GYPSUM PRODUCTS 1.pptGYPSUM PRODUCTS 1.ppt

GYPSUM PRODUCTS 1.ppt

2. GYPSUM PRODUCTS
PRESENTER:Dr. ARUN RAJENDRAN
GUIDED BY:Dr. VINAYA KUMAR G

3. INTRODUCTION
GYPSUM
CLASSIFICATION
MANUFACTURE
SETTING PROCESS
PROPERTIES
MANIPULATION
COMPATIBILITY
INFECTION CONTROL
RECENT DEVELOPMENTS
CONCLUSION
REFERENCES
CONTENTS

4. INTRODUCTION
Auxiliary dental material.
Glossary of Prosthodontic terms:
Model:Facsimile used for display purposes, a miniature
representation of something.
Dental Cast:A positive life-size reproduction of a part or parts of
the oral cavity.
Die:The positive reproduction of the form of a prepared toothin
any suitable substance.
Direct link between the clinical phase of treatment and the technical
laboratory procedures.
Contributes significantly to the ultimate success of the prosthesis.

5. GYPSUM:
Originates from the Greek word ‘Gypos’, which means Chalk.
Dihydrate of calcium sulfate.
Chemical formula CaSO
4
. 2H
2
O.
Sulfate mineral 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 Statesis the largest producer as well as the biggest consumer
of gypsum. Others are Canada, France, Japan, and Iran.

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

7. 2. ALABASTER:Large, fine-grained, white Stones. Often used for
carving into vases and ornaments. Also used in building of King
Solomon’s Temple.

8. KING SOLOMON’S TEMPLE

9. 3. SELENITE:as transparent crystals.

10. 4. SATIN SPAR: as fibrous crystals.

11. SYNTHETIC GYPSUM:
•Also produced as a Byproduct of manufacture ofPhosphoric acid.
•CHEMICAL GYPSUM
3 H
2
SO
4
(l) + Ca
3
(PO
4
)
2
(s) + 6 H
2
O(l) 2 H
3
PO
4
(s) + 3 CaSO
4
·2H
2
O(s)

12. PROPERTIES:
Found as prismatic, curved or twisted, monocliniccrystals of
vitreous luster.
Specific gravity: 2.3
It cleaves perfectly in one direction.
Moh hardness no: 2, which means that it can be scratched by a
fingernail.

13. 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 fertilizerfor
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 into ornaments andvases.
6.Selenite is sometimes used as an optical material.

14. Gypsum produced for dental application is nearly pure calcium sulfate
dihydrate.
DENTAL USES OF GYPSUM PRODUCTS:
•Impression plaster is used to make theimpressionof the edentulous
mouth.
•For preparation of study modelsof oral and maxillofacial structures.
•To form cast and dies,on which dental prosthesis are constructed.
•As a mold materialfor processing complete dentures.
•For mountingof casts on the articulator.
•Also used as abinderfor silica, gold alloy casting investment, soldering
investment and investment for low melting point nickel-chromium
alloys.

15. 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.
ADA No: 25 ISO No: 6873

16. Gypsum products can be classified into 5 Types:
GYPSUM PRODUCTS
IMPRESSION MODEL DENTAL STONE
PLASTER PLASTER [ISO TYPE-III]
[ISO TYPE-I] [ISO TYPE-II]
DENTAL STONE, HIGH
STRENGTH, LOW EXPANSION
[ISO TYPE-IV]
DENTAL STONE, HIGH
STRENGTH, HIGH EXPANSION
[ISO TYPE-V]

17. CHEMISTRY OF GYPSUM:
3phase changes occur in the CaSO
4
-H
2
Osystem.
110
0
-130
0
C
1) CaSO
4
.2H
2
O CaSO
4
.1/2H
2
O + Water
(Calcium Sulfate
Hemihydrate)
130
0
-200
0
C
2) CaSO
4
.1/2H
2
O g-CaSO
4
+ Water
(Hexagonal Form)
(Soluble anhydrite)
200
0
-1000
0
C
3)g-CaSO
4
CaSO
4
(Orthorhombic CaSO
4
)
(Insoluble anhydrite)
.

18. HEXAGONAL ORTHORHOMBICMONOCLINIC
Calcium Sulfate Hemihydrate
Calcium Sulfate Dihydrate
Soluble anhydrite Insoluble anhydrite

19. MANUFACTURE:
From the conversion temperature given before, it can be seen that
calcium sulfate hemihydrate would be produced, by heating gypsum to
temperatures in the range of 110
o
to 130
o
C. However, at these
temperatures, the reaction is slow; hence, complete conversion takes
about 12 hours (Khalil et. al., 1971). Therefore, in commercial
processes, temperatures higher than this are used for shorter times.
The stable phase at these higher temperatures is Hexagonal Calcium
Sulfate, so the initial product of calcination is largely of this Anhydrous
form. However, on cooling to lower temperatures and exposure to
atmospheric moisture, the Hexagonal Calcium Sulfate rehydrates to form
the Hemihydrate.

20. dehydration by heat
or other means
Mineral Gypsum
formulation
Plasters Dental plaster
Synthetic Gypsum Hydrocal Dental Stone
Densite 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 (SiO
2) are removed, and then
it is subjected to Calcination.

21. PLASTER OF PARIS:
Traditional Hemihydrate Plaster.
Produced by the dry calcinationof ground gypsum in open containers
(pan, kettles or rotary kilns) at temperatures in the range of 110
o
to
130
o
C.

22. DIFFERENT FORMS OF HEMIHYDRATES:
Depending on the method of Calcination, different forms of
Hemihydrate can be obtained.
a–Hemihydrate
a -modified Hemihydrate
b–Hemihydrate
The aand bdesignations 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 aand bHemihydrates are a result of
Differences in the Crystal Size, Surface Area, and Degree of Lattice
Perfection.

23. b-HEMIHYDRATE a-HEMIHYDRATE
Crystal sizeLARGER SMALLER
Shape IRREGULAR PRISMATIC
PackingLOOSELY PACKED CLOSELY PACKED
AMPLE SPACE B/W LITTLE SPACE B/W
CRYSTALS CRYSTALS
W/P ratioMORE LESS
StrengthLESS MORE
SurfaceMORE LESS
area/wt.
ExampleDENTAL PLASTER DENTAL STONE
IMPRESSION PLASTER

24. The a-modified hemihydrateis made by boiling Gypsum in a 30%
aqueous solution of Calcium Chloride and Magnesium Chloride. This
process yields the smoothest, most dense powder particles of the three
types, and the powder is used primarily for Dies.

25. POWDER PARTICLES OF b-HEMIHYDRATE ( X 400.)

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

27. PARTICLES OF a-MODIFIED HEMIHYDRATE ( x 400 )

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

29. 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 asDissolution-Precipitation Theory.
Based on the dissolution of Hemihydrate powder and instant
recrystallization to Dihydrate, 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.

30. The Setting Mechanism may be viewed as follows:
Step 1: INDUCTION
CaSO
4.½H
2O CaSO
4 (aq)
Step 2: CRYSTALLISATION
CaSO
4 (aq) CaSO
4.2H
2O

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

32. 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/Lat 20
o
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 40
o
Cis the Dihydrate,
which is even less soluble than the Hemihydrate [2.4 g/L at 20
o
C].
The aqueous solution is therefore supersaturated with respect to the
Dihydrate, which crystallizes out at suitable Nucleation-sites in the
Suspension.
The Gypsum-crystals normally are Acicular in nature, and often
radiate out from the nucleation centers in the form of Spherulitic-
aggregates.

33. As the Dihydrate precipitates, the solution is no longer saturated with
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.

34. NUCLEI FORMATION
GROWTH OF NUCLEI
SPHERULITE
FORMATION

35. EARLY STAGES IN SETTING OF A DIE STONE (X 400.)

36. The Setting Reaction is the reverse of the first stage of Dehydration, and
so, it is exothermic.
CaSO
4
.
1
/2H
2
O + 1
1
/2H
2
O CaSO
4
.2H
2
O. + 3900cal/gmol.
This chemical reaction takes place, regardless of whether the Gypsum
product is used as Impression material or a Die material.

37. 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 of Water 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.

38. 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

39. Some X-ray Diffraction Studies suggests that Hemihydrate particles
remain in the Set product.
Approx. less than 50% Dihydrate is present in Type-IVand Type-V
Stones.
Approx. 60%in Type-IImaterials.
Approx. more than 90% in Type-IPlasters.
These results demonstrate a higher concentration of Dihydrate in the
weaker set-material, or more Conversion Rate is seen in weaker Set-
material.

40. 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.

41. WATER REQUIREMENTS:
CaSO
4
.
1
/2H
2
O + 1
1
/2H
2
O CaSO
4
.2H
2
O + Heat
1 g. mol 1.5 g. mol 1 g. mol.
145 gm 27 gm 172 gm
100 gm 18.6 gm 118 gm
This 18.6 gm of Water is called as Gauging Water.
In practice, Powder cannot be mixed with such 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.

42. GYPSUM MIXING
WATER
(ml/100g
powder)
REQUIRED
WATER
(ml/100 g
powder)
EXCESS
WATER
(ml/100 g
powder)
MODEL
PLASTER
37 –50 18.6 18 –31
DENTAL
STONE
28 –32 18.6 9 –13
HIGH
STRENGTH
STONE
19 –24 18.6 0 –5
REQUIRED AND EXCESS WATER FOR
GYPSUM MATERIALS

43. Different Water requirements.
Mainly due to the Differences in the Crystal Shape and Surface Area.
In case of b-Hemihydrate Particles, the factors that promote the
Adhesiveness of the particles in the Dry Powder persist, when they are
suspended in Water. For this reason, the b-Hemihydrate Powder
particles produces a Flocculated Suspension, and needs a relatively
high proportion of Mixing Water, to give a Mix of workable Viscosity.

44. RATE OF SETTING REACTION:
Water/Powder Ratio
Spatulation
Temperature
Colloidal system and P
H
Additives:
Accelerators
Retarders

45. W/P RATIO
Within limits, the Rate of Hydration during Setting is independent of
the W/P ratio.
However, the Rate at which the associated Physical Changes
described above 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 Mix (low W/P Ratio) hardens more quickly, because available
Nucleation Sites are concentrated in a smaller volume; interaction of
the growing Solid Phase occurs earlier.
Manipulation and Setting Time are, thus, directly proportional to W/P
Ratio.

46. MATERIAL W/P
RATIO
( ml/g )
SPATULATION
TURNS
INTIAL (VICAT)
SETTING TIME
(min)
MODEL
PLASTER
0.45
100
8
0.50 11
0.55 14
DENTAL
STONE
0.27
100
4
0.30 7
0.33 8
HIGH
STRENGTH
DENTAL
STONE
0.22
100
5
0.24 7
0.26 9
EFFECT OF WATER/PODWER RATIO ON
SETTING TIME

47. 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 placed 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.

48. MATERIALW/P RATIO
( ml/g )
SPATULATION
TURNS
SETTING
TIME
( min )
MODEL
PLASTER
0.50 20 14
0.50 100 11
DENTAL
STONE
0.30 20 10
0.30 100 8

49. TEMPERATURE:
Evidently, the Temperature has 2 main effects on the Setting Reaction
of Gypsum Products:
Change in the Relative Solubilitiesof Calcium Sulfate
Hemihydrate and Calcium Sulfate Dihydrate.
Change in the Ion Mobility.

50. EFFECT ON SOLUBILITIES:
The Ratio of the Solubilities of Calcium Sulfate Dihydrate and
Calcium Sulfate Hemihydrate at 20
o
C is about 4.5
As the Temperature increases, the Solubility Ratio decreases, until
100
o
C 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.

51. SOLUBILITY OF CALCIUM SULFATE
HEMIHYDRATE AND CALCIUM SULFATE
DIHYDRATE AT DIFFERENT TEMPERATURES
Temperature
(
0
C )
CaSO
4.
1
/
2H
2O
( g/100g
water)
CaSO
4.2H
2O
( g/100 g
water)
RATIO
20 0.90 0.200 4.5
25 0.80 0.205 3.9
30 0.72 0.209 3.4
40 0.61 0.210 2.9
50 0.50 0.205 2.4
100 0.17 0.170 1

52. ION MOBILITY:
As the Temperature increases, the Mobility of the Calcium and Sulfate
ions increases, which tends to increase the Rate of Reaction, and shorten
the Setting Time.
NET EFFECT:
Practically, the Effects of these 2 Phenomena are superimposed, and the
Total Effect is observed.

53. THE EFFECT OF TEMPERATURE ON SETTING TIME
OF DENTAL STONE
TEMPERATURE
(
0
C )
SETTING TIME AT
W/P RATIO 0.3
( min )
5 65
25 75
30 60
35 50
40 45
45 40
50 35

54. COLLOIDAL SYSTEMS AND P
H
:
Colloidal Systems, such as Agar and Alginate, retard the Setting of
Gypsum Products.
Retard the reaction by Nuclei Poisoning.
They get adsorbed on the CaSO
4
.2H
2
O Nucleation Sites or on the
CaSO
4
.1/2H
2
O, 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 P
H
, such as Saliva, retard the Setting Reaction.
Liquids with high P
H
accelerate Setting.

55. ADDITIVES:
Used in the formulation on Dental Plasters and Stones for many years,
mainly on an empirical basis, because atleast in some cases, their mode
of action is not completely understood.
ACCELERATORS RETARDERS
Sodium Chloride (<2%) Sodium Chloride (>20%)
Potassium Sulfate Sodium Sulfate
Terra Alba Citrates
Tartarates
Acetates

56. 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, 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.

57. POTASSIUM SULFATE :
Reacts with Water and Hemihydrate, to form
Syngenite:K
2
(CaSO
4
)
2
.H
2
O
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.

58. TERRA ALBA:
Finely-Powdered Gypsum.
In small amounts, it will provide additional Sites for Nucleation,
decreasing the Working Time and Setting Time.

59. 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.
When the limit of Solubility is exceeded, the Salt precipitates on the
Nuclei of Crystallization, thus poisoning them.
BORAX:
Sodium Tetraborate Decahydrate [Na2B4O5(OH)4 .8H2O]
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 Ca2B6O11 .5H2O (‘Colemanite’), which is
very insoluble and delays the Dissolution of the Powder.

60. CITRATES AND TARTARATES:
Reaction of some Additives with Hemihydrate may occur; Soluble
Tartrates and Citrates precipitate Calcium Tartarate and Citrate
respectively.
These act by Nuclei Poisoning.
For a given Anion, particular Cation employed appears to affect the
Setting Reaction markedly.
Ca
2+
< K
+
< H
+
ACETATES:
May act by:
Nuclei Poisoning.
reducing the Rate of Solution of Hemihydrate.
inhibiting the Growth of the Dihydrate Crystals.

61. In formulating Dental Products, Manufactures adjust the Rate of Setting
of 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.

62. 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.

63. 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 5to
20mm.
The Aggregate exhibits Inherent Porosity, on a microscopic scale, which
is of 2 distinct Types:
1.Microporosity caused by the presence of Residual Unreacted Water.
Roughly Spherical.
Occur between Clumps of Gypsum Crystals.
2. Microporosity resulting from the Growth of Gypsum Crystals.
Associated with the Setting Expansion.
Smaller.
Appear as Angular Spaces, between Individual Crystals in the
Aggregate.

64. Microporostity due to presence of Unreacted Water

65. Microporosity caused due to Growth of Gypsum Crystals

66. 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, and so
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.

67. In any W/P Ratio, the total proportion of Inherent Porosity in the Set
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).

68. VOIDS:
Areas of Air in the Mix.
-2 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.

69. PICTURE SHOWING INTERNAL VOID

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

71. TYPE SETTING
TIME
(min)
SETTING
EXPANSION
RANGE
(%)
COMPRESSIVE
STRENGTH
(MPa)
Min. Max.
REPRODUCTION
OF DETAIL
(mm)
Type I2.5-5.00.0-0.154.08.0 75 +8
Type II+20 %0.0-0.309.0- 75 +8
Type III+20 %0.0-0.2020.0- 50 +8
Type IV+20 %0.0-0.1535.0- 50 +8
Type V+20 %0.16-0.3035.0- 50 +8
Property Requirements for Gypsum Products,
according to ADA Specification No. 25:

72. 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 20to
30 seconds. Hand Spatulation generally requires atleast a minute, to
obtain a Smooth Mix.
WORKING TIME:
The Time available to use a Workable Mix, one that maintains a Uniform
Consistency, 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.

73. 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 INITIAL SET:
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.

74. GILLMORE NEEDLES:
2 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 1 lb mass (453.6 g) on a diameter of
1/24"(1.06 mm).
The corresponding Stresses are 0.3 MPa and5 MPa respectively.

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

76. GILLMORE-TEST FOR INITIAL-SET:
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 SETTING-TIME:
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.

77. 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.

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

79. 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.

80. 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 atleast 80% of that which
would be attained at 1 hour.
Most Modern Products reach the ready-for-use state in approx. 30
minutes.

82. 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, beforethe 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, beforethe Mix loses Plasticity and enters Friable Stage.
There is no recognized objective method of measuring this time.

83. SETTING EXPANSION:
VOLUME CHANGES DURING SETTING:
Theoretically, Calcium Sulfate Hemihydrate should contract
volumetrically during the Setting Process.
(CaSO
4
)
2.H
2
O + 3 H
2
O 2 CaSO
4
.2H
2
O
Molecular 290.284 54.048 344.332
mass(g)
Density(g/cm
3
) 2.75 0.997 2.32
Eq. Volume(cm
3
) 105.556 54.211 148.405
T. Volume(cm
3
) 159.767 148.405
The Volume of the Calcium Sulfate Dihydrate formed is about 7% less
than the Sum of the Volumes of Calcium Sulfate Hemihydrate and Water.

84. However, Experiments have determined that All Gypsum Products
expand linearly, during Setting.
Instead of 7% Contraction, about 0.2% to0.4 % Linear Expansion is
obtained.
The Setting Reaction causes a Decrease in the True Volume of the
Reactants. 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.

85. STAGES:
INITIAL MIX
INITIAL CRYSTAL GROWTH
SOLID PHASE CONTACT
EXPANSION
TERMINATION

87. 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.

88. LINEAR SETTING EXPANSION OF GYPSUM
PRODUCTS SETTING IN AIR
TYPE W/P RATIO SETTING
EXPANSION
( % )
IMPRESIONION
PLASTER
0.60 0.15
LABORATORY
PLASTER
0.50 0.30
DENTAL STONE 0.30 0.20
DIE STONE 0.23 0.10
From Wlliam J.O Brien: Dental materials-Properties and selection

89. 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.

91. 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.
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 Setting of
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%.
Nevertheless, this Difference may be sufficient to cause the Misfit of
the Denture or similar device on the Cast.

92. 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 Tartarate -Sodium Citrate.
Combinations of Accelerators and Retarders, in solution, to control the
Setting Expansion, are known as ANTI-EXPANSION SOLUTIONS .
.

93. 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 Dimensional 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.

94. 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..

95. W/P RATIO:
Setting Expansion is inversely proportional to the W/P Ratio.
Reducing the Relative Amount of Aqueous phase in the Mix allows
more effective interaction of growing Gypsum Crystals during Setting,
and so increases the Setting Expansion.
Because of their lower Water Requirement, the Raw Hemihydrates
used to produce Dental Stones and Die Stones, have a higher Inherent
Setting Expansion in Normal Mixes, than does plaster. This effect is
masked, however by the Additives in their formulation.

96. 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 for Fracture Resistance.
Compressive Strength gives a better indication of Surface Hardness.
FACTORS AFFECTING STRENGTH:
•WATER/POWDER RATIO
•SPATULATION
•ADDITIVES

97. WATER/POWDER RATIO:
Strength properties are inversely related to the W/P Ratio and to the total
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
be used.

98. MATERIAL W/P RATIO
( ml/g )
COMPRESSIVE
STRENGTH
( Mpa )
MODEL
PLASTER
0.45 12.5
0.50 11.0
0.55 9.0
DENTAL
STONE
0.27 31.0
0.30 20.5
0.50 10.5
HIGH
STRENGTH
DENTAL
STONE
0.24 38.0
0.30 21.5
0.50 10.5

101. EFFECT OF DRYING:
The Free-Water Content of the Set Product definitely affects its Strength.
For this reason, 2 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.

102. STRENGTH PROPERTIES OF GYPSUM PRODUCTS
TYPE W/P
RATIO
TENSILE
STRENGTH
(Mpa)
Wet Dry
COMPRESSIVE
STRENGTH
(Mpa)
Wet Dry
Impression
Plaster
0.601.3 - 5.9 -
Dental
Plaster
0.502.34.112.4 24.9
Dental
Stone
0.303.57.625.5 63.5
Die Stone0.234.39.940.7 80.7
From William J.O Brien: Dental Materials-Properties and Selection 5
th
ed.

103. EFFECT OF SPATULATION :
With an Increase in the Mixing Time, the Strength is increased.
But, this Increase is only seen uptoa 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 OF ADDITIVES:
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 IntercrystallineCohesion.

104. 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.

105. 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.

106. 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.

107. •Mixing High-Strength Dental Stone with a Commercial
HardeningSolution 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/mm
2
. When Hardening Solution was used, these values
increased to 62 and79 kg/mm
2
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.

108. 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 lessthan 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.

109. REPRODUCTION OF DETAIL:
ADA Specification No. 25requires that:
TypeI andII reproduce a Groove of 75mmin Width.
Type III, IV andV reproduce a Groove of 50mmin Width.
Gypsum Dies do not reproduce Surface Details very well, because the
Surface of the Set Gypsum is Porous at microscopic level.

110. INFECTION CONTROL:
-There is always a risk of Cross Contamination to a Dental Office
Personnel by Microorganisms, including Hepatitis-B & HIV, via Dental
Impressions.
-Thus, the Impressions should be disinfected using Spray and Immersion
Disinfecting Techniques.
-If the Impression is not disinfected, it is prudent to disinfect Stone Cast.
-Usual Disinfectants for Stone Cast include Spray Disinfectants,
Hypochlorites, and Iodophores.
-Disinfecting Solutions that are used should not adversely affect the
Quality of the Gypsum Cast.
-Alternatively, Dental Stone containing Disinfectant may be employed.
-When Patients with known cases of Infection are being treated, Overnight
Ethylene Oxide Gas Sterilization is an option.

112. REFERENCES
1). Phillips’ Science of Dental Materials, 12
th
Edition.
2). Craig’s Dental Materials –Properties and Manipulation, 11
th
Edition.
3). Marcia Gladwin’s Clinical Aspects of Dental Materials –Theory,
Practice, & Cases.
4). McCabe’s Applied Dental Materials.
5). E.C Combe’s Notes on Dental Materials.
6). William J.O Brien’s Dental materials -Properties and Selection.