1. COLLEGE OF DENTAL SCIENCES
DEPARTMENT OF CONSERVATIVE DENTISTRY
AND ENDODONTICS
Seminar On
DENTAL INVESTMENTS
Presented by : -
Dr. Niju Aelias

2. DENTAL INVESTMENTS
INTRODUCTION :
The principal laboratory technique of making metal inlays, onlays,
crowns and bridges, is based on casting practice. This application of casting
practice is one of the major advances in restorative dentistry. This is mainly
based on “Lost Wax Technique”.
This process of casting involves some basic steps.
1. Preparation of a wax pattern.
2. Preparation of mold – It is done by pouring the mixed investment material
around the wax pattern and allow it to set.
Burnout : Wax is eliminated from he investment by boiling (or) burning it
in oven.
3. Then casting is done by melting the alloy and forcing the molten metal into
the mold cavity.
HISTORY :
This meticulous procedure of casting was used by various craftsmen to
produce jewelary and ornaments. Its history can be traced back around 3000
B.C. But origin of lost wax technique, when viewed history makes its presence
in the writings of Theophilus (11th
century).
11th
Century Theophilus  described lost wax technique, which was a
common practice prevailed in 11th
century.
1558  Benvenuto Cellini  claimed to have attempted, use of wax and clay
for preparation of castings.
1884  Aguilhon de saran  used 24K gold to form inlay.
1887  J.R. Knapp  invented Blowpipe.
1897  Phillibrook described a method of casting metal filling.
1907  Taggart  devised a practically useful casting machine.
Various studies conducted on the properties of investment materials and casting
alloys have led to a path for better, practical and useful processing methods.
2

3. DEFINITIONS :
1. Investing : The process of covering, enveloping, wholly (or) in part an
object such as denture tooth, wax form, crown etc with a suitable material
before processing, casting.
2. Dental Casting Investment : Material consisting principally of an allotrope
of silica and a bonding agent. The bonding substance may be gypsum (for
use in lower casting temperature) or phosphates and silica (for use in higher
casting temperatures).
3. Refractory : Difficult to fuse / corrode, capable of enduring high
temperatures.
4. Refractory investment : An investment that can withstand high temperature
using a soldiering /casting.
5. Allotropic phase : Phases of similar composition but different
crystallographic structures, with different properties.
6. Casting :
Noun : Something that has been cast in a mold, an object formed by the
solidification of a fluid that has been formed / injected into a mold.
Verb : the art of forming an objet in a mold.
IDEAL REQUIREMENTS OF AN INVESTMENT MATERIAL :
1. The powder should have a fine particle size to ensure a smooth surface on
the casting.
2. The mixed unset material should have a smooth consistency.
3. It should be easy to manipulate – easy to mix and also harden within a
relatively short time.
4. It should have sufficient strength at room temperature
- Should exhibit sufficient strength at high temperature.
- Inner surface of the mold should not break at a high temperature.
- Should exhibit sufficient strength to withstand the force of molten alloy
entering the mold.
5. Inner surface of mold should be smooth.
3

4. 6. At higher temperatures
- It should be stable without any decomposition of investment.
- Should show sufficient expansion enough to compensate for shrinkage
of wax pattern and solidification of molten metal.
7. The material should be sufficiently porous enough to permit escape of
air/other gases from the mold cavity during casting of molten metal.
8. It should show ease of divestment.
- It should not react with metal
- It should easily break away from the surface of casting.
9. It should be economical
No single material is known that can fulfill all the ideal requirements. So
various ingredients / modifiers are added to get the desired properties.
CLASSIFICATION OF DENTAL INVESTMENT :
I. Based on Processing Temperatures :
A. High temperature casting investments
- Phosphate bonded investments
- Silicate bonded investments
B. Low temperature casting investments
- Gypsum bonded investments (for low temperature gold alloy)
II. Depending on type of refractory used (Silica)
- Quartz investment
- Cristoballite investment
III. Based on type of binder used
A. Gypsum bonded investments : According to ADA Specification 2
Uses Shrinkage compensation
Type I Inlay, Crown Purely thermal expansion
Type II Inlay and Crowns Purely hygroscopic expansion
Type III R.P.D. Frame work Thermal
B. Phosphate bonded investments
C. Silicate bonded investments
4

5. BASIC COMPOSITION OF INVESTMENTS :
In general, an investment is a mixture of the following 3 distinct types of
materials,
A. Refractory Materials : A material that will withstand high temperatures
without decomposing on disintegrating. The most commonly used
refractory material is silicon dioxide such as quartz, tridymite, or
cristoballite or a mixture of these.
Function :
- Resist the heat and forces of casting.
- To expand and compensate for casting shrinkage
B. Binder : The refractory material alone do not form a coherent solid mass,
so some kind of binder is needed. Commonly used binders are ;
- α - Calcium sulfate hemihydrate
- Others are – Sodium silicate, Ethyl silicate, Ammonium sulfate, Sodium
phosphate.
C. Other Chemical Modifiers : Usually a mixture of refractory materials and a
binder alone is not enough to produce all the desirable properties required
of an investment. Other chemicals such as sodium chloride, boric acid,
potassium sulfate, graphite, copper powder or magnesium oxide are often
added in small quantities to modify various physical properties.
Eg. Small amounts of chlorides or boric acid enhance the thermal expansion
of investment bonded by calcium sulfate.
INVESTMENTS FOR LOW CASTING TEMPERATURE :
Gypsum Bonded Investments :
The gypsum based material represent the type traditionally used for
conventional gold alloys.
5

6. ADA Specification No.2 for casting investment for dental gold alloys
comes in three types.
Type I : For inlay and crown – Use mainly thermal expansion
Type II : For inlay and crown – Use mainly hygroscopic expansion
Type III : For R.P.D. frame work – Use mainly thermal expansion
Common Brands :
Baker’s Sterling, Begocast, Cristobalite, Inlay – Vest, Luster cast.
Composition :
1. Binder :
α - Calcium sulfate hemihydrate (25 – 45%)
2. Refractory Material
Silica – Quartz or Cristoballite or a blend of two in varying proportions (65
– 75%)
3. Chemical modifiers
a) Carbon / Copper powder (Reducing agents) (2-3%)
b) Boric acid and sodium chloride (Balancing agents)
Functions of Each Constituent :
1. Gypsum α Hemihydrate :
- The α hemihydrate form of gypsum is the binder for investments used in
casting gold containing alloys with melting ranges below 1000o
C.
- Hold other ingredients together and provide rigidity.
- Gypsum after heating undergoes dehydration with shrinkage and
fracture of mold.
Effect Of Temperature On Calcium Sulfate Binders :
The binder used for gold investments in dentistry is α - calcium sulfate
hemihydrate.
6

7. During the investing process some of the water mixed with the
investment reacts with the hemihydrate and in converted to calcium sulfate
dihydrate, whereas the remainder of excess water is uniformly distributed in the
mix.
During the heating process the excess water is evaporated. As the
temperature rise about 105o
C, calcium sulfate dihydrate starts losing water and
expands, then shrink / contract considerably after dehydration between 200o
C
and 400o
C. A slight expansion then occurs between 400o
C and approximately
at 700o
C a large contraction occurs.
This latter shrinkage in most likely caused by decomposition, and sulfur
gases such as sulfurdioxide are emitted. This decomposition not only causes
shrinkage but also contaminate the castings with the sulfides of the non noble
alloying elements such as silver and copper. So gypsum bonded investment
should not be heated above 700o
C.
These properties are explained as follows ; Upto about 105o
C, ordinary
thermal expansion occurs. Above 105o
C, the calcium sulfate dihydrate is
converted to anhydrous calcium sulfate.
Dehydration of the dihydrate and a phase change of the calcium sulfate
anhydrate cause a contraction. The α form of tridymite (which might be
present as an impurity) is expanding and sufficiently compensates for the
contraction of the calcium sulfate to prevent the investments from registering a
serious degree of contraction.
At elevated temperature the α forms of silica present in the investment
are converted to the β forms, which cause some additional expansion.
7

8. 2. Silica :
Silica (SiO2) is added to provide a refractory during the heating of the
investment and to regulate the thermal expansion and also counter shrinkage of
gypsum.
Usually the wax pattern is eliminated from the mold by heat. During
the heating, the investment is expected to expand thermally, to compensate
partially or totally for the casting shrinkage of the gold alloy.
Gypsum shrinks considerably when it is heated. If the proper form of
silica is employed in the investment, this contraction during heating can be
eliminated and changed to an expansion.
 Silica exist in four allotropic forms :
- Quartz
- Tridymite
- Cristobalite and
- Fused quartz
Effect Of Temperature On Silicon Dioxide Refractories :
The most commonly used refractory material is silica (SiO2).
Each of the polymorphic forms of silica, quartz, tridymite and
cristoballite expands when heated, but the percentage of expansion varies from
one type to another.
Pure crystoballite expands to 1.6% at 400o
C, whereas quartz expands
about 1.4% at 600o
C and the thermal expansion of tridymite at 600o
C is less
than 1%.
The percentage of expansion of the 3 types of silica versus temperature
shows, none of the three forms of silica expands uniformly, instead they all
show a break (non linearity) in their thermal expansion.
8

9. In case of crystoballite the expansion is somewhat uniform to about
200o
C. At this temperature its expansion increases sharply from 0.5% to 1.2%,
and then above 250o
C it again becomes more uniform.
At 573o
C quartz also shows a break in the expansion and tridymite
shows a similar break at a much lower temperature.
The breaks on the expansion versus temperature indicate that
cristoballite and quartz, each exist in two polymorphic forms. One of which is
more stable at a higher temperature and the other at a lower temperature.
The form that is more stable at room temperature is called the α form,
and the more stable form at higher temperature is designated as the β from.
Tridymite has three stable polymorphic forms. Thus the temperatures of
220o
C for cristobalite, 573o
C for quartz and 105o
and 160o
C for tridymite are
displacive transition temperatures.
A displacive change involves expansion or contraction in the volume of
the mass without breaking any bonds. In changing from the α form (which is
the more stable form at room temperature) to the β form (which is stable at
higher temperatures), all three forms of silica expand.
The amount of expansion is highest for cristoballite and lowest for
tridymite.
The quartz form of silica is found abundantly in nature, and it can be
converted to cristobalite and tridymite by being treated through a reconstructive
transition during which bonds are broken and a new crystal structure is formed.
 The α quartz is converted to β quartz at a temperature of 573o
C.
 If the β quartz is heated to 870o
C and maintained at that temperature, it is
converted to β tridymite.
9

10.  From β tridymite obtaining either α tridymite or β cristoballite is possible.
 If β tridymite is cooled rapidly to 120o
C and hold at that temperature, it is
changed to α - tridymite, which is stable at room temperature.
On the other hand, if β tridymite is heated to 1475o
C and hold at that
temperature, it is converted to β - cristoballite. Further heating of β
cristoballite produces fused silica, but if it is cooled to 220o
C and held at that
temperature, α cristoballite is formed.
870o
C 1475o
C 1700o
C
β quartz β tridymite β cristobalite Fused Silica
160o
C
573o
C Middle tridymite 220o
C
105o
C
α-Quartz α-tridymite α-cristobalite
All forms of silica are in either α forms in the investment and during the
heating process they are converted completely or in part to their corresponding
β forms. This transition involves an expansion of the mass, which helps to
compensate for casting shrinkage.
 Fused quartz is amorphous and glass like in character and it exhibits no
inversion at any temperature below its fusion point. It has an extremely low
linear coefficient of thermal expansion and is of little use in dental
investments.
 Quartz, cristobalite or a combination of the two forms may be used in a
dental investment. Both are now available in pure from.
 Tridymite is no longer an expected impurity in cristobalite.
3. Modifiers :
10

11. a) Reducing agents : Carbon and powdered copper provide a non oxidizing
atmosphere in the mold when the gold alloy is cast.
b) Balancing agent : Boric acid and sodium chloride
Regulate
- Setting time
- Setting expansion
- Prevent shrinkage of gypsum when it is heated above 300o
C
Setting Reaction of Gypsum Bonded Investments :
Same as Dental stone.
CaSO4 ½ H2O + ½ H2O  CaSO4•2H2O + 3900 Cal / gmol.
(Ca. Sulfate hemihydrate) (Ca. Sulfate dihydrate)
When calcium sulfate hemihydrate is mixed with water, calcium sulfate
hemihydrate is converted back to calcium sulfate dihydrate which sets to form
a solid mass which binds the silica particles together.
The reaction is exothermic and whenever 1 gm mol of calcium sulfate
hemihydrate is reacted with 1.5 gm mol of water. 1 gm mol of calcium sulfate
dihydrate is formed and 3900 calories of heat are developed.
The microstructure of set material shows; rod like particles of gypsum
intermeshed with large irregular particles of silica refractory.
Setting Time :
It is dependent on the gypsum content and upon the type of gypsum
employed.
Initial setting time : 8 – 15 min
Final setting time : 12-25 min.
This can be altered by addition of K2SO4, NaCl (Increase setting time),
Borax and potassium citrate (decrease setting time).
11

12. ADA Specification No.2 ; States that setting time should not be less
than 5 minutes and nor longer than 25 minutes. Modern inlay investments set
initially in 9-18 minutes. This provides sufficient time for mixing and
investing the pattern.
Factors Controlling Setting Time :
• Manufacturing process – finer the particle size – faster setting
• Mixing time and rate - ↑ mixing time - ↓ setting time.
• Water / powder ratio - ↑ water : powder ratio - ↑ setting time
• Temperature - ↑ temperature - ↑ setting time
• Modifier – Accelerator and retarders – Accelerates ↑ setting time, Retarders
↓ setting time
Properties of Gypsum Bonded Investment :
1. Expansion
2. Contraction
3. Strength
4. Other consideration
1. Expansion :
Expansion aids in enlarging the mold. This property of investment is
needed for compensation of casting shrinkage of alloy.
Expansions are of 3 types :
1. Normal setting expansion.
2. Hygroscopic setting expansion
3. Thermal expansion
Normal Setting Expansion :
The setting expansion of an investment, is the linear expansion that take
place during the normal setting of the investment in air. A mixture of silica and
gypsum hemihydrate results in setting expansion greater than that of the
12

13. gypsum product when it is used alone. It is because of interference of silica
with the growing crystals.
The silica particles interfere with the intermeshing and interlocking of
the crystals as they form. Thus, the thrust of the crystals is outward during
growth and therefore more effective in the production of expansion.
ADA Specification No.2 for Type I investment permits a maximum setting
expansion in air of only 0.6%. Modern investments show setting expansion of
approximately 0.4%. It is regulated by retarders and accelerators.
Effect of Wax Pattern on Normal Setting Expansion :
The setting reaction of gypsum bonded investment is exothermic in
nature (3900 cal/gmol). The heat causes expansion of wax pattern leading to
expansion of the mold.
The amount of heat liberated depends on
The ratio of gypsum – ↑gypsum  ↑heat - ↑ S.E.
W:P Ratio  ↓ W:P ratio  ↑ Heat  ↑ S.E.
B. Hygroscopic Setting Expansion :
Hygroscopic expansion is the linear expansion of the investment that
occurs if the investment is in contact with water from any source during the
setting process, after investing the wax pattern.
Contact with water can be achieved by placing the casting ring in a
water bath, before initial set is complete or by putting some water on the
surface of the investment in the ring or by using a wet liner inside the casting
ring.
Distinguishing between a setting expansion and hygroscopic expansion
is difficult because both take place almost at the same time and end at the same
time. In practice a sum of the hygroscopic and setting expansion of the
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14. investment is obtained, which is six times more than the normal setting
expansion.
Hygroscopic expansion may occur because of
• Gypsum
• Refractory
Gypsum :
According to some theories,
• Addition of water during the setting of an investment increases the surface
film thickness on the inert particles and gypsum crystals, thereby forming
them apart.
• Added water during the setting process permits further hydration of calcium
sulfate, thus causing expansion of the investment.
• Added water may force gypsum gel to swell.
• Addition of water provides additional volume in to which gypsum crystals
can grow.
Refractory :
The water physically seperates the fine parities of silica by capillary
action leading to expansion of mass. This is reversible in case of absence of
binder. But if binder is present and set, the expansion is retained.
The decreased expansion is affected by :
• Amount of silica  ↑ amount of silica - ↑ expansion
• W:P ratio  ↑ W:P ratio  ↓ Expansion
• Time of insertion – Investment immersed in water after initial setting cause
decreased expansion.
ADA Specification No.2 for Type II Investments requires a
Minimum expansion of 1.2%
14

15. Maximum expansion of 2.2%
15

16. FACTORS AFFECTING HYGROSCOPIC EXPANSION :
1. Composition : HSE is directly proportional to the amount of silica present.
Finer the particle size of silica : Increased hygroscopic expansion
HSE is greater in α hemihydrate than β hemihdyrate.
A dental investment should have enough hemihdyrate binder with the
silica to provide sufficient strength after hygroscopic expansion. Otherwise, a
shrinkage occurs during the subsequent drying of the set investment. At least
15% of binder is necessary to prevent a drying shrinkage.
2. Water Powder Ratio : Increased water powder ratio  Decreased
Hygroscopic exposure.
3. Spatulation : Increased missing time : Increased hygroscopic expansion.
4. Ratio of Spatulation : Increased spatulation : Increased hygroscopic
expansion.
5. Shelf life of investment : Old investment : Decreased hygroscopic
expansion.
6. Time of investment : Immersion of setting investment.
Before initial setting : Increased hygroscopic expansion
After initial setting : Decreased hygroscopic expansion
7. Effect of confinement : Both the normal and the hygroscopic setting
expansion are confined by opposing forces, such as the walls of the
container in which the investment is placed or the walls of the wax pattern.
The confining effect in the hygroscopic expansion is more pronounced than
the similar effect on the normal setting expansion.
8. Water bath temperature: Although the water bath temperature has little
effect on the hygroscopic expansion of the investment, it has a definite
effect on the wax pattern. At higher water bath temperature, the wax
pattern expands, requiring less expansion of the investment to compensate
for the total casting shrinkage.
16

17. In addition higher water bath temperature soften the wax. The softened
wax then gives less resistance to the expansion of the investment, thus
making the setting and hygroscopic expansion more effective. The net
effect is higher expansion of the mold with higher water bath temperature.
During the setting process, dental casting investments actually absorb water
from their surroundings and expand. If during setting more water, an
investment is permitted to take up from any source, the higher the
hygroscopic expansion, upto a point where further addition of wax do not
create any additional expansion. This degree of expansion or its
corresponding quantity of water is called the critical point.
For an investment to reach its maximum hygroscopic expansion, sufficient
water should be available.
If hygroscopically expanding investments are in contact with less water
than they are able to absorb, but they will not exhibit their maximum
hygroscopic expansion.
9. Amount of water added: More amount of water added during setting period,
more is the expansion.
Thermal Expansion :
In case of gypsum investments, thermal expansion is achieved by
placing the mould in a furnace at a temperature not greater than 700o
C. When
the investment ring with the investment is heated two events take place.
a. Gypsum undergoes shrinkage, as it becomes calcium sulfate anhydrate
losing water.
b. Silica undergoes a thermal expansion. Cristobalite contributes more to such
expansion.
Thermal expansion of a gypsum bonded investment is directly related to
the amount of silica present and to the type of silica employed.
A considerable amount of quartz is necessary to counterbalance the
contraction of gypsum during heating.
17

18. • When the quartz content of the investment is increased to 60% with the
balance being hemihydrate binder, the initial contraction of gypsum is not
eliminated.
• The contraction of gypsum is entirely balanced when the quartz content is
increased to 75%.
• If a sufficient amount of setting expansion had been present, the casting
made at 700o
C would have fit the die reasonably well.
• Quartz expands to 1.4% at 575o
C
• Cristobalite expands to 1.6% at 250o
C
Cristobalite produce adequate mold expansion at lower temperature
because of the lower inversion temperature of the cristobalite in comparison
with that of quartz. Thus the normal contraction of the gypsum during heating
is easily eliminated.
A reasonably good fit of the casting is obtained when the gold alloy is
cast into the mold at temperature of 500o
C and higher.
The amount of thermal expansion of a dental investment depend on it use.
If the hygroscopic expansion is used to compensate for the contraction
of the gold alloy for Type II investments, ADA specification No.2 requires that
the thermal expansion be between 0% and 0.6% at 500o
C.
However, for the Type I investments which rely principally on thermal
expansion for compensation, the thermal expansion must be not less than 1%
nor greater than 1.6%.
Another desirable feature of an inlay investment is that its maximal
thermal expansion be attained at a temperature not higher than 700o
C. Thus
when a thermal expansion technique is used, the maximum mold temperature
for the casting of gold alloy should be less than 700o
C. The gold alloys can
become contaminated at a mold temperature higher than this.
18

19. FACTORS AFFECTING THERMAL EXPANSION :
1. Water Powder Ratio : The magnitude of thermal expansion is related to the
amount of solids present.
↑ W:P ratio  ↓ thermal expansion.
2. Effect of chemical modifiers : Disadvantage of an investment that contains
sufficient silica to prevent any contraction during heating is that the
weakening effect of silica in such quantities is likely to be too great.
• Addition of small amounts of sodium, potassium or lithium chlorides to the
investment.
- Eliminate the contraction caused by the gypsum.
- Increases expansion without the presence of an excessive amount of
silica.
• Boric acid has a similar effect
- Increases expansion
- Hardens the set investment
- It apparently disintegrates during the heating of the investment and a
roughened surface on the casting may result.
• Silica do not prevent gypsum shrinkage but counter balance it.
• Chlorides actually reduce gypsum shrinkage below temperatures of
approximately 700o
C.
2. Thermal Contraction :
When the investment is allowed to cool from 700o
C the refractory and
binder contact (This contraction is because of gypsum when it is first heated).
On cooling to room temperature, the investment exhibits an overall
contraction compared with its dimension before heating.
19

20. On reheating to the temperature previously attained, the investment does
not expand thermally to the previous level. Moreover the process of cooling
and reheating causes internal cracks in the investment that can affect the quality
of the casting.
3. Strength :
The strength of the investment must be adequate to prevent fracture or
chipping of the mold during heating and casting of the gold alloy.
The total thermal contraction of the investment is similar to that of the
gold alloy from the casting temperature to room temperature, the contraction of
the investment is fairly constant, until it cools to below 550o
C. Thus, when the
alloy is still quite hot and weak, the investment can resist alloy shrinkage by
virtue of its strength and constant dimension.
This can cause distortion and even fracture in the casting if the hot
strength of the alloy is low. Although this is rarely a factor with gypsum-
bonded investments, it can be important with other types of investments.
The compressive strength of the investment mold is a primary factor to
be considered in addition to the expansion when evaluating the dimensional
accuracy of dental castings. Ideally, the investment should have sufficient
expansion to compensate for all of the thermal contraction of the alloy.
However, after burnout of the mold, the strength need be no greater than that
required to resist the impact of the metal entering the mold.
According to ADA Specification No.2, the compressive strength for the
inlay investment should not be less than 2.4 MPa (350 psi) when tested 2 hours
after setting.
Factors Affecting Strength :
1. Increased W:P ratio :decreased compressive strength.
2. Heating the investment to 700o
C may increase or decrease strength by 65%.
20

21. Greatest reduction in strength on heating is found in investments containing
NaCl.
3. After the investment has cooled to room temperature, its strength decreases
considerably, because of fine cracks that form during cooling.
4. The use of α hemihydrate instead of plaster increases the compressive
strength.
5. The use of chemical modifiers increases the strength because more of
binder can be used without a marked reduction in the thermal expansion.
4. OTHER CONSIDERATIONS :
a. Fineness : Fineness of the investment affects its setting time, surface
roughness of the casting.
Fine the silica particle – greater the hygroscopic expansion than a coarse
silica.
Finer the investment – Smaller are the surface irregularities on the casting.
b. Porosity : During the casting process, the molten metal is forced into the
mold under pressure. As the molten metal enters the mold, the air must be
forced out ahead of it. If the air is not completely eliminated, a
backpressure builds up, which prevent the gold alloy from completely
filling the mold. The common method for venting the mold is through the
pores of the investment.
The amount of porosity depends on :
• Gypsum crystals. More gypsum crystals are present in the set investment 
less porosity.
Therefore lower the amount of hemihdyrate and greater the amount of water
used to mix the investment  more porous it becomes.
• More uniform the particle size  Greater is the porosity.
A mixture of coarse and fine particles  Less porosity than an investment
composed of a uniform particle size.
21

22. c. Storage : Under conditions of high relative humidity, the setting time may
change. Under such conditions, the setting expansion and the hygroscopic
expansion may be altered so that the entire casting procedure may be
adversely affected. Therefore the investments should be stored in airtight
and moisture proof containers. During use, the containers should be opened
for short time as possible.
All investments are composed of a number of ingredients each of which
possess a different specific gravity. Therefore there is tendency for these
components to separate according to the specific gravity. This separation may
sometimes influence the setting expansion and other properties of the
investment so it is advisable to purchase the investment in relatively small
quantities.
The investment should be weighed and the water should be measured
according to the proportion of the investment mix. Only in this manner can
one expect to control the setting or the thermal expansion in relation to the
compensation needed for the casting shrinkage and other important properties.
Some manufacturers supply their investment in preweighed packages so that
one needs only to measure the gauging water.
Uses :
For casting of inlays, bridges, removable partial denture frame works
using gold alloys and other low fusing alloys.
MODIFIED TYPES OF GYPSUM – BONDED INVESTMENTS :
Hygroscopic Thermal Inlay Casting Investment :
A new inlay casting investment that can be used as a hygroscopic or
thermal type has became available. This investment contains a blend of quartz
and cristobalite as the refractory. When hygroscopic casting technique is used,
the investment is heated to 482o
C after setting in accordance with the normal
water immersion technique. When used in the thermal casting technique, the
22

23. investment is not immersed in water, but after setting it is heated to 644o
C, then
the appropriate expansion is achieved.
INVESTMENTS USED FOR HIGH TEMPERATURE CASTING
PROCEDURE :
Most palladium and base metal alloys used for partial dentures and
porcelain fused-to-metal restoration have high melting temperatures. They are
cast in moulds at 850 to 1100o
C.
So, gypsum bonded investments cannot be used, because it disintegrates
at such high temperature. To withstand these high temperatures, the molds
require different types of binders such as silicate and phosphate compound.
The investment used for this purpose are ;
- Phosphate bonded investments
- Silica bonded investments
Phosphate Bonded Investments :
The most common type of investment for casting high melting alloys is
the phosphate bonded investment.
Common Brands :
Aurobond, Calsite, Cerafine, Deguvest, DVP, Eurocent, Nirobond,
Roma exalet etc.
Composition :
1. Binder : 20%
It consists of 2 components.
- Acidic part  Ammonium diacid phosphate (NH4H2PO4)
- Basic part  Magnesium oxide (mgo)
23

24. Which react at room temperature to form a phosphate binder 
Ammonium magnesium phosphate (NH4MgPO4.6H2O) which gives the
investment green strength / room temperature strength.
2. Refractory : Silica. Either cristoballite / quartz or a mixture of two in a
concentration of approximately 80%. They function as refractory (i.e. to
provide high temperature thermal shock resistance) and provide thermal
expansion at high temperature.
3. Modifiers :
Carbon – Act as a reducing agent to produce clean casting and facilitate
devesting of the casting from the mould. It is used for high temperature gold
casting alloys. But with silver palladium or base metal alloy, carbon embrittles
the alloy even though the investment is heated to temperatures that burnout the
carbon.
Palladium reacts with carbon at temperatures above 1504o
C. Thus if the
casting temperature of a high palladium alloy exceeds this critical point, a
phosphate investment without carbon should be used. s
Mode of Supply :
Powder in packets with special Liquid :
1. Powder contain NH4H2PO4, MgO, Silica, Traces of carbon. This powder
may be mixed with water to form the investment.
2. Special liquid  Contain colloidal silica.
Colloidal silica suspensions are available for use with phosphate
investments in place of water. This liquid shows increased setting expansion,
produce significant amount of hygroscopic expansion (as with pure water the
amount of hygroscopic expansion is less) and increases its strength.
For base metal alloys a 33% dilution of colloidal silica is required.
Setting Reaction :
24

25. At room temperature ammonium diacid phosphate reacts with
magnesium oxide to give the investment green strength or room temperature
strength.
1. NH4H2PO4 + MgO + 5H2O  NH4 MgPO4. 6H2O
(Ammonium diacid phosphate) (Magnesium ammonium phosphate)
The ammonium diacid phosphate is used in a greater amount than is
necessary for this reaction, so that the additional amount can react with silica at
an elevated temperature. At higher temperature there is probably a superficial
reaction between P2O5 and SiO2 to form silica phosphate, which increases
strength of investment at higher temperature.
Phosphates are quite complex and the reaction is not simple. The
stoichometric (determination of the relative proportion of the compounds
involved in a chemical reaction) quantities are equal molecules of magnesia
and ammonium diacid phosphate, an excess of magnesia is usually present and
some of it is never fully reacted. The product formed is predominantly
colloidal multimolecular (NH4MgPO4.6H2O)n which aggregate around excess
MgO and fillers. After the initial setting reaction, the set colloid undergoes
various thermal reactions on heating.
2. MgO + NH4 H2PO4 + H2O
↓
(NH4MgPO4. 6H2O)n
MgO Colloidal type particles
NH4H2PO4
H2O
Prolonged setting at room temperature or
dehydration at 50o
C
(NH4MgPO4.6H2O)n
H2O Dehydrated at 160o
C
(NH4 MgPO4.H2O)n
25

26. Heated from 300 – 650o
C
(Mg2P2O7)n Non crystalline polymeric phase
Above 690o
C
Mg2P2O7 Crystalline in nature
Heated above 1040o
C
Mg3(P2O4)2
The final products are crystalline Mg2P2O7 and some excess MgO, along
with essentially unchanged quartz, cristobalite or both. Some Mg3(PO4)2 may
be formed if the investment is grossly overheated or when the molten metal
contacts the mold cavity surface.
Manipulation :
The powder is mixed with a measured amount of liquid using a bowl
and spatula. Hand mixing or mechanical mixing under vacuum can be done.
The mixed material is vibrated into the casting ring.
Setting and Thermal Expansion :
Theoretically, the setting reaction should show shrinkage but in practice
there is slight expansion when colloidal silica solution is used instead of water.
When phosphate investments are mixed with water, they exhibit a shrinkage at
the same temperature range as gypsum bonded investments (200o
C to 400o
C).
This contraction is practically eliminated when a colloidal silica solution
replaces the water.
The shrinkage is due to decomposition of the binder, magnesium
ammonium phosphate with evolution of ammonia, which is readily apparent by
its odor, but this shrinkage is masked by expansion of cristoballite.
According to ADA specification No.42 there are 2 types of phosphate bonded
investments.
Type I : Inlays, crown and other fixed restorations.
Type II : Partial dentures and other cast removable restorations
26

27. Working and Setting Time :
Working time : 2 minutes
Setting time : 1 hour
Factors Affecting Setting Time :
1. Increased Temperature  Fast set
The setting reaction itself is exothermic, and this further accelerates the rate
of setting.
2. Increased mixing time  Fast set
Generally mechanical mixing under vacuum is preferred.
3. Increased L : P ratio  Increased working time
Miscellaneous Properties :
Compressive Strengths  Type I  Should not be less than 2.5 MPa
 Type II  3.0 MPa
Setting expansion  0.4%
Hygroscopic expansion  0.6 – 0.8%
Thermal expansion  With water  0.8%
 With special liquid  1.2%
Advantages :
1. High green strength
2. High fired strength – Less mold cracking and few fins on casting.
3. They can withstand temperatures upto 1000o
C for short periods of time.
Disadvantages :
1. At temperatures greater than 1375o
C  Cause mold breakdown and
roughen the surface of casting.
2. Due to high strength devesting is defect.
27

28. 3. To increase expansion, with use of special liquid  Cause less porous mold
 Incomplete casting
4. High tendency for reaction with non-precious alloy producing oxides which
is difficult to remove from castings.
Surfactant Containing Phosphate Bonded Investment :
Addition of surfactants to phosphate bonded investment can increase the
hygroscopic setting expansion. The surfactant also makes the unset investment
more viscous and reduces the compressive strength.
Ethyl-Silicate Bonded Investments :
Another type of binding material for investments used with casting high
melting alloys is silicate bonded investments. This investment material are
being used since 1930. But now it is loosing popularity due to complicated and
time consuming procedures involved.
Common Brands :
- Nobilium rapid set (low iron and sodium investment)
- Saddle lock (a ferruginous investment)
- Howmet Vary rapid (intermediate iron and sodium)
Composition :
A. Binder  Silica gel that reverts to silica (cristobalite) on heating.
B. Refractory  Silica (cristobalite)
C. Additive  Magnesium oxide – Make it alkaline, strengthen the gel
D. Wetting agent  To reduce accumulation of air bubbles on surface of wax
pattern.
Various methods of producing silica or silicic acid gel binder.
28

29. 1. pH of sodium silicate is lowered by the addition of an acid or acid salt 
Silicic acid gel forms.
2. Aqueous suspension of colloidal silica can be converted to gel by addition
of an accelerator such as ammonium chloride  Silicic acid gel.
Another system for binder formation is based on ethyl silicate.
3 Stages :
Stage I : Hydrolysis :
A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the
presence of hydrochloric acid, ethyl alcohol and water.
HCl, C2H5OH
Si(OC2H5)4 + 4H2O Si(OH4) + 4C2H5OH (Ethyl alcohol)
Ethyl silicate Sol of polysilicic acid
Because of the use of polymerized form of ethyl silicate, a colloidal sol
of polysilicic acid is expected instead of simpler silicic acid sol.
Stage II : Gelation
The sol is then mixed with quartz or cristoballite, to which is added a
small amount of finally powdered magnesium oxide to render the mixture
alkaline.
nSi (OH)4 + Mgo  Mgo [Si(OH)]n
A coherent gel of polysilicic acid forms, accompanied by a setting
shrinkage.
Stage III : Drying (<168o
C)
This soft gel is dried at a temperature below 168o
C. During the drying
process, the gel loses alcohol and water to form a concentrated hard gel. A
volumetric contraction accompanies the drying, which reduces the size of the
29

30. mold. This contraction is known as “Green Shrinkage” and it occurs in
addition to the setting shrinkage.
This gelatin process is slow and time consuming. An alternative and
faster method for the production of silica gel is employed. Certain types of
amines can be added to the solution of ethyl silicate so that hydrolysis and
gelation occur simultaneously.
Simultaneous hydrolysis
Ethyl silicate + Piperidine Silica gel
(amine) Gelation
With an investment of this type, the mold enlargement before casting
must compensate not only for the casting shrinkage of the metal but also for the
green shrinkage and setting shrinkage of the investment.
Mode of Supply :
1. Powder  Refractory particles of silica and glass
 Calcined MgO
 Other refractory oxide
2. Liquid  Single liquid of stabilized alcohol solution of silica gel.
or
With 2 liquids
- One bottle contains a properly diluted water soluble silicate solution
- Other bottle contains a properly diluted acid solution such as solution of
hydrochloric acid.
Before use equal volume of each bottle should be mixed and allowed to
stand for a prescribed time according to the manufacturers instruction. So that
hydrolysis can take place and freshly prepared silicic acid formed.
Manipulation :
30

31. Powder is added to the hydrolyzed ethyl silicate liquid, mixed quickly
and vibrated into a mold. This allows the heavier particles to settle quickly
while the excess liquid and some of the fine particles rise to the top.
In about 30 minutes  The accelerator (NH4Cl) in the powder hardens
the settled part, and the top excess is poured off. Thus the liquid: powder ratio
in the settled part is greatly reduced and the setting shrinkage is reduced to
0.1%.
It is little more complicated than phosphate type in that care must be
exercised in handling and burnout, because flammable alcohol is given off. If
it is heated high enough, some silica converts to quartz and provides added
expansion.
This type of investment can be heated to 1090o
C to 1180o
C and is
compatible with the higher fusing alloys. Its low setting expansion minimizes
distortion.
Properties :
Compressive strength  Not less than 1.5 MPa
Setting contraction  0 – 0.4%
Thermal expansion  1.5 – 1.8% (This material has only thermal
expansion and no other expansion)
Can withstand high temperature  1090o
C to 1180o
C
Porosity  The particles in the set material are very closely packed leading to
low porosity. Air space / vents must be left in investment to permit escape of
air from the mould.
Advantages :
1. High permeability, yields sharply defined castings.
2. Low setting expansion
3. The investment has more refractory  Form smooth castings
31

32. 4. Low burnout strength results in easy removal of castings and cleaning of
oxides from the castings.
Disadvantages :
1. Limited shelf life of liquid
2. Must wait for substantial period of time, prior to using freshly mixed liquid.
3. Potential of cracking exits during burnout, owing to high thermal
expansion.
4. Very expensive
5. Gives off flammable components during processing.
Uses :
1. They are mainly used for casting Co-Cr R.P.D. frame work.
2. Accurate casting of Nickel based alloys.
Divestments (By Whipmix Corporation) :
It is a combination of die stone and gypsum bounded investment
material. The powder is mixed with colloidal silica.
Properties:
Setting expansion : 0.9%
Thermal expansion : 0.6% (at 977o
C)
Advantages : The wax pattern and die are invested simultaneously without
removal of pattern. Useful with gold alloys. (Used for casting minute patterns)
Divestment Phosphate (Dvp) : Similar to divestment, but used for casting
post and core, crowns of base metal alloys without any need of removal of wax
pattern.
Brazing Investment :
ADA Sp. No.93 : Type I – Gypsum bonded dental brazing investment
Type II – Phosphate bonded
32

33. Steps :
1) Broken parts are stabilized by sticky wax.
2) The broken parts are then embedded in investment with portion to be solder
is left exposed and free of investment.
They should have low setting and thermal expansion. Particle size is
usually not fine. They possess usually a compressive strength of 2-10 MPa.
CONCLUSION :
It should be emphasized that several investment techniques can produce
comparable results. The dentist or technician should become familiar with
different method and different investment materials. In any technique the
fundamentals of that techniques should be applied.
33