gypsum and gypsum products for dental use

2. INTRODUCTION
GYPSUM
CLASSIFICATION
MANUFACTURE SETTING
SETTING PROCESS
STAGES IN SETTING
FACTORS AFFECTING RATE OF SETTING REACTION
MICROSTRUCTURE OF CAST
PROPERTIES
SETTING EXPANSION
INFECTION CONTROL
REFRENCES
Contents

3. INTRODUCTION
Terms to know:
Model: A positive replica of an object.
Dental Cast: A reproduction of the shape and features of a
surface made from an impression of the surface.
Die: The positive reproduction of the form of a prepared
toothin any suitable substance.

4. GYPSU
M:
• Originates from the Greek word ‘Gypos’ which meanschalk.
• Dihydrate of calcium sulfate.
• Chemical formula CaSO4.2H2O.
• Sulfate mineral are most commonly found
• Usually white to yellowish white in color.
• Large beds of gypsum were formed when seawater evaporated,
leaving dissolved Calcium and Sulfate ions to form deposits of
gypsum.
• United States is the largest producer as well as the biggest consumer of
gypsum. Others are Canada, France, Japan and Iran.

5. Gypsum mineral can be found in various forms:
1. ROCK–GYPSUM: widely occurring massive dull coloured
rocks.

6. 2. ALABASTER: Large fine-grained white stones.
Often used for carving into vases and ornaments.

7. 3. SELENITE: as transparent crystals.

8. 4. SATIN SPAR: as fibrous crystals.

9. SYNTHETIC GYPSUM:
• Also produced as a by-product of manufacture of
phosphoric acid.
• CHEMICAL GYPSUM.
3 H2SO4(l) + Ca3(PO4)2(s) +
6 H2O(l)
2 H3PO4(s) + 3
CaSO4·2H2O(s)

10. PROPERTIES:
 Found as prismatic, curved or twisted monoclinic
crystals forming a rectangular prism with a pallelogram
base of vitreous luster.
 Specific gravity: 2.3
 It cleaves perfectly in one direction.
 Moh hardness no: 2

11. USES:
1. As a raw material for making Plaster of Paris. Plaster of
Paris is called so, because the gypsum that was used to
manufacture it came from a village called Montamarte, near
Paris. It is used extensively in construction purposes.
2. Grounded gypsum (land plaster) is sometimes used as a
fertilizer for soil that needs calcium.
3. Raw gypsum is also used to keep Portland cement from
hardening too quickly.
4. It is also used to make paint (as a filler), filters, insulation
and wall plaster.
5. Alabaster is used for carving ornaments and vases.
6. Selenite is sometimes used as an optical material.

12. DENTAL USES OF GYPSUM PRODUCTS:
• Impression plaster is used to make the impression of the
edentulous mouth.
• For preparation of study models of oral and maxillofacial
structures.
• To form cast and dies on which dental prosthesis or crowns are
constructed.
• As a mold material for processing complete dentures.
• For mounting of casts for various purpose.
• Also used as a binder for silica, gold alloy casting investment,
soldering investment and investment for low melting point
nickel-chromium alloys.
Gypsum produced for dental application is nearly pure
calcium sulfate dihydrate.

13. GYPSUM PRODUCTS:
Refers to the various forms of calcium sulfate, hydrous and
anhydrous.
Manufactured by the calcination of calcium sulfate dihydrate.
Calcination can be controlled to produce partial or complete
dehydration.
ADANo: 25. ISO No: 6873.

14. Gypsum products can be classified into five types:
GYPSUM PRODUCTS
HIGH STRENGTH PLASTER
IMPRESSION PLASTER
( ISO TYPE I )
PLASTER
( ISO TYPE II )
DENTAL STONE
( ISO TYPE III )
DIE STONE, HIGH
STRENGTH, LOW
EXPANSION
( ISO TYPE IV )
STONE, HIGH STRENGTH,
HIGH EXPANSION
( ISO TYPE V )

15. CHEMISTRY OF GYPSUM:
In the temperature range of 110o to 1000oC that is important in
dental manipulation of gypsum products, three phase changes
occur in the CaSO4.2H2O system.
o o
110 – 130 C
1) CaSO4.2H2O Water
CaSO4.1/2H2O +
(Calcium Sulfate
Hemihydrate)
o o
130 – 200 C
2) CaSO4.1/2H2O -CaSO4 + Water
(Hexagonal Form)
(Soluble anhydrite)
o o
200 – 1000 C
3) -CaSO4 CaSO4
(Orthorhombic CaSO4)
(Insoluble anhydrite)

16. dehydration by heat
or other means
Mineral Gypsum
formulation
Plasters
Synthetic Gypsum Hydrocal
Densite
Dental plaster
Dental Stone
High strength
dental stone
Low High
expansion expansion
In the production of plaster, the gypsum is ground to a fine powder,
impurities such as sulphur (S) and quartz (SiO2) are removed, and then it
is subjected to calcination.

17. PLASTER OF PARIS:
Traditional hemihydrate plaster.
Produced by the dry calcination of ground gypsum in open
containers (pan, kettles or rotary kilns) at temperatures in the
range of 120o to 180oC.

18. DIFFERENT FORMS OF
HEMIHYDRATES:
 Depending on the method of calcination, different forms of
hemihydrate can be obtained.
 – hemihydrate
 – hemihydrate
 The  and  designations are retained because of tradition and
convenience. They are without chemical significance and their
use is solely to indicate the particular morphological
appearance of the crystals.
 The differences between the  and  hemihydrates are a
result of differences in the crystal size, surface area, and
degree of lattice perfection.

19.  HEMIHYDRATE  HEMIHYDRATE
Crystal size LARGER SMALLER
Shape IRREGULAR PRISMATIC
Packing LOOSELY PACKED
AMPLE SPACE B/W
CRYSTALS
CLOSELY PACKED
LITTLE SPACE B/W
CRYSTALS
W/P ratio MORE LESS
Strength LESS MORE
Surface
area/wt.
MORE LESS
Example DENTALPLASTER DENTALSTONE ,
IMPRESSION PLASTER DIE STONE

20. The  hemihydrate is made from gypsum by carefully
controlled calcination under steam pressure in closed
container forming more uniform particles called as dental
stone or Type III.
The modified hemihydrate is made by boiling gypsum in a
30% aqueous solution of calcium chloride and magnesium
chloride. this process yields the smoothest, most dense
powder particles, and the powder is used primarily for dies
called the die stone or Type IV.
The Type V dental stone is same as Type IV added with
accelerators, retarders and surface tension reducing agent like
lignon sulphate used for making very strong dies.

21. POWDER PARTICLES OF -HEMIHYDRATE ( X 400.)

22. POWDER PARTICLES OF DENTAL STONE ( x 400.)

23. PARTICLES OF -MODIFIED HEMIHYDRATE ( x 400 )

24. THE SETTING PROCESS:
THE SETTING REACTION:
Crystalline theory.
Gel theory.
Hydration theory.

25. Crystalline theory:
Proposed in 1887 by Henry Louis Le Chatelier.
In 1907, it received the full support of Jacobus Hendricus
vant’s Hoff.
Also called as Dissolution-precipitation theory.
Based on the dissolution of hemihydrate powder and instant
recrystallization of gypsum, followed by interlocking of the
crystals to form the set solid.
The differences in the solubilities of calcium sulfate dihydrate
and hemihydrate causes the setting of these materials.

26. The setting mechanism may be viewed as follows:
Step 1. INDUCTION
CaSO4.½H2O CaSO4(aq)
Step 2. CRYSTALLISATION
CaSO4(aq) CaSO4.2H2O

27. STAGES:
DISSOLUTION
SUSPENSION
SATURATION
SUPERSATURATION
NUCLEI FORMATION
GROWTH OF NUCLEI
SPHERULITE FORMATION

28.  When the hemihydrate powder is mixed with water in the
correct proportions it forms a thick slurry.
 The hemihydrate is sparingly soluble in water (6.5g/L at 20o C), so
only a small amount can dissolve. Initially, therefore, the mix is a
two- phase suspension of hemihydrate particles in a saturated
aqueous solution.
 The stable hydrate at temperatures below 40o C is the dihydrate
which is even less soluble than the hemihydrate (2.4 g/L at 20oC).
 The aqueous solution is therefore supersaturated with respect to
the dihydrate, which crystallizes out at suitable nucleation sites
in the suspension.

29. The gypsum crystals normally are acicular in habit, and
often radiate out from the nucleation centers in the form of
spherulitic aggregates.
As the dihydrate precipitates, the solution is no longer
saturatedwith the hemihydrate, so it continues to dissolves.
Dissolution of the hemihydrate and the precipitation of the
dihydrate proceeds as either new crystals forms or further
growth occurs on the crystals already present.
The process is continuous and continues until no further
dihydrate precipitates out of the solution.

30. GEL THEORY:
 Also known as the colloidal theory.
 Proposes that when mixed with water, plaster enters into the
colloidal state through the sol-gel mechanism.
 In the sol state, hemihydrate particles are hydrated to form
dihydrate, thereby entering into an active state. As the
measured amount ofwater is consumed, the mass converts to
a solid gel.
HYDRATION THEORY:
 Suggests that rehydrated plaster particles join together through
hydrogen bonding to the sulfate groups to form the set
material.

31. STAGES IN
SETTING:
The setting process is continuous, from the beginning of mixing
until the setting reaction is complete, by which time the material
has reached its full wet strength. However, important physical
changes can be recognized during this process.
The stages in setting may be designated as:
 Fluid
 Plastic
 Friable
 Carvable

32. Initially, there is a continuous aqueous phase present, and the
mix is a viscous liquid, exhibiting pseudoplasticity so that it
flows readily under vibration; in this stage the mix has a
glossy surface giving specular reflections.
As the setting reaction proceeds, gypsum crystals continue to
grow at the expense of the aqueous phase, and the viscosity of
the mix increases. When clumps of the growing gypsum
crystals interact, the mix becomes plastic; it will not flow
under vibration but can be readily molded. At this time the
glossy surface disappears as the aqueous phase is drawn into
the pores formed when the growing gypsum crystals thrust
apart.
Continued crystal growth converts the plastic mass into a
rigid solid, weak and friable at first but gaining strength as
the relative amount of solid phase increases.

33. WATER
REQUIREMENTS:
In practice, powder cannot be mixed with small amount of water
and still develop a mass suitable for manipulation.
Some excess water is required for mixing to obtain the desired
viscosity of the mix, which can be easily manipulated.
At the completion of reaction, the excess unreacted water
remains in the set mass. This residual water weakens the cast.

34. RATE OF SETTING REACTION:
Water/Powder ratio
Spatulation
Temperature
Colloidal system and PH
Additives
Accelerators
Retarders

35. W/P RATIO
Within wide limits, the rate of hydration during setting is independent
of the W/P ratio.
However, the rate at which the associated physical changes described
earlier occur is highly dependent on the W/P ratio of the mix, because
these changes occur from the interaction of clumps of gypsum crystals
growing from nucleation centers in the slurry.
Physical changes associated with the setting of the mix take place
more rapidly as the W/P ratio is decreased.
Thick mixes (low W/P ratios) harden more quickly becauseavailable
nucleation sites are concentrated in a smaller volume; interaction of the
growing solid phase occurs earlier.
Manipulation and setting times are thus directly proportional toW/P
ratio.

36. SPATULATION
The mixing process, called spatulation has a definite effect on
the setting time and setting expansion of the material.
An increase in the amount of spatulation (either speed of
spatulation or time or both) shortens the setting time.
When the powder is place in water, the chemical reaction starts,
and some calcium sulfate dihydrate is formed. During
spatulation, the newly formed calcium sulfate dihydrate breaks
down to smaller crystals and starts new centers of nucleation,
around which calcium sulfate dihydrate can be precipitated.

37. TEMPERATURE:
Evidently the temperature has two main effects on the setting
reaction of gypsum products:
Change in the relative solubilities of calcium
sulfate hemihydrate and calcium sulfate
dihydrate.
Change in the ion mobility

38. EFFECT ON
SOLUBILITIES:
 The ratio of the solubilities of calcium sulfate dihydrate and
calcium sulfate hemihydrate at 20 oC is about 4.5.
 As the temperature increases, the solubility ratio
decreases, until 100 oC is reached and the ratio becomes
one.
 As the ratio of the solubilities become lower, the reaction is
slowed, and the setting time is increased.

39. ION MOBILITY:
As the temperature increases, the mobility of the calcium
and sulfateions increases, which tends to increase the
rate of reaction and shorten the setting time

40. COLLOIDAL SYSTEMS AND
PH:
Colloidal systems such as agar and alginate retard the setting of
gypsum products.
Retard the reaction by nuclei poisoning.
They get adsorbed on the CaSO4.2H2O nucleation sites or on the
CaSO4.1/2H2O and thus interfering in the setting reaction.
The adsorption of these materials on the nucleation sites retards
the setting reaction more effectively than the adsorption on the
calcium sulfate hemihydrate.
Liquids with low Ph, such as saliva, retard the setting reaction.
Liquids with high Ph accelerate setting.

41. ADDITIVES:
Used in the formulation on dental plasters and stones for many
years.
ACCELERATORS
Sodium Chloride(<2%)
Potassium sulfate
Terra alba
RETARDERS
Sodium Chloride(>20%)
Sodium sulfate
Citrates
Tartrates
Acetates

42. SODIUM CHLORIDE:
 Provides additional sites for crystal formation. The increased
number of sites for nucleation also decreases the setting time
of the material.
 Also increases the solubility of hemihydrate, so that it
dissolves rapidly, thereby also decreasing the setting time.
 If it is present in high concentrations( >20%), the sodium
chloride will deposit on the surface of crystals and prevents
further growth. This decreases the reaction rate.

43. POTASSIUM SULFATE:
 Reacts with water and hemihydrate to
form. Syngenite:K2 (CaSO4) 2.H2O.
 This compound crystallizes very rapidly and encourages the
growth of more crystals, thus decreasing the setting time.
 When present as a 2 % solution in water, the setting time is
decreased from 10 minutes to 4 minutes.

44. TERRA
ALBA:
Finely Powdered gypsum.
In small amounts, it will provide additional sites for nucleation,
decreasing the working time and setting times.
SODIUM
SULFATE:
Salts of relatively low solubility, such as sodium sulfate,
acts as retarders in higher concentrations, by nuclei
poisoning.
As the setting proceeds, the amount of free water in the mix
decreases and the concentration of the additive increases.

45. When the limit of solubility is exceeded, the salt precipitates
on the nuclei of crystallization, thus poisoning them.
BORAX:
Sodium tetra borate decahydrate (Na2B4O5(OH)48H2O)
When the plaster, or stone, powder is mixed with an appropriate
aqueous solution of borax (0.9%w/w), the powder particles
become coated with a thin layer of Ca2B6O115H2O
(‘colemanite’) which is very insoluble and delays the
dissolution of the powder.

46. CITRATES AND
TARTRATES:
Reaction of some additives with hemihydrate may occur;
soluble tartrates and citrates precipitate calcium tartrate
and citrate respectively.
These act by nuclei poisoning.
For a given anion, particular cation employed appears to
affect the setting reaction markedly.
ACETATES:
May act by:
 nuclei poisoning.
 reducing the rate of solution of hemihydrate.

47. In formulating dental products, manufactures adjust the rate of
setting or raw hemihydrates by adding accelerators and
retarders, often as a balanced mixture.
Many accelerators and retarders reduce the setting expansion,
in some cases by changing the crystal habit of the growing
gypsum crystals, thereby reducing the effect of growth
pressure. This is accompanied by a reduction in the strength of
the set material.

48. IMPURITES:
 If calcination is not complete and the gypsum particles remain,
or if the manufacturer adds gypsum, the setting time is
shortened because of the increase in the potential nuclei of
crystallization.
 If orthorhombic anhydrite is present the setting reaction is
delayed.
 If hexagonal anhydrite is present, the setting reaction is faster.
FINENESS:
 The finer the particle size, the faster the mix hardens as:
 Rate of hemihydrate dissolution is increased.
 Gypsum nuclei are more numerous.

49. THE MICROSTRUCTURE OF CAST
GYPSUM:
The set material consists of a tangled aggregate of monoclinic gypsum
crystals, usually acicular in shape, with lengths in the range of 5 to
m.
The aggregate exhibits inherent porosity, on a microscopic scale, which
is of two distinct types:
1. Micro-porosity caused by the presence of residual unreacted water.
 Roughly spherical.
 Occur between clumps of gypsum crystals.
2. Micro-porosity resulting from the growth of gypsum crystals.
 Associated with the setting expansion.
 Smaller
 Appear as angular spaces between individual crystals in the
aggregate.

50. Micro-porostity due to presence of unreacted water

51. Micro-porosity caused due to growth of gypsum crystals

52. EFFECT OF W/P RATIO:
The Relative amounts of both types of porosity are affected by
the W/P ratio of the mix, but in two opposite ways:
1.A low W/P ratio leaves less residual water in the set mass andso
decreases the amount of the first type of porosity.
2.A low W/P ratio increases the effect of the crystal growth
during setting, because available nucleation sites are
concentrated in a smaller total volume of mix; interaction of
growing gypsum crystals occur earlier and is more effective so
that the amount of the second type of porosity is increased.

53. In any W/P ratio, the total proportion of inherent porosity
in theset mass is the sum of these two types.
The effect of the first type predominates, so for any given
plaster or stone there is always a decrease in the total
inherent porosity of the set mass (i.e. an increase in apparent
density) as the W/P ratio of the mix is reduced.
Inherent porosity represents about 40% of the total cast
volume at a W/P ratio of 0.5. and about 20% at a W/P ratio
of 0.25. (Lautenschlager and Corbin, 1969).

54. VOIDS:
Areas of air in the mix. Two types:
•Internal Voids: weaken the material.
•External Voids: do not record impression anatomy in that
area.
Ryerson NV(2000), investigated the effect of pressurized
atmosphere on the size and number of voids in dental stones
while setting. Increased atmospheric pressure reduced the
size and number of voids. This method produces improved
cast surfaces and fewer, smaller voids. Several pressure
vessels are available for this purpose.

55. PICTURE SHOWING INTERNALVOID

56. PROPERTIES:
MIXING, WORKING AND SETTING TIMES
SETTING EXPANSION
STRENGTH
SURFACE HARDNESS
ABRASION RESISTANCE
REPRODUCTION OF DETAIL
SOLUBILITY

57. MIXING, WORKING AND SETTING TIMES:
MIXING TIME:
Defined as the time from the addition of the powder to the water
until the mixing is completed.
Mechanical mixing of stones and plaster is usually completed in
20 to 30 seconds. Hand spatulation generally requires at least a
minute to obtain a smooth mix.
WORKING TIME:
The time available to use a workable mix, one that maintains a
uniform constituency to perform one or more tasks.
It is measured from the start of mixing to the point where the
consistency is no longer acceptable for the product’s intended
purpose.
Generally, a 3-minute working time is adequate.

58. INDUCTION TIME:
Time from the beginning of mix till the exothermic heat is felt.
SETTING TIME:
The time that elapses from the beginning of mixing until the
material hardens is known as the setting time.
An arbitrary setting time can be determined by using suitable
penetrometers( e.g. Gillmore or Vicat needles).
LOSS OF GLOSS TEST FOR INITIALSET:
As the material sets, the mix loses its gloss. This occurs as some of
the excess water is taken up in forming the dihydrate. This loss of
the gloss is also sometimes considered as a indication of initial set
of the material.

59. GILLMORE NEEDLES:
 Two types of Gillmore needles.
 The lighter Gillmore needle is constructed from a brass
cylinder, of mass 0.25 lb (113.4 g), attached to a needle with a
flat disk end of diameter 1/12" (2.12 mm).
 The larger needle consists of a 1lb mass (453.6 g) on a
diameter of 1/24" (1.06 mm).
 The corresponding stresses are 0.3 MPa and 5 MPa,
respectively.

60. GILLMORE NEEDLES
USED TO DETERMINE THE
SETTING TIME OF
GYPSUM PRODUCTS

61. GILLMORE TEST FOR INITIALSET:
 The mixture is spread out, and the smaller needle is lowered
onto the surface. The time at which it no longer leaves an
impression is called the initial set.
 This event is marked by a definite increase in strength.
 Acts as a guide to the time when the rigid material is strong
enough to handle and, in particular, when it can be carved or
trimmed to the final shape.
 The setting reaction continue for some time after this initial set.
GILLMORE TEST FOR FINAL SETTINGTIME:
 Measured by the use of the heavier Gillmore needle.
 The elapsed time at which this needle leaves only a barely
perceptible mark on the surface is called the final setting time.

62. VICAT TEST FOR SETTING TIME:
 Used to measure the initial setting time of gypsum products.
 Consists of a rod weighing 300 gm with a needle of 1 mm
diameter.
 A ring container is filled with the mix. The needle with a
weighted plunger rod is supported and held just in contact
with the mix, then is the needle is released and allowed to
penetrate the mix. The time elapsed until the needle no longer
penetrates to the bottom of the mix is known as Vicat setting
time.

63. VICAT
PENETROMETER
USED TO
DETERMINE
THE SETTING
TIME OF
GYPSUM
PRODUCTS

64. In some cases, the Vicat and initial Gillmore measurements
occur at the same time, whereas in other instances, there is
small difference.
If a dental manufacturer specifies a setting time, it will
be a Gillmore or Vicat initial setting time.

65. READY-FOR –USE
CRITERION:
A subjective measure of the time at which the set material may
be safely handled in the usual manner.
Not determined by any designated test but by the ability to
judge readiness improves with experience.
Technically, the set material may be considered ready for use at
the time when the compressive strength is at least 80% of that
which would be attained at 1 hour.
Most modern products reach the ready-for-use state in approx.
30 minutes.

66. MANIPULATION TIME:
Recognition of the physical changes occurring in the mix
during setting is important in the manipulation of plaster and
stone.
1.When casting (e.g. pouring casts or dies), manipulation must
be completed before the mix loses its fluidity. This change is
marked by the disappearance of the glossy surface from the mix.
2.When molding (e.g. taking impressions or jaw registrations,
articulating casts, flasking wax pattern dentures), manipulation
must be completed before the mix loses plasticity and enters
friable stage.There is no recognized objective method of
measuring this time.

67. SETTING EXPANSION:
VOLUME CHANGES DURING SETTING:
Theoretically, calcium sulfate hemihydrate should contract
volumetrically during the setting process.
(CaSO4)2. H2O + 3 H2O 2 CaSO4.2H2O
Molecular 290.284
mass
54.048 344.332
Density 2.75 0.997 2.32
Eq. Volume 105.556 54.211 148.405
T. Volume159.767 148.4.05
The volume of the calcium sulfate dihydrate formed is about 7%
less than the sum of the volumes of calcium sulfate hemihydrate
and water.

68. However, experiments have determined that all gypsum products
expand linearly during setting.
Instead of 7% contraction, about 0.2% to 0.4 % linear
expansion is obtained.
The setting reaction causes a decrease in the true volume of the
reactants and under suitable conditions this contraction can be
observed early in the setting process, when the mix is still fluid.
However, once the mix begins to attain rigidity, marked by the
loss of surface gloss, an isotropic expansion is observed,
resulting from growth pressure of the gypsum crystals that are
forming.

69. The initial contraction is unlikely to affect the important dimensions
of a gypsum cast, because in the still fluid mix it will occur mainly in
the vertical direction. Gravity will keep the mix adapted to the
anatomical portion of an impression.
The expansion that is observed after the mix attains rigidity takes
place in all directions and will affect the dimensions of the cast.
The point at which the initial contraction ceases is used as zero in
the laboratory measurements of effective setting expansion.
The observed expansion that occurs when plaster or stone sets
is a volumetric one.
In dental testing a value is determined for linear setting expansion
and the assumption is made that the expansion is isotropic. This
assumption is not always justified; if restraint is imposed in some
directions and not in others( e.g. by a rigid impression) setting
expansion can be far from uniform.

70. EFFECT OF IMMERSION:
Gypsum products exposed to additional water while
setting ( e.g. by immersion) show a greater expansion than
when setting in air, a phenomenon commonly called
Hygroscopic expansion.
The most well accepted reason for the increased expansion
when the hemihydrate reacts under water is the additional
crystal growth permitted by allowing the crystals to grow
freely, rather than being constrained by the surface tension
when crystals form in air, also known as the Crystal
interlocking theory.

71. It follows, therefore, that the basic mechanism of crystal growth is
the same in both instances, and both phenomena are true setting
expansions.
The hygroscopic setting expansion is physical and is not caused
by a chemical reaction any more than is the normal setting
expansion.
The reduction in W/P ratio increases the hygroscopic setting
expansion and the normal setting expansion in the same manner.
Increased spatulation results in increased hygroscopic setting
expansion as well.
The Hygroscopic setting expansion obtained during the
settingof dental plaster or stone is generally small in
magnitude.
A dental stone may exhibit a normal setting expansion of 0.15%,
with a maximum hygroscopic setting expansion of not more than
0.30%.

72. FACTORS AFFECTING SETTING EXPANSION:
 ADDITIVES
 WATER/POWDER RATIO
ADDITIVES:
Manufactures can reduce the setting expansion and at the same
time control setting time by the addition of a balanced blend of
accelerators and retarders to the raw hemihydrate base plaster.
Typical combinations are:


Potassium sulfate- borax
Potassium sodium tartrate- sodium citrate.
Combinations of accelerators and retarders, in solution, to control
the setting expansion, are known as ANTI-EXPANSION
SOLUTIONS.

73. The additives also reduce the strength of the set material. This is
not a disadvantage in impression plaster, but in stones and die
stones strength as well as dimension accuracy is important;
formulation of the latter materials therefore involves striking a
compromise between a desirable reduction in setting expansion
and an undesirable reduction in strength

74. SODIUM CHLORIDE:
Provides additional sites for crystal formation.
The higher density of crystals limits the growth of crystals and hence
reduces their ability to push each other apart.
This results in decreased setting expansion.
POTASSIUM SULFATE:
Reacts with water and hemihydrate to form ‘ Syngenite’
This compound crystallizes very rapidly and encourages the growth of
more crystals.
Results in decreased setting expansion..

75. STRENGTH:
Brittle material.
Weaker in tension than in compression.
For set plaster, the tensile strength is about 20% of the compressive
strength; for set die stone about 10%.
In practice, fracture of set gypsum typically occurs in tension, tensile
strength is a better guide to fracture resistance.
Compressive strength gives a better indication of surface hardness.
FACTORS AFFECTING STRENGTH:
• WATER/POWDER RATIO
• SPATULATION
• ADDITIVES

76. WATER/POWDER
RATIO:
Strength properties are inversely related to the W/P ratio and to
thetotal amount of inherent porosity.
When maximum strength is required, a given material should be
mixed with as low a W/P ratio as practicable.
The limiting factor is the viscosity of the mix because it
increases with decreasing W/p ratio and can become so high
that the ability to pour casts is prejudiced.
With any plaster or stone, using a low W/P ratio to obtain
maximum strength properties also gives an increased setting
expansion, which must be accepted. But in applications where
dimensional accuracy is more important than strength (e.g.
impressions), higher W/P ratio can beused.

77. EFFECT OF DRYING:
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 the green strength.
 It is the strength obtained when the water in excess of that
required for hydration of hemihydrate is left in the test specimen.
 The dry strength:
 When the excess water has been driven off by drying, the
strength obtained is dry strength.
 The dry strength may be two or more times as high as wet
strength.

78. EFFECT OF
SPATULATION:
With an increase in the mixing time, the strength is increased.
But this increase is only seen upto a hand mixing of 10 minutes.
If the mixture is overmixed, the gypsum crystals formed are broken up,
and less crystalline interlocking results in the final product.
EFFECT OFADDITIVES:
The addition of an accelerator or retarder lowers both the wet and the dry
strengths 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 intercrystalline cohesion.
Prombonas A, Vlissidis D(1994) measured the Compressive strength
and setting temperatures of mixes with various proportions of plaster to
stone. The mix made with 1:1 ratio of stone to plaster, may be the
material of choice for filling the upper half of the flask during complete

79. TENSILE
STRENGTH:
Important in structures in which bending tends to occur because of lateral
force applications, such as removal of cast from impressions.
Because of the brittle nature of gypsum materials, the teeth on the cast
may fracture rather than bend. For brittle materials like gypsum products,
diametral compressive strength test is used to determine the tensile
strength of such products.

80. SURFACE HARDNESS AND ABRASION RESISTANCE:
In general, hardness is defined as the resistance to penetration.
For dental purposes, the surface hardness of a material is
generally measured in terms of its resistance to indentation.
The surface hardness of unmodified gypsum products is related in
a general way to their compressive strength. High compressive
strength of the hardened mass corresponds to high surface
hardness.
After the final setting occurs, the surface hardness remains
practically constant until most excess water is evaporated from
the surface, after which its increase is similar to increase in
compressive strength.
The surface hardness increases at a faster rate than the
compressive strength, because the surface of the hardened mass
reaches a dry state earlier than the inner portion of the mass.

81. Attempts have been made to increase the hardness of gypsum
products by:
• Impregnating the set gypsum with epoxy or methyl
methacrylate monomer that is allowed to polymerize.
According to Craig, increase in hardness was obtained for
model plaster but not for dental stone or high-strength dental
stone.
Generally, impregnating set gypsum with resin increases
its abrasion resistance, but decreases compressive strength
and surface hardness.
• Soaking the gypsum dies or casts in glycerine or different
oils does not improve the surface hardness but rather makes
the surface smoother, so that a wax carver or other
instrument will not cut the stone as it slides over the surface.

82. Mixing high strength dental stone with a commercial hardening
solution containing colloidal silica (about 30%) improves the
surface hardness of set gypsum. The Knoop hardness numbers of
two commercial high-strength dental stone were 54 and 77
kg/mm2. When hardening solution was used, these values
increased to 62 and 79 kg/mm2 respectively.
The abrasion resistance of gypsum product is an important
property in certain dental procedures. For example, if a wax
pattern is to be carved and finished on a stone die, the metal
instrument used to carve the wax may abrade off and destroy
adjacent areas of the stone.
Abrasion is a major concern when gypsum products are used for
dies, leading to the frequent recommendation that surface
hardeners should be used before waxing or scanning.

83. HARDENING SOLUTION:
Composed of:
Colloidal Silica:
30 % Water
Modifiers
May be used in place of water to mix gypsum products.
Amount of solution used is less than if water were used alone
because surface-active modifiers allow the particles to be more
easily wetted.
Affects the hardness and setting expansion of gypsum dies.

84. REPRODUCTION OF
DETAIL:
ADA specification no. 25 requires that:
 Type I and II reproduce a groove 75m in width.
 Type III, IV and V reproduce a groove 50m in width.
Gypsum dies do not reproduce surface details very well
because the surface of the set gypsum is porous at microscopic
level.

85.  Infection control (IC) is an important part of dentistry.
 Potential pathogens can be transmitted via orally soiled
impressions, dental prostheses.
 Transfer of oral microorganisms into and onto
impressions and dental casts has been documented.
Certain microbes have been demonstrated to remain
viable within gypsum cast materials for <7 days.

86. Dental stone containing a
disinfectant may also be used.
Disinfection solutions that do
not adversely affect the quality
of the gypsum product can be
used.

87. Disinfectants commonly used :
 Sodium Hypochloride.
 Iodophors.
 Chlorine Dioxide.

88.  Gypsum products should be
disposed properly. Inhaled
fine particles can cause
respiratory problems.

89. References
Phillips’ Science of Dental Materials 12th Edition
Ryerson NV. Effect of pressurized atmosphere on void size and quantity in dental
stone and fine-grained phosphate-bonded investment. J Dent Technol. 2000 Jul-
Aug;17(6):13-5.