistering.  If possible, always add porcelain at high bisque stage. Avoid adding porcelain to a glazed surface, it may peel or blister.  Fewer the number of bakes, always better the product. Repeated firing cause layering & porosity due to contamination. www.indiandentalacademy.com
  40. 40. Classification of stage in maturity:  Low Bisque :   •  Surface of porcelain very porous •  Will easily absorb a water soluble die. •  Grains of porcelain start to soften •  Shrinkage minimal •  Fired body extremely weak + friable. Medium Bisque:  •  Still slightly porous •  Flow of glass grain increased •    Any  entrapped  furnace  atmosphere  that  hasn’t  escaped  via  grain    boundaries will be trapped and become sphere shaped •  Definite shrinkage occurs.   www.indiandentalacademy.com
  41. 41. High bisque:  •   Surface of porcelain should be completely sealed. •   Smooth surface with a slight shine •   Shrinkage complete, increased strength •   Any corrections made before glazing. •   Glazing should not alter the anatomical accuracy.   Aluminous porcelain special precautions:    •  Do not fire in vaccum for long periods. Glass phase of the alumina crystal /  glass composite melts at much lower temperature than the alumina, Prolonged    •    Vaccum  firing  cause  the  glass  to  bloat  /  swell.  Common  cause  of  many  alumina core porcelains finishing up like a “honeycomb”. Break vaccum when  the porcelain reaches maximum temperature & then air – fire. www.indiandentalacademy.com
  42. 42. •  Prolonged firing of alumina porcelain in normal atmosphere (air firing)  improve strength of ceramic.   •  Air firing at temperature around 11000C for 15 to 20 minutes produce  best strength figures. Slow firing reduce risk of the core fissuring.  •    Vaccum  firing  of  enamel  porcelain  over  aluminous  core  will  not  damage the core porcelain since differences in maturing temperatures  (150 – 2000C) are too great. www.indiandentalacademy.com
  43. 43. DEVITRIFICATION :    •  Vitrification in ceramic terms is the development of a liquid phase, by  reaction  or  melting  which,  on  cooling  provides  the  glassy  phase.  The  structure is termed “Vitreous”. •    Glass  phase  (silica)  disrupted  -  addition  of  too  much  modifiers  (oxides/ alkali such as soda (Na2O) Mobility of molecules increases –  crystallization (or) devitrification occurs (cloudiness appearance).   •  A correctly fired porcelain crown should preserve the glass phase in  dental porcelain and consist of a dense mass of glass powder fused at  its  grain  boundaries  giving  porcelain  a  translucent  and  prismatic  appearance. www.indiandentalacademy.com
  44. 44. Thermal shock:     Caused by uneven or rapid heating or cooling of the fired crown.     Cracking  of  enamel  veneers  occurs  because  of  a  differential  thermal expansions stresses that will set up.     Thermal shock is more severe on reheating or glazing a crown than  when cooling it. Insert the crown very slowly in to hot zone of furnace &  give it a thorough pre-heat.      Cool  the  crown at  the  muffle  entrance. Donot remove  it and  place  under a glass jar or cool rapidly.       Even  thickness  of  porcelain  over  the  metal  or  core  porcelain    maintained to balance any discrepancies in the thermal diffusivity,      Never handle a hot crown. www.indiandentalacademy.com
  45. 45. Lighting: •    Northern  day  –  light  is  the  best  light  for  seeing  colour  in  porcelain  crown. •  Artificial day light lamps – colour corrected lights the also used. •  Waldman Leuchten lamp (Laboratory)   Firing Temperatures :   High fusing 13000C (23720F) Medium fusing 1101-13000C (2013 – 20720F) Low fusing 850 – 11000C (1562 – 20120F) Ultra Low fusing – 8500C (15620F) www.indiandentalacademy.com
  46. 46. Furnace    There are 2 types of furnace available today    1.    Horizontal Muffle 2.    Vertical muffle   The requirements for firing a crown are    1. Pre – drying ==> The “green” crown should be very slowly dried   to prevent steam explosions and cracking.   2. Firing    ===>    After drying the muffle should be capable of   rapid temperature rise. The dried green crown should be able to be placed under  vaccum out increased in temp. So that there is no risk of     high temperature sealing the surface of causing blistering.   3. Muffle chamber should have no hot or cold spots (i.e) even heat distribution.    4. Control of time / temperature cycle should be automatic. www.indiandentalacademy.com
  47. 47. 5. Firing programme should be able to be discontinued during the firing  cycle if required.   6.  Vaccum  pump  should  be  able  to  be  switched  off  manually  during  firing cycle without altering the programme.   7.  Firing  temperatures  should  be  completely  controllable  independent  of age of muffle winding.   8. Muffle should be large enough to accommodate two or three six unit  bridges with out losing heat control.   9.  Automatic  compensation  for  line  voltage  fluctuations  and  a  timer  control  over  24  hrs  to  allow  the  furnace  to  be  switched  on  in  the  absence of operator. www.indiandentalacademy.com
  48. 48.     Types of Furnaces (i) Vita- Vaccumat “S”: Horizontal Muffle. 1.    Semi-automatic furnace with a horizontal muffle with a mechanically  operator firing platform to transport the ceramic work in to the muffle. 2.    No controlled pre-drying system apart from introducing the work into  the  furnace  opening  and  delaying  the  introduction  of  the  work  into  the firing platform. 3.    The firing platform has a surface of 75 X 83 mm which allows large  bridges to be fired in one piece. 4.     Firing  controlled  by  pre-selection  of  firing  temperature  and  is  therefore  automatic.  Vaccum  is  applied  prior  to  the  introduction  of  the work into the muffle. www.indiandentalacademy.com
  49. 49. (ii) DeTrey Biodent Systomat: Vertical Muffle:   1. The muffles on this furnace are mounted vertically above a moving platform.    2.  They’re  cylindrical  and  so  give  a  better  heat  distribution  than  the  horizontal  types.   1.  Preheating the green porcelain 3.    Two muffles  2. Vaccum firing.    4.     When  preheating  the  porcelain  the  drying  muffle  radiates  heat  in  to  the  moving platform. After 5 minutes the plat form automatically introduces itself into  the muffle which has been set at a temperature of about 6000C.   5.     It  remains  in  the  muffle  for  a  length  of  time  which  is  controlled  by  the  operator. When the specified time is completed, the platform automatically drops  down  and  the  articles  to  be  fired  is  transferred  to  the  other  platform  by  the  technician. www.indiandentalacademy.com
  50. 50. (iii) Unitek Ultra – Mat Furnace    Horizontal muffle furnace – fully automatic.   Single muffle with a firing table of diameter 83.mm    Muffle will rise from 00C to a working temperature of 7000C in about  4  minutes  and  since  the  muffle  insulation  reflects  heat  rather  than  absorbing  it  the  muffle  will  cool  rapidly  upon  completion  of  any  firing  cycle.     Pre-drying  and  all  subsequent  firing  operations  are  carried  out  in  automatic sequences by preselected programmes.     Two  push-buttons  are  pressed  and  if  the  selected programme  has  to be cancelled there is another push-button for this purpose.    Firing table movement is set to give a slow rise of 5.5 minutes and a  fast rise of 12 seconds.  www.indiandentalacademy.com
  51. 51. (iv) Rapid cycle Furnace (Doxc Euromat):    1. The term “rapid cycle: does not mean quick firing. 2. In  this  furnace  the  heat  is  brought  to  the  porcelain,  not  the  porcelain to the heat. 3. Muffle is of the vertical type but the work to be fired is inserted via  the top of the furnace muffle which greatly assists viewing. 4. The programme will not start if the temperature is above 200’C. 5. To programme the furnace there are 5 settings to be made: 6. Drying time 5-10 minutes according to bulk 7. Temperature to be set for introduction of vaccum 8. Time required to reach firing temperature 9. Firing temperature 10. Time set after release of vaccum.  www.indiandentalacademy.com
  52. 52. www.indiandentalacademy.com Leader in continuing dental education www.indiandentalacademy.com

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