Production of Gypsum Products
Setting of Gypsum Products
Strength of Set Gypsum Products
Types of Gypsum Products
Manipulation of Gypsum Products
Gypsum (CaSO4•2H2O; calcium sulfate dihydrate) is a
mineral mined in various parts of the world, but it is also
produced as a by-product of flue gas desulfurization in some
coal-fired electric power plants.
Albaster- pure white ,fine grained and translucent.
4. Satin spar - fibrous needle like with silky lustre.
Selenite - colourless, crystalline and transparent.
APPLICATION IN DENTISTRY
For cast preparation.
Models and dies.
As Investment Material
Mounting of Casts
As a mold material for processing of complete dentures.
7. PRODUCTION OF GYPSUM PRODUCTS
These materials are produced by calcining calcium sulfate
Commercially, the gypsum is ground and subjected to
temperatures of 110 °C to 130 °C (230 °F to 266 °F) in open
containers to drive off part of the water of crystallization.
8. The resulting particle is a fibrous aggregate of fine crystals with
capillary pores known as plaster of Paris or dental plaster in
As the temperature is further raised, it becomes an anhydrite.
This process is known as calcination.
When gypsum is heated in a kettle, vat, or rotary kiln that
maintains a wet environment; a crystalline hemihydrate called
dental stone is produced in the form of rods or prisms.
9. Because of differences in crystal size, surface area, and degree
of lattice perfection, the resulting powders are often referred to
as α-hemihydrate for dental stone and β-hemihydrate for
plaster of Paris.
If the calcination process occurs under pressure in a 30%
calcium chloride solution or in the presence of more than 1%
of sodium succinate, the resulting hemihydrate crystals will be
shorter and thicker than those produced in a closed container.
Residual calcium chloride or sodium succinate is removed by
washing the powder with hot water. This type of gypsum-
producing product is called modified α-hemihydrate or die
stone. These crystals require even less water for mixing.
10. α-hemihydrate β-hemihydrate
• Formed when dihydrate is
heated under steam pressure.
Formed when dihydrate is
heated in an open kettle or kiln.
• α-hemihydrate crystals are
denser and have a prismatic
β -hemihydrate crystals are
characterized by their
“sponginess” and irregular
•When hemihydrate particles are
mixed with water the α-
hemihydrate produces a much
stronger and harder dihydrate
β -hemihydrate produces a less
stronger and harder dihydrate
• It requires less water as
compared to β-hemihydrate.
• β-hemihydrate crystals require
more water to wet the powder
12. SETTING OF GYPSUM PRODUCTS
The reaction between gypsum products and water produces solid
gypsum, and the heat evolved in the exothermic reaction is
equivalent to the heat used originally for calcination.
Set gypsum products probably never attain 100% conversion
unless they are exposed to high humidity for a long time.
Therefore, there are unreacted hemihydrates remaining in the set
13. SETTING REACTIONS
There are three theories of gypsum setting.
The colloidal theory proposes that, when mixed with water,
hemihydrate enters into the colloidal state through a sol-gel
In the sol state, hemihydrate particles are hydrated to form
dihydrate, thereby entering into an active state. As the measured
amount of water is consumed, the mass converts to a solid gel.
The hydration theory suggests that rehydrated plaster particles
unite through hydrogen bonding with sulfate groups to form the
14. The most widely accepted mechanism is the dissolution-
precipitation theory, which is based on dissolution of the
hemihydrate particles in water followed by instant
recrystallization to the dihydrate.
This reaction has become possible because the solubility of
hemihydrate in water is four times greater than that of the
dihydrate near room temperature.
Thus, the setting reactions occur as follows:
When the hemihydrate is mixed with water, a suspension is
formed that is fluid and workable.
The hemihydrate dissolves until it forms a saturated solution
of Ca2+ and (SO4) 2−.
This saturated hemihydrate solution is supersaturated with
respect to the solubility of the dihydrate; precipitation of
15. As the dihydrate precipitates, the hemihydrate continues to
dissolve. The process proceeds as either new crystals form or
further growth occurs on the crystals already present until no
further dihydrate can precipitate out of solution.
The setting reaction takes time to complete and changes in the
mixture begin as soon as the hemihydrate and water are mixed
There is less than 50% dihydrate present in Type IV and V
stones, about 60% in Type II die materials, and over 90% in
Type I plasters.
16. QUANTIFYING SETTING REACTIONS
The time from addition of powder to the water until mixing is
completed is called the mixing time.
Mechanical mixing is usually completed in 20 to 30 seconds.
Hand spatulation generally requires at least a minute to obtain a
The time from the start of mixing to the point where the
consistency is no longer acceptable for the product’s intended
purpose is the working time.
Generally, a 3-minute working time should allow sufficient time
for mixing, pouring an impression and a spare impression, and
cleaning the equipment before the gypsum becomes
17. The elapsed time for each stage varies from material to
material. The manufacturers usually provide this information
with the product.
The time that elapses from the beginning of mixing until the
material hardens is known as the setting time.
18. TESTS FOR SETTING OF GYPSUM PRODUCTS
The tests done for setting of gypsum products are listed as
o Loss of gloss test.
o Gillmore’s test.
o Vicat test for setting time.
Loss of gloss test: As the reaction proceeds, the excess water on
the surface is taken up in forming the dihydrate, so that the mix
loses its surface gloss and gains strength.
19. Gillmore’s test: When the mix no longer leaves an impression
when penetrated by Gillmore needle, which has a tip 2.12 mm
in diameter and weighs 113.4 g, the time elapsed is called the
initial setting time.
At this point, the mass still has no measurable compressive
strength and the cast cannot be safely removed from the
The elapsed time at which a heavier Gillmore needle,
weighing 453.6 g and with a tip 1.06 mm in diameter,
leaves only a barely perceptible mark on the surface is
called the final setting time.
20. Vicat’s test for setting time: After initial setting the further
reaction is determined by an instrument called Vicat
The needle with a weighted plunger rod is supported and held
just in contact with the mix .
The time elapsed until the needle no
longer reach the bottom of the mix is
known as setting time.
The elapsed time for each stage varies from
material to material. The manufacturers usually
provide this information with the product.
21. CONTROL OF THE SETTING TIME
Theoretically, there are at least three mechanisms that can
achieve such control. These include:
Solubility of the hemihydrate- If the solubility of the
hemihydrate is increased, supersaturation of dihydrate is
achieved faster, which accelerates rate of dihydrate crystal
Number of nuclei of crystallization- Nucleation is the first
step at which Ca2+ and SO4
2− in solution start to assemble
into clusters on the nanometer scale, becoming stable under
the operating conditions.
These stable clusters constitute the nuclei. The greater the
number of nuclei of crystallization, the faster the dihydrate
crystals will form and the sooner the mass will harden. Any
preexisting fine dihydrate particles will also serve as nuclei.
22. Rate of crystal growth—Increasing or decreasing the rate
of crystal growth will accelerate or retard the setting time.
In practice, these mechanisms have been incorporated in
the formulation of the material by the manufacturer and by
manipulation techniques performed by the operator.
Impurities : Fine gypsum particle residues from incomplete
calcination or addition by the manufacturer will shorten the
setting time because of the increase in the number of nuclei.
Fineness :Grinding of hemihydrate particles during
manufacturing increases not only the rate of dissolution of
the hemihydrate solution but also the number of nuclei. This
increase in nuclei density results in a more rapid rate of
23. Water/powder ratio : The weight (or volume) of the water
divided by the weight of the hemihydrate powder is known as
the water/powder ratio.
o The use of a higher W/P ratio decreases the number of nuclei
per unit volume. Consequently, the setting time is prolonged.
24. Mixing : Within practical limits, the longer and the more
rapidly the gypsum product is mixed, the shorter is the
setting time as the crystals are broken up by the
spatulation process, which results in more nuclei of
Temperature: the difference in solubility between
hemihydrate and gypsum becomes smaller with increasing
temperature, and this condition lowers the driving force
for forming the dihydrate; it also results in a slower setting
25. Material W/P Ratio Spatulation
Model Plaster 0.50 20 14 min
0.50 100 11 min
0.50 200 8 min
Dental Stone 0.30 20 10 min
0.30 100 8 min
Effect of Spatulation on Setting Time
26. MODIFIERS FOR CONTROLLING SETTING TIME
Chemical modifiers have been used extensively to increase
or decrease the setting time of gypsum products; they are
called retarders and accelerators, respectively.
The chemical that increases the rate of hemihydrate
dissolution or precipitation of dihydrate accelerates the
The most commonly used accelerator is potassium sulfate,
which is particularly effective in concentrations greater than
Slurry water flowing out from a model trimmer contains
numerous fine gypsum particles that act as nuclei of
crystallization and that can serve as an effective accelerator.
27. At a concentration of 2% of the hemihydrate, sodium
chloride is an accelerator. Sodium sulfate has its
maximum acceleration effect at approximately 3.4%.
Borax, a known retarder for gypsum setting, has been
shown also to promote the growth of dihydrate crystals,
but only at a concentration lower than 0.2 mM (about
28. SETTING EXPANSION
Regardless of the type of gypsum product selected, an expansion of
the mass can be detected during the change from the hemihydrate to
Depending on the composition of the gypsum product, this observed
linear expansion may be as low as 0.06% or as high as 0.5%.
MECHANISM OF SETTING EXPANSION
The crystallization of dihydrates can be pictured as an outgrowth of
crystals from nuclei of crystallization. Crystals growing from the
nuclei can intermesh with and obstruct the growth of adjacent
When the process repeats itself with thousands of the crystals during
growth, an outward stress or thrust develops, producing an
expansion of the entire mass.
29. Therefore, the structure immediately after setting is composed
of interlocking crystals between which are micropores and
pores containing the excess water required for mixing.
The onset of initial setting occurs at approximately the minimal
point of the curve, the point at which expansion begins.
30. Expansion under normal setting conditions
STAGE I: Imagine that the initial mix is represented in the top
row of by the three round particles of hemihydrate
surrounded by water. Under normal setting conditions, the
crystals of the dihydrate begin to form on the nuclei.
STAGE II :The water around the particles is reduced by
hydration and these particles are drawn more closely
together because surface tension of the water keeps the
water surface area at a minimum .
31. STAGE III: As the crystals of dihydrate grow, they contact
each other and the water around the particles again
STAGE IV: Further dihydrate growth consumes more water
and should draw the crystals together as before, but the
outward thrust of the growing crystals opposes this
STAGE V : Eventually, the crystals become intermeshed and
32. Hygroscopic setting expansion
The hygroscopic setting expansion is a physical
phenomenon and is not caused by a chemical reaction any
more than is the normal setting expansion.
STAGE I: shows an identical mixture of hemihydrate (the
area delineated by the dashed circle) under water (the area
outside of the dashed circle). The hydration of hemihydrate
particles here would proceed as usual.
STAGE II: Since they are under water, the water consumed
by hydration will be immediately replenished by the
immersion water and the distance between the particles
would remain the same .
33. STAGE III: As the dihydrate crystals continue to grow
and contact each other, no reduction in the distance
between crystals is expected.
STAGE IV: The crystals grows much more freely
during the early stages, before the intermeshing finally
prevents further expansion (stage V).
34. CONTROL OF SETTING EXPANSION
A lower W/P ratio and a longer mixing time will
increase the setting expansion.
Potassium sulphate (accelerator) — 4% solution
decreases setting expansion from 0.5% to 0.06%
Sodium chloride 2%(accelerator) and ground
gypsum increases setting expansion.
35. STRENGTH OF SET GYPSUM PRODUCTS
EFFECT OF WATER CONTENT
The strength of plaster or stone increases rapidly as the material
hardens after the initial setting time. However, the free water
content of the set product definitely affects its strength.
For this reason, two strength properties of gypsum are reported:
the wet strength (also known as green strength), and the dry
The wet strength is the strength that is determined when water
in excess of that required for hydration of the hemihydrate
remains in the test specimen.
When such excess water is removed by drying, the strength
obtained is the dry strength.
36. o The dry strength may be two or more times as
high as the wet strength.
o Microwave irradiation has been used to speed up
the drying of gypsum casts.
37. EFFECT OF W/P RATIO
As previously noted, the set plaster or stone is porous, and
the greater the W/P ratio, the greater the porosity. The
greater is the W/P ratio, the less is the dry strength of the
Material that is mixed at a high W/P ratio has a diametral
tensile strength as high as 25% of the corresponding
When materials are mixed at low W/P ratios, the diametral
tensile strength is less than 10% of the corresponding
38. The compressive strength is inversely proportional to the
Model plaster has the greatest quantity of excess water,
whereas high-strength dental stone contains the least
High-strength dental stone is the densest and thus shows
the highest compressive strength. Model plaster is the
most porous and thus shows the lowest compressive
39. A plot of the strength as a function of the W/P ratio for the
five different types of gypsum products used in dentistry.
The strength ranges represent the wet strength at 1 hour.
The strength increases as the specimens dry and it can
double in a week.
40. EFFECT OF MANIPULATION AND ADDITIVES
The spatulation time also affects the strength of the plaster, an
increase in mixing time increases the strength to a limit that is
approximately equivalent to that of hand mixing for 1 minute.
If the mixture is overmixed, the gypsum crystals will be
broken up and the final product will hold less crystalline
The addition of an accelerator or retarder lowers both the wet
strength and the dry strength of the gypsum product.
Such a decrease in strength can be partially attributed to the salt
added as an adulterant and to the reduction in inter crystalline
41. SURFACE HARDNESS & ABRASION RESISTANCE
Surface hardness of gypsum materials is related to their
Surface hardness increases at a faster rate than the compressive
Abrasive Resistance of gypsum products (for high-strength
dental stone) increases by 15-41% when impregnated with
Surface hardness of set gypsum is improved by mixing stone
with a hardening solution containing colloidal silica( about
42. TYPES OF GYPSUM PRODUCTS
ADA Specification No. 25 classifies five types of
gypsum products, as shown in Table 9-5, with the
property requirement for each type.
43. IMPRESSION PLASTER (TYPE I):
Impression plaster is a β-calcium sulfate hemihydrate
used at a water/powder ratio of approximately 0.5 to 0.6.
Its fluidity makes it suitable for making impressions of
soft tissues in the uncompressed state, a characteristic of
mucostatic impression material.
Because of its rigidity, the use of impression plaster has
been suggested for making preliminary impressions or
splinting transfer coping utilized to produce long-span
44. Potassium sulfate is added as an anti-setting expansion agent
to reduce the setting expansion and a retarder like borax is
added to the powder to balance the setting acceleration
caused by the potassium sulfate and to bring the setting time
A pigment, such as alizarin red, may be added to make a
clear distinction between the impression and the model after
casting of the model.
As an alternative, an antiexpansion solution containing
potassium sulfate, borax, and pigment may be used with a
standard white plaster.
45. Manipulation Because freshly mixed plaster is too fluid to
be retained in a stock tray, a custom tray can be constructed
using a 1- to 1.5-mm spacer with acrylic resin or shellac.
Preliminary impressions can be made with dental
compound, and impression plaster can be used as the wash
material. The technique for inserting the impression into the
mouth involves “puddling” the impression into place.
With the remaining plaster in the tray, the tray is seated in a
single movement. Then the tray is gently moved from side
to side and anteroposteriorly to take advantage of the
fluidity of the material.
In view of the fluidity of the material, the resulting
impression may be difficult to remove. The plaster
impression material is very brittle and fractures easily.
46. When the impression involves an undercut area, it is
necessary to fracture the impression to facilitate removal
from the mouth. The fragments are then reconstructed to
form the completed impression.
Long, narrow strips of wax can be adapted around the
periphery of the impression. This is called beading. The
impression is then coated with a thin layer of separating
medium and cast in fresh plaster; otherwise, separation
would be impossible.
Disinfection of a plaster impression can be achieved with
a 10-min soak in sodium hypochlorite solution, as
47. MODEL PLASTER (TYPE II)
This model plaster or laboratory Type II plaster is now
used principally to fill a flask used in denture construction
when setting expansion is not critical and the strength is
adequate according to the limits cited in the ADA
specification or ISO standard.
It is usually marketed in the natural white color, thus,
contrasting with stones, which are generally colored.
48. DENTAL STONE (TYPE III)
With the advent of hydrocolloid impression material , the
improved hardness of α-hemihydrate made stone dies
workable and the indirect wax pattern possible.
Type III stone has a minimal 1-hour compressive strength
of 20.7 MPa (3000 psi), but it does not exceed 34.5 MPa
49. It is intended for the construction of casts in the fabrication
of full dentures to fit soft tissues. For this application, a
slight setting expansion can be tolerated in casts that
reproduce soft tissues, but not when teeth are involved.
Type III stones are preferred for casts used to process
dentures because the stone has enough strength for this
purpose and the denture is easier to remove after processing
50. DENTAL STONE, HIGH STRENGTH (TYPE IV)
The principal requisites for a die material are strength,
hardness, and minimal setting expansion. To obtain these
properties, modified α-hemihydrate is used.
The cube-shaped particles and the reduced surface area
produce such properties without undue thickening of the mix.
This material is also called die stone.
51. A hard surface is necessary for a die stone because
the tooth preparation is covered with wax and carved
flush with the margins of the die.
A sharp instrument is used for this purpose;
therefore, the stone must be resistant to abrasion.
It is fortunate that the surface hardness increases
more rapidly than the compressive strength because
the surface dries more rapidly.
This is a real advantage in that the surface resists
abrasion, whereas the core of the die is tough and
less subject to accidental breakage.
52. DENTAL STONE, HIGH STRENGTH, HIGH
EXPANSION (TYPE V)
This gypsum product exhibits an even higher compressive
strength than the Type IV dental stone by lowering the W/P
ratio even further than that used for Type IV stone.
In addition, the setting expansion has been increased from a
maximum of 0.10% to 0.30%.
Thus, higher expansion is required in the stone die to aid in
compensating for the alloy solidification shrinkage.
The use of a Type V stone may also be indicated when the
expansion achieved during the fabrication of cast crowns is
53. SPECIAL GYPSUM PRODUCTS
The orthodontist prefers a white stone or plaster for study
models and may even treat the surface with soap to increase
their sheen. These products generally have a longer working
time, which reduces void formation and facilitates trimming.
The use of an articulator makes it necessary to mount the
casts using a gypsum-producing product. These materials are
referred to as “mounting” stones or plasters. They are fast
setting and have low setting expansion.
The mounting plaster has a sufficiently low strength to
permit easy trimming and facilitate separating the cast from
the articulator mounting plates
54. MANIPULATION OF GYPSUM PRODUCTS
In practice, clinicians and technicians must not only produce a
cast using a gypsum-producing material, but they must also store
the powder properly and maintain the cast in its best condition for
CARE OF GYPSUM PRODUCTS
The hemihydrate of gypsum absorbs water from the air readily.
For example, if the relative humidity of the surroundings exceeds
70%, the plaster absorbs sufficient moisture from the air to start a
The first hydration probably produces a few crystals of gypsum
on the surface of the exposed hemihydrate crystals. These
gypsum crystals can act as nuclei of crystallization and accelerate
the setting reaction when they are mixed with water.
55. If the hydration is allowed to continue, this process results
in the hemihydrate crystals being completely covered with
dihydrate crystals. Under these conditions, the water
penetrates the dihydrate coating with difficulty and the
setting time is prolonged.
Therefore, it is important that all gypsum products be
stored in a dry atmosphere. The best means of storage is to
seal the product in a moisture-proof metal container.
When gypsum products are stored in closed containers,
the setting time is generally retarded only slightly,
approximately 1 or 2 min per year. This may be
counteracted by a slight increase in the mixing time if
The recommended W/P ratio should be used. The water and powder
should be measured by using an accurate graduated cylinder for the
water volume and a weighing balance for the weight of powder.
The powder should not be measured by volume (as by using a scoop) as
it does not pack uniformly. This characteristic may vary from product to
product, and it will pack more densely if the container remains
When the container is shaken, the packed particles will be loosened and
the volume will increase as a result of air entrapment.
If mixing is performed by hand, the bowl should be
parabolic in shape, smooth, and resistant to abrasion.
The spatula should have a stiff blade and a handle that is
convenient to hold.
Method of mixing:-
Add measured water
Gradual addition of the preweighed powder
The mixture is then vigorously stirred, with periodic wiping of
the inside of the bowl with the spatula.
59. The mixing should continue until a smooth mix is
obtained, usually within a minute.
Trapping of air should be avoided while mixing to avoid
porosity – weak spots & surface inaccuracies.
Longer spatulation = increases working time.
After mixing, the use of a vibrator of high frequency and
low amplitude is helpful in reducing air entrapment.
61. CARE OF THE CAST
Once the setting reactions in the cast have been
completed, its dimensions will be relatively constant
under ordinary conditions of room temperature and
However, it is sometimes necessary to soak the gypsum
cast in water in preparation for other procedures. When a
dry cast is immersed in water, negligible expansion may
occur if the water is saturated with calcium sulfate.
If the water is not saturated, dissolution of gypsum will
occur. For example, a stone cast immersed in a container
under running water will lose approximately 0.1% of its
linear dimension for every 20 min of immersion
62. INFECTION CONTROL
If an impression has not been disinfected, it is necessary
to disinfect the stone cast.
Disinfection solutions that do not adversely affect the
quality of the gypsum product can be used.
Dental stone containing a disinfectant may also be used.
Useful disinfectants for stone casts include spray
disinfectants, hypochlorites, & iodophores.
63. The safest method for soaking the cast is to place it in a
water bath with gypsum debris remaining on the bottom
of the container to provide a saturated solution of calcium
If the storage temperature is raised to between 90 °C and
110 °C (194 °F to 230 °F), shrinkage occurs, along with
loss of strength as the water of crystallization is removed
and the dihydrate reverts to the hemihydrate form.
As a rule of thumb, it is not safe to store or heat a stone
cast in air at a temperature higher than 55 °C (130 °F).
The hemihydrate and water must be mixed uniformly to ensure a homogeneous reaction through the entire mixture. It must also maintain its workable state as long as possible to facilitate pouring of a model or the casting investment.
One should recognize that significant acceleration of the reaction time could compromise the optimal flow characteristics of the material. The manufacturer has added accelerators, retarders, and other controlling agents, so it is not wise to add other ingredients that may counteract the effects of components that are already incorporated into the product. Therefore, one should select a product from the manufacturer’s catalogue with the specific setting time that the operator needs
However, the most effective method by which to control setting expansion is through the addition of chemicals by the manufacturer.
). Consequently, the observed setting expansion that occurs when the gypsum product sets under water is greater than that which occurs during setting in air.
). Consequently, the observed setting expansion that occurs when the gypsum product sets under water is greater than that which occurs during setting in air.
Excess water is uniformly distributed in the mix and contributes to the volume and not the strength.
dental gypsum products contain additives that reduce the setting expansion, increase the working time, and provide a rapid final set. The addition of more chemicals can upset the delicate balance of these properties. Thus if a change is desired in the setting time, it should be controlled by modest alterations in the W/P ratio and/or the spatulation time.
Because the surface of the hardened mass reaches a dry state earlier than the inner portion of the mass.
To make plaster a suitable impression material, the expansion and the setting times should be controlled by incorporating compounds designed to mediate the handling properties.
The average dry surface hardness of the Type IV stones is approximately 92 (Rockwell hardness) and that of Type III stone is 82.
The rationale for increasing setting expansion is that certain newer alloys, such as base metal, have a greater casting shrinkage than do the traditional noble metal alloys.
to ensure wetting of all of the powder and breaking up of any agglomerates or lumps
to ensure wetting of all of the powder and breaking up of any agglomerates or lumps