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1. INVESTMENTS
An investment is any material used in dentistry to invest an object. It is
also defined as the material used to enclose or surround a pattern of a dental
restoration for casting or molding or to maintain the relations of metal parts
during soldering.
For casting procedures, the wax pattern of an inlay or crown is
embedded or invested in a heat resistant material, which is capable of setting to
a hard mass. The wax is removed from such a mould usually by burning out,
before casting the molten alloy. There are in general two types of investments:
casting investments and soldering investments.
Requirements for an ideal investment material
1. Should be easily manipulated. It should be easy to mix and paint on to the
wax pattern, and it should harden within a short time.
2. Should produce a smooth surface as well as fine detail and margins on the
casting.
3. On being heated at high temperatures, it should not decompose and give off
corrosive gases that may chemically injure the surface of gold alloys.
4. Should be porous enough to permit the air or other gases in the mould
cavity to escape easily during the casting procedure.
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2. 5. The mould must expand to compensate for the shrinkage on cooling of the
alloy.
6. Should be strong enough to withstand the force of the molten alloy entering
the mould.
7. After casting, it should break away readily and should not attach itself to the
surface of the metal.
8. Casting temperature should not be critical. Preferably the thermal expansion
versus temperature curve should have a plateau of thermal expansion
expansion over a range of casting temperatures.
9. Should be comparitively inexpensive.
Types of casting investments
1. Gypsum bonded investments
2. Phosphate bonded investments
3. Silica bonded investments
Gypsum bonded investments are the oldest materials and are used for
casting conventional gold alloys. The phosphate bonded investments are used
for metal ceramic restorations. Silica bonded investments are principally used
for the casting of base metal alloy partial dentures.
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3. Gypsum bonded investments
American Dental Association specification no: 2 for casting investments
used for gold alloys, lists 3 types of investments:
Type I – for inlays and crowns utilizes thermal expansion of the mould.
Type II – for inlays and crowns utilizes hygroscopic setting expansion.
Type III – for gold partial denture utilizes thermal expansion.
Composition
Gypsum bonded investment is a mixture of 4 types of materials.
a. Refractory materials: usually a form of silica (quartz, crystobalite or
tridymite), this withstands high temperatures and provides mould expansion
by thermal expansion.
b. Binder : autoclaved calcium sulphate hemihydrate is used.
i. To react with water and on hydration binds the silica together.
ii. To impart sufficient strength to the mould and
iii. To contribute to the mould expansion by its setting expansion.
c. A reducing agent: such as powdered charcoal, reduces any oxide formed on
the metal.
d. Modifying chemicals : such as boric acid or sodium chloride to inhibit
shrinkage on heating.
3

4. Gypsum
The alpha hemihydrate is usually used because of its greater strength
compared to the beta variety. But this binder when heated to high temperatures
for complete dehydration, shrinks considerably and frequently fractures. The
shrinkage on heating is due to the dehydration of the set gypsum in two stages.
i. 2CaSO4 · 2H2O  (CaSO4)2 H2O + 3H2O
ii. (CaSO4)2 · H2O  2CaSO4 + H2O
Shrinkage is due to the transformation of calcium sulphate from the
hexagonal to the orthorhombic configuration.
A thermal expansion curve of gypsum shows considerable shrinkage
from 200 to 400°C. A slight expansion then occurs to approximately 700°C and
then a tremendous contraction is caused, probably due to decomposition, and
sulphur gases like sulphur dioxide are emitted which contaminates the casting.
So gypsum investments should not be heated above 700°C and these untoward
effects can be minimized by ‘heat soaking’ the mould at 700°C for at least an
hour to allow the reactions to be completed before casting commences.
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5. Silica
Silica (Silicon dioxide, SiO2) is the refractory material and it regulates
thermal expansion. Gypsum contracts during heating and silica eliminates this
contraction and changes it to an expansion. Silica exists in at least four
allotropic forms: quartz, tridymite, cristoblalite and fused quartz. When quartz,
tridymite or Cristobalite are heated, a change in crystalline form occurs, called
“inversion”, at a transition temperature characteristic of the particular form of
silica.
Quartz inverts from a “low” form or alpha quartz to a “high” form called
beta quartz at 575°C, Cristobalite inverts from 200 to 270°C from low or alpha
cristobalite to high or beta cristobalite. Two inversions of tridymite occur, one
at 117°C and the other at 163°C. Fused quartz exhibits no inversion at any
temperature below its fusion point. Due to its very low coefficient of thermal
expansion it is of little use in dental investments. The beta allotropic forms are
stable only above the transition temperature and an inversion to the lower form
occurs upon cooling. In powdered form inversion occurs over a range of
temperature rather than instantaneously.
870°C 1475°C 1700°C
Beta quartz Beta Beta Fused silica
tridymite cristobalite
575°C 117°C 220°C
Alpha quartz Alpha Alpha
Tridymite Cristobalite
The dentistry decreases and volume increases as the alpha form changes
to the beta form and a rapid increase in linear expansion occurs. This is
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6. probably due to a straightening of chemical bonds to form a less dense crystal
structure.
Only quartz and cristobalite or their combination are used as
investments. Depending upon the type of silica principally employed,
investments are classified as quartz investments or cristobalite investments.
Modifiers
Modifying agents, colouring matter and reducing agents such as carbon
or powdered copper are also added. Some modifiers such as boric acid and
sodium chloride not only regulate the setting expansion and setting time, but
prevent most of the shrinkage of gypsum when it is heated above 300°C.
The usual composition of gypsum bonded investment is:
Refractory : 60 to 65%
Binder : 30 to 35%
Modifiers
& Pigments : 4 to 7%
Setting time
ADA specification no.2 recommends a minimum setting time of 5 mins.
and a maximum of 25 minutes. The modern investments usually have a initial
setting time between 9 to 18 minutes.
Normal setting expansion
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7. A mixture of silica and gypsum investment results in a greater setting
expansion than a gypsum product used alone. The silica particles interfere with
the intermeshing and interlocking of the crystals as they form, resulting in a
outward thrust and so an expansion. Setting expansion of modern investments
is about 0.4%.
Setting expansion of the mould compensates partially for the casting
shrinkage of gold. The ‘normal setting expansion’ as determined in the
laboratory is different from the ‘effective setting expansion’ occuring in
practical usage.
Factors determining the effective setting expansion
a. Greater the gypsum content of the investment, greater the exothermic heat
transmitted to the wax pattern and greater the mould expansion.
b. Lower the W/P ratio for the investment, greater the exothermic heat and
greater the setting expansion.
c. Thinner the walls of the wax pattern, greater the setting expasion of the
investment.
d. Softer the wax, greater the setting expansion. If a wax softer than Type B
inlay wax is used, the setting expansion may cause a serious distortion of
the pattern.
Hygroscopic setting expansion
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8. This occurs when the gypsum is allowed to set under or in contact with
water. Hygroscopic setting expansion of an investment may be more than six
times the normal setting expansion. The ADA specification no.2 for Type II
investments requires a minimal hygroscopic setting expansion of 1.2% and a
maximal expansion of 2.2%. In one, method known as the ‘water immersion’
technique, the investment mould is placed into water. Another method is the
‘water added’ technique. Here a measured volume of water is placed on the
upper surface of the investment material within the casting ring. This produces
a more readily controlled expansion.
Theory of hygroscopic setting expansion
Hygroscopic setting expansion is a continuation of normal setting
expansion and occurs as the immersion water replaces the water of hydration
used up by the crystals. This prevents the confinement of the growing crystals
by the surface tension of water. (Mahler D.B. and Ady A.B. ‘An explanation
for the hygroscopic setting expansion of dental gypsum products’ J. Dent. Res.
39 : 578, 1960).
Factors affecting hygroscopic setting expansion
a. Composition
Increase in silica content increases the hygroscopic setting expansion.
Finer the silica particles greater the expansion. Alpha hemihydrate produces
more expansion in the presence of silica, than beta hemihydrate. The
hygroscopic setting expansion of stone or plaster alone is very slight.
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9. The investment should have at least 15% binder to provide strength after
hygroscopic setting expansion, and to prevent drying shrinkage.
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10. b. W/P ratio
Higher the W/P ratio of the original investment – water mixture, less the
hygroscopic setting expansion.
c. Spatulation
Shorter the mixing time, less the hygroscopic expansion. This factor is
important in controlling the effective setting expansion as well.
d. Shelf life of investment
The older the investment, less is its hygroscopic setting expansion. The
material should be stored in air tight containers and should not be exposed to
humidity. It is better to purchase small amounts of the investment at a time.
e. Time of immersion
The greatest amount of hygroscopic setting expansion takes place if the
immersion in water takes place before the initial set. The longer the time of
immersion is delayed after the initial set, lesser the expansion. Also the longer
the time for which it is kept immersed, greater the hygroscopic setting
expansion.
f. Confinement
Both the normal and hygroscopic setting expansions are confined by
opposing forces, such as the walls of the casting ring or the walls of the wax
pattern. This confinement can be avoided largely by placing damp asbestos as a
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11. liner on the inner wall of the ring. The water in the asbestos also is utilized for
hygroscopic expansion.
g. Temperature of the water bath
Higher the temperature of the water bath used for immersion, greater the
hygroscopic expansion. The softening of the surface of the wax results in
increased effective setting expansion.
h. Amount of added water
An increase in the amount of water added, increases the hygroscopic
setting expansion upto a certain point, after which further addition of water
does not create any expansion. This degree of maximum expansion is called the
“critical point”. This critical point can be raised or lowered easily by changing
the manipulative conditions like W/P ratio, time of spatulation, age of
investment etc.
i. Particle size of silica
Finer particles of silica produce greater hygroscopic expansion. The
hemihydrate particles have little effect on this expansion.
j. Silica/binder ratio
If this ratio increases, greater will be the hygroscopic expansion and
lesser the strength. This is because the added water can easily diffuse through
the silica particles.
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12. Thermal Expansion
Thermal expansion is directly related to the amount and type of silica
employed. The contraction of gypsum is entirely balanced when the quartz
content is about 75%.
Cristobalite produces a much greater thermal expansion during its
inversion and so the normal shrinkage on heating gypsum is easily avoided.
Further, the expansion occurs at a lower temperature because of the lower
inversion temperature of cristobalite as compared to quartz. Some of the
modern investment contain both quartz and cristobalite.
The ADA specification no. 2 requires the thermal expansions of type II
investments to be between 0.0 to 0.6% and of Type I investments which rely
principally on thermal expansion for compensation, to be between 1.0 to 2.0%.
It is desirable that the maximal thermal expansion be attained at a
temperature not greater than 700°C. Gold alloys are apt to be contaminated
above this temperature.
Factors affecting thermal expansion
a. W/P ratio
The magnitude of thermal expansion is related to the amount of solids
present. So more the water used in mixing, less is the thermal expansion.
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13. b. Chemical modifiers
When the silica content of an investment is high enough to prevent any
contraction during heating, the investment becomes weak. The addition of
small amounts of chlorides of sodium, potassium or lithium to the investment
eliminates the contraction and increases expansion without adding excessive
amounts of silica. (Earnshaw R. ‘The effects of additives on the thermal
behaviour of gypsum bonded casting investments’. Part I. Aust. Dent. J. 20 :
27, 1975).
Silicas do not prevent shrinkage of gypsum, but counterbalance it,
whereas chlorides actually reduce gypsum shrinkage below temperatures of
700°C.
The total expansion of the mould from normal setting expansion,
hygroscopic setting expansion or thermal expansion is generally sufficient to
compensate for the shrinkage on cooling of gold alloys, which is about 1.5% by
volume.
PROPERTIES
Thermal Contraction
When an investment is allowed to cool from 700°C, its contraction
curve follows the expansion curve, during inversion of the beta quartz to its
stable form at room temperature. It shrinks to less than its original dimension
because of the shrinkage of the gypsum when first heated.
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14. Strength
The investment must be strong enough to prevent fracture or chipping of
the mould during heating and casting. But the compressive strength should not
be unduly high. Sometimes a distortion of the casting may result from a
directional restraint by the strong investment to the thermal contraction of the
casting, as the alloy cools to room temperature.
(Teteruck W.R. Mumford G. ‘The fit of certain dental casting alloys
using different investing materials and techniques’. J. Prosthet. Dent. 16 : 910,
1966).
This rarely occurs in gypsum bonded investments, but may occur with
other types of investments. The distortion can even lead to fracture of the
casting if the hot strength of the alloy is low.
The compressive strength of the investment depends on the type and
amount of gypsum binder used. Modifiers aid in increasing the strength as
more of the binder can be used without much reduction in thermal expansion.
ADA specification no. 2 requires a minimum compressive strength of
2.5 Mpa, 2 hours after setting of the investment. However, higher strength is
required for a Type III partial denture investment.
More the W/P ratio employed, lower the compressive strength.
The greatest reduction in strength upon heating is found in investments
containing sodium chloride. The strength decreases on cooling to room
temperature, because of the fine cracks that develop.
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15. Fineness
Fineness of the investment affects the setting time and surface roughness
of the casting. Fine silica particles increase the hygroscopic setting expansion
and gives smoothness to the casting.
Porosity
When the molten metal enters the mould, air must be forced out ahead
of it or else back pressure builds up and prevents the alloy from completely
filling the mould, causing back pressure porosity. Pores in the investment offer
adequate venting.
Less the hemihydrate content and more the gauging water used, more
porous is the investment.
The more uniform the particle size, greater is its porosity. A mixture of
coarse and fine particles exhibits less porosity.
Manipulation
Before investing the wax pattern, it is washed with a non-foam detergent
to remove any oil or grease and to facilitate the wetting of the pattern by the
investment mix. The casting ring is lined with wet asbestos strip. Investing the
pattern may be done:
i) under vacuum, to prevent trapping air on the surface of the pattern, or
ii) painting investment material on to the pattern with a brush before gently
forcing it into the filled casting ring.
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16. If the hygroscopic technique is employed, (Type II) the casting ring,
with the crucible former end down, is immediately immersed in a water bath at
a temperature of 37 to 38°C. For the thermal expansion technique (Type I and
III) the investment is allowed to harden in the ring placed on the bench.
Divestment: It is a die stone-investment combination in which the die material
and investing medium have a comparable composition. Divestment is a
commerical gypsum-bonded material, it is mixed with colloidal silica liquid.
The die is made from this mix and the wax pattern constructed on this. Then
the entire assembly (die and pattern) is invested in the Divestment, thereby
eliminating the possibility of distortion of the pattern upon removal from the
die or during the setting of the investment. The setting expansion of the
material is 0.9% and the thermal expansion is 0.6% when heated to 677°C.
Since divestment is a gypsum-bonded material, it is not recommended
for high-fusing alloys, as used with metal ceramic restorations. However, it is a
highly accurate technique for use with conventional gold alloys, especially for
extra-coronal preparation. Wax elimination is started only after atleast one
hour.
Limitations: Above 1200°C, a reaction occurs between calcium sulphate and
silica: CaSO4 + SiO2  CaSiO3 + SO3. This sulphur trioxide gas evolved
causes porosity in the casting and contributes to the corrosion of the casting.
For this reason gypsum-bonded investments are not used for higher fusing
alloys like cobalt-chromium. In such cases, phosphate or silica bonded
materials are chosen.
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17. Phosphate bonded investments
The popularity of metal-ceramic restorations, which use higher melting
gold alloys and the trend toward the use of inexpensive alloys have resulted in
the greater use of phosphate bonded investments.
Composition
The filler is silica, in the form of cristobalite or quartz or a mixture of
these two, and forms approximately 80%. It provides, refractoriness and a high
thermal expansion.
The binder is magnesium oxide (basic) and a phosphate that is (acidic).
Originally phosphoric acid was used, but at present mono-ammonium
phosphate is used as it can be incorporated into the powdered investment. The
powder is mixed with an aqueous colloidal silica suspension and invested.
As alloys used in metal ceramic restorations have higher melting points,
their contraction during solidification is greater and the aqueous colloidal silica
suspensions compensate for this.
Sometimes these silica solutions freeze in cold weather and so, some
phosphate investments have been produced, which utilize water as the gauging
liquid.
Carbon is often added to the powder to produce clean castings and to
facilitate the ‘dig-out’ of the casting. It is suggested that carbon embrittles
alloys like silver-palladium and base metal alloys. Recent evidence shows that
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18. palladium does not react with carbon at temperatures below 1504°C. (J.F.
Jelenko Co: Thermotrol Technician, 34 : No.1, Winter, 1980).
Thus if the casting temperature of a high palladium alloy exceeds this
critical point phosphate investment without carbon is used. A carbon crucible
should not be employed for melting the alloy; and even gold alloys used with
porcelain should not be premelted or fluxed on charcoal blocks, because the
trace elements that provide high strength may be removed.
Setting reactions
The chemical reaction is as follows:
NH4H2PO4 + MgO + 5H2O  NH4MgPO4 6H2O
The Magnesium ammonium phosphate formed is polymeric. (Neiman R
and Sarma A.C. ‘setting and thermal reactions of phosphate cements’. J. Dent.
Res. 9: 1478, 1980).
The MgO is never fully reacted. Thus a predominantly colloidal
multimolecular (NH4MgPO4 6H)2n is formed, coagulating around excess MgO
and fillers.
Upon heating, the binder of the set investment undergoes thermal reactions, as
suggested by J. F. Jelenko Co., in the following way:
18

19. MgO + NH4H2PO4 + H2O
Room Temperature
(NH4MgPO4 6H2O)n
MgO
NH4H2PO4 Colloidal type particles
H2O
Prolonged setting at room temperature or dehydration at 50°C
(NH4MgPO4 6H2O)n
Dehydrated at 160°C
(NH4MgPO4 · H2O)n Ammonia which is liberated gives off its
characteristic smell.
Heated at 300-650°C
(Mg2P2O7)n Non-crystalline polymeric phase
Heated above 690°C
Mg2P2O7
+MgO
Heated above 1040°C
Mg3(PO4)2
The final products are crystalline Mg2P2O7 and some excess MgO, along
with unchanged quartz and/or cristobalite. Some Mg3(PO4)2 may be formed if
the investment is grossly overheated or when the molten metal contacts the
mould cavity surfaces.
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20. Setting and thermal expansion
During the setting reaction a slight expansion occurs and this is more in
the colloidal silica-mixed materials than for water-mixed materials. The setting
expansion, as in the case of gypsum, results from the outward thrust of the
growing crystals. Thermal expansion is also greater for colloidal silica-mixed
materials. The combined setting and thermal expansion for phosphate
investments is around 2% if the special silica liquid is used
Similar to gypsum bonded investments, the phosphate investments
mixed with water show a shrinkage from 200° to 400°C. This contraction is
practically absent in the colloidal silica mixed materials.
Both the setting expansion and the thermal expansion increase as the
concentration of the special liquid increases.
A decrease in expansion can be achieved by increasing the
liquid/powder ratio rather than by decreasing the concentration of the liquid.
20

21. According to Engler et al the setting and thermal expansions do not
compensate totally for the shrinkage of the metal as was formerly thought, and
research need to be done to find other factors which contribute to compensate
for the metal shrinkage. The magnitude of shrinkage is of the order of 1.4% for
most gold alloys, 2.0% for Ni/Cr alloys and 2.3% for Co/Cr alloys.
Thermal shrinkage
This occurs due to decomposition of the binder, magnesium ammonium
phosphate, and is accompanied by the evolution of ammonia, which is readily
apparent by its odour.
300°C
2 MgNH4PO4 · 6H2O Mg2P2O7 + 2NH3 + 13H2O
However, some of the shrinkage is masked because of the expansion of
the filler, especially cristobalite.
At higher temperatures, some of the remaining phosphate reacts with
silica forming complex silicophosphates. These impart significant strength to
the material at the casting temperature.
Working and Setting time
Unlike gypsum investments, phosphate investments are markedly
affected by temperature. Warmer the mix, faster the set. The reaction gives off
heat, which further accelerates the setting. Increased mixing time and mixing
efficiency result in a faster set; these two factors give smoothness and accuracy
to the casting. Mechanical mixing under vacuum is preferred.
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22. An increase in liquid/powder ratio increases the working time.
Other properties
The cohesive strength of phosphate investments is so high that they do
not have to be contained in a metal casting ring. The material is allowed to set
inside a plastic ring which is removed before heating.
The porosity of the material is sufficient enough to prevent the build-up
of ‘back pressure’.
Even though it is possible to use hygroscopic setting expansion like for
gypsum bonded materials, it is rarely used in practice.
Technical consideration
Separate mixing bowls must be used for gypsum bonded and phosphate
bonded investments to prevent contamination which causes problems to the
setting of gypsum.
When a number of wax patterns are invested together, there should be at
least 3-4 mm of investment around each pattern and each of them must be in
different planes to prevent excess pressure developing and leading to fracture
of the investment. The rapid expansion of cristobalite at approximately 300°C
requires slow heating and holding at about 200-300°C for at least 30 minutes,
so as to prevent cracking of the investment. Thus a two-stage burnout is
employed.
22

23. Recovery and cleansing of the casting are more difficult when phosphate
investment is used. So ultrasonic cleansing may be necessary. Neither
phosphate binder nor silica is soluble in hydrochloric or sulphuric acid. Cold
hydrofluoric acid will dissolve silica refractory very well, without damage to a
gold-palladium-silver alloy.
Base metal alloys require light grit-blasting usually with fine aluminium
oxide. However, chromium based alloys are usually sand-blasted to remove the
investment. Acid should never be used for cleaning base metal alloy. (Allen
F.C and Asgar K. ‘Reaction of cobalt chromium casting alloy with
investments’. J. Dent. Res. 45 : 1516, 1966).
Silica bonded investments
These are used for casting base metal alloys at high temperatures.
The binder is a silica gel which reverts to silica (cristobalite) on heating.
Several methods are used to produce silica or silicic acid gel binders.
a. When a pH of sodium silicate is lowered by the addition of an acid or an
acid salt such as monoammonium phosphate, a bonding silicic acid gel
forms. The addition of Magnesium oxide will strengthen the gel. (Dootz
E.R., Craig R.C. and Peyton F.A.: ‘Simplification of Chrome-cobalt
partial denture casting procedure’ J. Prosthet. Dent. 17 : 464, 1967).
b. An aqueous solution of colloidal silica can be made to gel by the
addition of an accelerator, such as ammonium chloride.
23

24. c. A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the
presence of hydrochloric acid, ethyl alcohol and water, as follows:
Si(OC2H5) + 4H2O  Si (OH)4 + 4C2H5OH
Because a polymerized form of ethyl silicate is actually used, a colloidal
sol of polysilicic acids is formed instead of the simpler silicic acid shown in the
above reaction.
The formation of poly silicic acid constitutes the 1st
stage of the setting
reaction, called “hydrolysis”. Stage 2 is called “gelation”. Here the sol is mixed
with quartz or cristobalite to which is added a small amount of finely powdered
MgO to render the mixture alkaline. A coherent gel of polysilicic acid then
forms accompanied by a slight ‘setting shrinkage’.
Stage 3 is called “drying”. Here the soft gel is dried to a temperature
below 168°C. During drying, the gel loses alcohol and water to form a hard,
concentrated gel of silica particles tightly packed together. A considerable
volumetric contraction accompanies the drying. Which reduces the size of the
mould. This contraction is known as “green shrinkage” and it occurs in
addition to the setting shrinkage.
The gelation process is slow and time consuming. A faster method to
obtain silica gel is by the addition of amines such as piperidine to the solution
of ethyl silicate. Here hydrolysis and gelation occurs simultaneously. But an
unacceptable shrinkage may occur, mainly in the stage of hydrolysis.
24

25. Stock solutions of hydrolysed ethyl silicate binder may be prepared and
stored in dark bottles. The solution gels slowly on standing and its viscosity
may increase noticeably after 3-4 weeks when it has to be discarded.
The thermal expansion of silica investments is considerable as both the
binder and the refractory are forms of silica which can invert during heating.
Another shrinkage called ‘firing shrinkage’ occurs above 675°C, when the
polysilicic acid gel changes to silica.
As with other investments, the thermal expansion can be controlled by
the amount, particle size and type of silica filler employed.
Silica-bonded investments being more refractory than phosphate-bonded
investments, can tolerate higher burn-out or mould-casting temperatures.
Temperatures between 1090 and 1190°C are employed when the higher fusing
chromium containing alloys are cast. The common burn-out temperatures
generally employed.
Gold alloys : Slow burn-out 450°C
Rapid burn-out 700°C
Ni/Cr alloys : 700-900°C
Co/Cr alloys : 1000°C
(Mabie CP: ‘Petrographic study of the refractory performance of high-
fusing dental alloy investments: II silica – bonded investments. J. Dent. Res. 52
: 758, 1973).
25

26. After divesting and cleaning, the casting is commonly electrolytically
polished in an acid bath prior to a final mechanical polishing.
Porosity
The particles of the set material are packed so closely together that the
porosity is negligible. Air spaces or vents must be left in the investment to
permit escape of gases.
Soldering investment
In the process of assembling the parts of a restoration by soldering, such
as clasps on a removable partial denture, it is necessary first to surround the
parts with a suitable ceramic or investment material before the heating
operation. The assembled parts are temporarily held with sticky wax until they
are surrounded with the investment after which the wax is softened and
removed. The portion to be soldered is left exposed to permit removal of wax
and effective heating prior to being joined with a solder.
These investments contain quartz and a calcium sulphate hemihydrate
binder. They have lower setting and thermal expansions than casting
investments. So the assembled parts do not shift position during setting and
heating of the investment. Soldering investments do not have a fine particle
size as smoothness of the mass is less important.
26

27. Application of various types of investment materials
Investment Primary use
Dental plaster or stone
Gypsum-bonded materials
Phosphate-bonded materials
Silica-bonded materials
Mould for acrylic dentures.
Mould for gold casting alloys.
Mould for base metal and gold casting
alloys, mould for cast ceramics and
glasses; mould for soldering.
Mould for base metal casting alloys.
Discussion
Of the three main types of casting investment materials, the phosphate
bonded products are becoming most widely used. Silica-bonded materials are
rarely used nowadays due to the fact that they are less convenient to use than
the other products and that the ethanol produced in the liquid can
spontaneously ignite or explode at elevated temperatures. As phosphate bonded
investments can be used even for gold castings it can be considered a universal
investment material.
References
1. Mahler P.B. and Ady A.B. ‘An explanation for the hygroscopic setting
expansion of dental gypsum products’. J. Dent. Res. 39: pp 578, 1960.
2. Earnshaw R. ‘The effects of additives on the thermal behaviour of
gypsum bonded casting investments part I. Aus. Dent. J. 20 : pp27,
1975.
27

28. 3. Teteruck W.R. and Mumford G. ‘The fit of certain dental casting alloys
using different investing materials and techniques’. J. Prosthet. Dent.
16 : pp910, 1966.
4. J. F. Jelenko Co: Thermotrol Technician, 34 : No.1, Winter, 1980.
5. Neiman R. and Sarma A.C. : ‘Setting and thermal reactions of phosphate
cements’. J. Dent. Res. 9 : pp1478, 1980.
6. Allen F.C. and Asgar K : ‘Reaction of cobalt chromium casting alloy
with investments’. J. Dent. Res. 45 : pp1516, 1966.
7. Dootz E.R., Craig R.C. and Peyton F.A. : ‘Simplification of chrome-
cobalt partial denture casting procedure’. J. Prosthet. Dent. 17 : pp464,
1967.
8. Mabie C.P. : ‘Petrographic study of the refractory performance of high-
fusing dental alloy investments: II silica-bonded investments. J. Dent.
Res. 52 : pp758, 1973.
28