CONDENSATION AND FIRING IN PORCEALIN
INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com www.indiandentalacademy.com
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.
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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.
Distance between porcelain particles
translucency, esthetics, and
porosity of the entire mass Strength of porcelain (
density)
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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
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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
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
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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.
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• 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.
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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.
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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 )
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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.
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• 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
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.
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• 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.
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• 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
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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
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• 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.
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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
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.
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• 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.
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• 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.
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• 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.
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•
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
CONDENSATION TECHNIQUES:
1.
Spattering
2.
Fixation by adding water
3.
Tapping
4.
Spatulation
5.
Vibration
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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
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.
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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.
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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.
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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).
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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.
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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
“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.
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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
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
ď ś Small cavities produced when an ultrasonic waver is emitted into water (cavitations). ď ś
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
ď ś 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 ď ś The framework is held by a locking tweezer and being in contact with an ultrasonic applicator instead of a Lecron carver. ď ś Tweezer must be held on place where vibrations are present, only then is audible sound heard & sufficient condensation can take place.
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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
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
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)
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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.
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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.
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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.
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• 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.
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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
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.
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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)
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Furnace There are 2 types of furnace available today 1. 2.
Horizontal Muffle 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.
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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
Types of Furnaces
(i) Vita- Vaccumat â&#x20AC;&#x153;Sâ&#x20AC;?: 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.
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(ii) DeTrey Biodent Systomat: Vertical Muffle: 1. The muffles on this furnace are mounted vertically above a moving platform. 2. Theyâ&#x20AC;&#x2122;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
(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.
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(iv) Rapid cycle Furnace (Doxc Euromat):1. The term â&#x20AC;&#x153;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 200â&#x20AC;&#x2122;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
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