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

CONDENSATION AND FIRING IN PORCEALIN

INDIAN DENTAL ACADEMY
Leader in continuing dental education
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.


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)


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


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.

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


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.


• 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.




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.


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 )


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.


• 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.

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.


• 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.


• 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


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


• 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.


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

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.


• 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.


• 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.


• 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.


•

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.

CONDENSATION TECHNIQUES:

1.

Spattering

2.

Fixation by adding water

3.

Tapping

4.

Spatulation

5.

Vibration


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.

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.


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.


 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.


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).


 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.


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).



“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.


 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.

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.

 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.


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.

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.

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)


 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.


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.


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.


• 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.


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.

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.


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)


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.


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.

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.


(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.

(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.


(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.

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