This document provides an overview of gypsum products used in dentistry. It discusses the properties and manufacturing of various types of gypsum, including plaster, dental stone, die stone. It describes how gypsum sets through a hydration reaction and how factors like water-powder ratio, temperature, and additives can control the setting time. The document also outlines the uses of different gypsum products for impressions, models, dies and other dental applications.

1. GYPSUM PRODUCTS
Presented by: Rukhsar Showkat
MDS, Ist Year

2. Contents
 Introduction to Gypsum
 Uses of Gypsum Products
 Manufacturing methods
 Types of Gypsum Products
 Setting reaction
 Mixing time, Working time and
Setting time
 Modifiers: Retarders and accelerators
 Strength of Gypsum products
 Manipulation
 Care of the cast
 Infection control
 Specialized Gypsum Products
 Conclusion

3. Gypsum
• Discovered by Egyptians who used it in Egyptian Pyramids
• Egyptians called it “Gypsos” meaning Chalk
• Because water returns gypsum to its original rock form it has earned its
reputation as the “wonder mineral”
• It was introduced in dentistry by Phillips Pfaff in 1756
• Gypsum is a mineral mined from earth that, like limestone, occurs as a
rock naturally in many parts of the world and is white in its absolutely
pure form.

4. Gypsum: Natural forms
ALBASTER SATIN SPAR SELENITE
Pure white, fine
grained &
translucent
Fibrous needle
like with silky
luster.
Colorless,
Crystalline,
transparent.

5. • Usual presence of impurities produces rocks in varying shades of gray,
brown and even black.
• It is the dihydrate form of calcium sulfate (CaSO4.2H2O), usually appears
white to milky yellowish and is found in compact mass in nature.
• Mining Cleaning of gypsum Grinding into fine powder
Calcination
• It is the 2nd Softest mineral on Moh’s hardness scale.

6. Cast, Die and Model.
 Cast: It is positive ,dimensionally accurate replica of oral soft and hard tissues of either
the maxillary or mandibular jaw and used for the construction of dental appliances
which fit on to the soft tissues of the oral cavity.
 Die: It is a positive , dimensionally accurate replica of a prepared natural tooth and used
for the construction of inlay , crown or bridges.
 Model: It is a positive, dimensionally accurate replica of oral soft and hard tissues used as a
study model or for patient education .The progress of treatment can be shown to the
patient with these models.

7. Gypsum Products: Uses
• Impression material: Impressions of mouth and face
• For making moulds, diagnostic cast,
master cast, and dies over which dental
prostheses and restorations are made.

8. To mount cast on
articulator
Dental Investments
Stone plus Silica.
Withstands high heat
and stress when
molten metal is forced
into moulds to form
indirect restorations

9. Gypsum products: Manufacturing methods
Calcination: The process of heating gypsum to manufacture plaster and stone
products is known as calcination.
i) Dry Calcination iii) Dehydration by boiling with chemicals
ii) wet Calcination iv) Synthetic method
CaSO4.2H2O CaSO4. ½ H2O CaSO4 CaSO4
- H20 (Water of crystallization)
Calcium Sulphate Calcium Sulpate Hexagonal Orthorhombic
Dihydrate hemihydrate anhydrite anhydrite
(Gypsum) (Plaster/Stone)
110-130 oC 130-200 oC 200-1000 oC

10. Dry Calcination:
 Commercially gypsum is finely ground and subjected to high
temperatures of 110-130oC in open kettle or kiln to eliminate the
water of crystallization
 Beta hemihydrates (Type I & II Gypsum products) are made with this
method
Wet Calcination:
 Gypsum is ground and steam heated to a temperature of 110-130 oC
in a closed kiln, kettle or autoclave
 Alpha hemihydrates (Type III) are made through this method

11. Dehydration by boiling chemicals:
 Gypsum is calcined by boiling with 30% CaCl2 solution . The chlorides
are then washed away in the presence of 0.5% Sodium Succinate.
 α-modified hemihydrate (Type IV & V) Improved stone is
manufactured by this method.
Synthetic Gypsum Products:
 alpha & beta Hemihydrate can also be formed from the by-products
or waste products of the manufacture of Phosphoric Acid.
 More expensive than natural gypsum products but when product is
properly made its properties are equal or exceed to natural gypsum
products.

12. Types of Hemihydrates based on method of Calcination
β form α form α-modified form
Large, irregular shaped
orthorhombic crystals
with capillary pores.
Type I & II Gypsum
- Dry Calcination
Small, regularly shaped,
crystalline rod or
prismatic particles
Type III Gypsum
- Wet Calcination
Smoothest, Cuboidal and
most dense powder particles
Type IV & V Gypsum
- Dehydration by Boiling
chemicals

13. Types of Gypsum Products
According to ADA Specification no. 25 there are 5 types of Gypsum Products
 Type 1- Plaster impression
 Type 2- Plaster model
 Type 3- Dental stone
 Type 4- Dental stone high strength
 Type 5- Dental stone High strength high expansion
(Identical chemical formulae- CaSo4. 1/2H2O)

14. Type
W/P Setting time Porosity
Compressive
strength (1 hr)
Abrasion
resistance
% Setting
Expansion
(2Hr.)
Impression Plaster 0.40-0.70 4 +/- 1 High 4.0 MPa Low 0.15
Model Plaster 0.45- 0.50 12 +/- 4 High 9.0 MPa Low
0.06-0.30
Dental Stone 0.28-0.30 12 +/- 4 moderate 20.6 MPa Moderate 0.20
High strength
Low-expansion stone
0.22-0.24 12 +/- 4 Low 34.3 MPa High 0.15
High strength
High- expansion stone
0.18-0.22 12 +/- 4 Low 48.0 MPa High
0.15-
0.30

15. • Plaster of Paris is called so because it was obtained by burning gypsum from
deposits near Paris.
• Seldom used as replaced by less rigid materials like hydrocolloids and elastomers
Type I: Impression Plaster
Type II: Model Plaster
 Used for
 Diagnostic cast
 Articulation of stone cast
 Art portion of working cast
 Flasking procedure for acrylic dentures (cast end)
The mix produces a weak cast compared to dental stone.
It’s available is fast and regular sets.

16. Also known as Class I stone or Hydrocal. Used for
 Full or partial denture models
 Orthodontic models
 Flasking procedure for acrylic dentures (teeth
end)
 It requires less water and is stronger than
plaster
Type III: Dental Stone
Dental stone
casts
Type IV: Dental Stone high strength
Also called as Class II Stone or densite or improved stone.
About 2 times stronger than type II. Uses,
 Used when high strength and surface hardness is required.
 CAD /CAM dies
 Dies for fabricating inlay, crown and bridge wax patterns.
Die stone cast
Dental stone

17.  Has higher compressive strength than type IV which
is attained by lower W/P ratio.
 Setting expansion has been increased from a
maximum of 0.10% to 0.30% to compensate for the
solidification shrinkage of some alloys used as base
metals used for dental casting.
 Most costly of all gypsum materials.
 Should be avoided in production of dies for inlays as
higher expansion may lead to tight fits.
Type V: Dental Stone high strength high Expansion

18. Setting Reaction
• When plaster is mixed with water it takes up 1 ½ molecule of water i.e. it
regains water of crystallization and becomes calcium sulphate dihydrate.
• The reaction is exothermic
• The reaction is continuous and repeated until hemihydrate is converted
to dihydrate
2CaSO4. ½ H2O + 3H2O 2(CaSO4. 2H2O) + Heat (3900 Cal)

19. • Initially there is little reaction and thus little or
no rise in temperature. This period is called
“Induction Period”
• Later there is a thickening of mix which allows
it to be poured- “semi-fluid consistency”
• As the amount of gypsum formed increases,
the mass thickens and hardens into needle like
clusters called “Spherulites”
• Finally intermeshing of crystals of gypsum
leads to a strong solid structure

20. Theories of Setting Reaction of Gypsum Products
 Colloidal/Gel theory
 Hydration theory
 Dissolution-Precipitation theory (Crystalline theory)
Colloidal/Gel theory:
Hemihydrate
Hemihydrate
Sol
Dihydrate
Sol
Dihydrate
Gel
+ H2O + H2O + H2O

21. Hydration theory:
Hemihydrate
Rehydrated
Particles
Sulphate
Group
Set
Dihydrate
+ H2O
Dissolution-Precipitation Theory (Crystalline Theory):
 Most widely accepted theory propounded by Henry Louis.
 Based on dissolution of plaster and instant crystallization of gypsum followed by
interlocking of the crystals to form set solid.
 Difference in Solubility of Gypsum and Gypsum products
Solubility
CaSO4. 2H2O 0.2
CaSO4. ½ H2O 0.9

22. Stage I
• Hemihydrate dissolves
• Suspension of Hemihydrate
Stage II
• Saturated Solution of Hemihydrate
• Formation of Dihydrate
Stage III
• Supersaturated solution of Dihydrate
• Dihydrate Crystal Precipitate
Stages when Hemihydrate is mixed with water Stages of
Crystallization
 Dissolution
 Suspension
 Saturation
 Super saturation
 Nuclear formation
 Growth of nuclei
 Crystallization
 Spherulite formation

23. Water-Powder Ratio and effect of low and high
water powder ratio on Gypsum Products
Low W/P Ratio (Thick Mix)
• Quick setting
• High Expansion
• More Strength
• More surface Hardness
• Less porosity
High W/P Ratio (Thin Mix)
• Slow setting
• Low expansion
• Less strength
• Less surface hardness
• More Porosity
Proportion of water and powder to make a workable mix. Water
required per 100 gram powder.

24. Effect of Temperature on Setting Reaction
Change in temperature produces two effects
1) Change in Temperature causes change in solubility of the hemihydrate and
dihydrate, which alters the rate of reaction.
 As temperature increases Solubility ratio of hemihydrate to dihydrate
decreases and vice versa
Solubility ratio Setting reaction & Setting time
2) Change in ion mobility:
Increase in
temperature
Increase in
mobility of
Ca & SO4 Ions
Increase in
rate of
reaction
Decrease in
setting time

25. Effect of Temperature on
Solubility
 As expected increase in water temperature
increases rate of reaction but this is not the case
with Gypsum products
 Effect of temperature on setting reaction varies
from stone to stone
 Little change occurs between 0 oC to 50 oC
 If temperature of water exceeds 50 oC, there is
gradual retardation of the reaction
 If the temperature approaches 100 oC, no
reaction takes place
 Between 50 to 100 oC, reaction will get reversed.

26. Mixing Time:
The time from addition of powder to the water until the mixing is completed.
Mechanical mixing : 20 to 30 seconds
Hand-Spatulation: 1 minute
Working Time:
The time available to use workable mix. It is measured from start of
mixing to the point where the consistency is no longer acceptable for the
product’s intended purpose. Generally 3 minutes is sufficient.
Setting Time:
Time that elapses from the beginning of mixing until the material hardens.

27. Initial Setting Time:
Time required for Gypsum products to reach a certain arbitrary stage of firmness
which is represented by semi-hard mass that is no more workable, but is not
completely set.
Final setting time:
Time required for the reaction to be completed. It is the time at which the material
can be separated from impression without any distortion or fracture.
Setting time is measured by various penetration tests.

28. Loss of gloss test for initial set
Some of the excess water is taken up
in forming the dihydrate so that the
mix loses its gloss. It takes 12 +/- 4
minutes approximately.
Initial Gilmore test for initial set
 The smaller one amongst the Gilmore
needles is used.
 Mixture is spread out, needle is
lowered onto the surface.
 Time at which it no longer leaves an
impression is called initial set.
Gillmore
needles
Loss of gloss

29. Vicat Test for setting time
 Instrument used: Vicat
Penetrometer
 The needle is held just in
contact with the mix. Soon
after the gloss is lost, the
plunger is released.
 Time elapsed from start of
mixing till the needle no
longer penetrates to bottom
of mix is k/a setting time.
 Only Vicat setting time is
listed under tables of
physical properties
Gilmore test for final setting time
 The heavier one amongst the Gilmore
needles is used.
 Time elapsed at which this needle
leaves only a barely perceptible mark
on the surface is k/a final setting time.
Vicat
Penetrometer
Gillmore
needles
300gm
1mm
1 lb, 1.06mm¼ lb, 2.1mm

30. Ready for Use Criterion
 It is a subjective measure of the time at which the
set material may be safely handled in the usual
manner.
 Technically material may be considered ready for
use at the time when compressive strength is at
least 80% of that which would be attained at 1 hr.
 Most products reach ready-for-use state in 30
minutes.
 Clinical tips:
 Before separating cast from impression, ensure that no
part of the tray is connected to the gypsum
 If alginate impression dried before cast separation,
soak in water for 15 minutes.

31. How to Control Setting time?
Theoretically there are 3 methods
 Solubility of hemihydrates can be increased or decreased
Solubility Setting reaction & Setting time
 Number of nuclei of crystallization can be increased or decreased
Nuclei of crystallization Gypsum crystal formation & Setting time
 By increasing or decreasing the rate of crystal growth, setting time can be
accelerated or retarded respectively
Impurities
By adding gypsum, setting time is shortened as it increases potential nuclei
of crystallization

32. Fineness
Finer the particle size of hemihydrate, faster the mix hardens. Fineness
increases both dissolution as well as number of nuclei of crystallization.
W/P Ratio
More the water used for mixing Fewer nuclei available per unit volume
Consequently setting time is prolonged.
Mixing
Longer and rapid mixing Shorter the setting time
As mixing begins, formation of crystals increases. At the same time crystals are
broken up by mixing spatula and are distributed throughout mixture resulting in
formation of more nuclei of crystallization.

33. Modifiers: Retarders and Accelerators
• Retarder
• Chemical added to increase setting time
• Organic materials: Glue, Gelatin and
gums. Act by forming adsorbed layer
on hemihydrate to reduce its solubility
and inhibit growth of gypsum crystals
• Salts: Borax, Potassium Citrate, NaCl
(20%). Form a layer of calcium salt that
is less soluble than sulphate
• Accelerator
• Chemical added to decrease setting time
• Materials like Potassium sulfate (>2%)
or NaCl (2%), Sodium sulfate ( 3-
4%), slurry water (gypsum <20%),
tera alba act as sites for crystallization.
Note: Blood, saliva, alginate act as retarders and If
left on impression, can affect surface details of
impression. Impression surface need to be
properly rinsed before being poured.
Balanced Stone: A stone with a setting
time established by addition of proper
quantities of both accelerators and retarders

34. Setting Expansion
• Occurs due to outward thrusting of growing crystals. It is observed based on the
mechanism of crystallization i.e., the process of outgrowth of crystals from the
nuclei of crystallization.
• It may range from 0.06% to 0.5%
• It is of two types
Normal setting expansion: refers to the setting expansion when a gypsum
product is allowed to expand in air (when placed on table or dry environment)
Hygroscopic setting expansion: refers to the expansion of a gypsum product
when it is allowed to set immersed in water.

35. Difference and similarities between normal and hygroscopic setting expansion:

36.  Increased expansion (more than twice) occurs in hygroscopic setting because of
additional crystal growth permitted by allowing crystals to grow freely rather than
being constrained by the surface tension when the crystals grow in air.
 There are five stages in the process of expansion explained below
 Stage I: Represented by 3 round particles of hemihydrate surrounded by water
which constitutes the initial mix
 Stage II: Crystals of dihydrate begin to form on the nuclei and water around the
particles is reduced by hydration and particles are drawn more closely together
because of surface tension.
In the hygroscopic setting, water of hydration is replaced and distance between the
particles remains the same.

37.  Stage III: As crystals grow, they contact each other and setting expansion begins.
Water around the particles decreases in normal expansion. The particles with their
attached crystals are drawn together but contraction is opposed by the outward
thrust of the growing crystals. However the crystals in hygroscopic setting are not
inhibited because of replenishment of water.
 Stage IV & V: More marked effect. Intermeshed and entangled crystals prevent
expansion in normal setting.
 Both types of expansion are physical in nature and do not involve chemical
reaction.

38. Control of setting expansion
 W/P Ratio:
 At higher W/P ratios, fewer nuclei of crystallization per unit volume are
present compared to low W/P ratio.
 Inter nuclear spaces will be more in case of higher W/P ratio and there is
lesser growth interaction of dihydrate crystals resulting into lesser outward
thrust.
 Most effective method of controlling setting expansion is by adding chemicals.
 Increased spatulation increases setting expansion
 Setting expansion can be reduced by adding Potassium Sulfate, Borax or
Sodium Chloride

39. Strength of Gypsum Products
 Strength of set Gypsum Products is expressed in terms of compressive strength,
although Diametral tensile strength can also be considered.
 Wet Strength or Green Strength is determined when water in excess of that
required for hydration of hemihydrate remains in the test specimen. When such
excess water is removed by drying, the strength obtained is dry strength.
.
a. Effect of water content:
Dry strength is 2 or more times the wet strength. Up to 8 hrs about 17.4% weight is lost.
Between 8-24 hours of drying only 0.6% weight is lost but strength almost doubles because
fine gypsum crystals that precipitate act as anchor between large crystals

40. b. Effect of W/P Ratio:
 As W/P ratio increases, compressive strength
decreases
c. Effect of Manipulation and additives:
 An increase in Spatulation increases strength
proportionally up to the limit of 1 minute equivalent
to hand mixing. Beyond this overmixing results in
break of crystals and decrease in strength.
 Additives decrease the strength as they act as
impurities reduce intercrystalline cohesion.

41. Manipulation of Gypsum Products
 Selection: based on the desired properties and dental application. e.g.:
 For a diagnostic cast plaster can be used.
 For a working cast, strength and accuracy is required, dental stone is the gypsum product of
choice
 Working models for cast restorations require die stone.
 For casting procedures dental investment is required.

42. Proportioning
Golden rule: Manufacturer instructions should always be followed. Variations in W:P
ratio affect the set materials’ properties such as strength and accuracy.
 The strength of a stone is inversely proportional to the w/p ratio.
 It is better to keep the amount of water as low as possible.
 Once the optimum proportion is noted , the same must be used subsequently
 An accurate graduated cylinder has to be used for water and a proper weighing
balance for powder has to be used
 Powder should NOT BE MEASURED USING SCOOP as it doesn’t pack
uniformly.

43. Proportioning
Golden rule: Manufacturer instructions should always be followed. Variations in W:P
ratio affect the set materials’ properties such as strength and accuracy.
 The strength of a stone is inversely proportional to the w/p ratio.
 It is better to keep the amount of water as low as possible.
 Once the optimum proportion is noted , the same must be used subsequently
 An accurate graduated cylinder has to be used for water and a proper weighing
balance for powder has to be used
 Powder should NOT BE MEASURED USING SCOOP as it doesn’t pack
uniformly.

44. MIXING
Manual: Rubber bowl and spatula.
 Powder is sifted into water to ensure good wetting and avoid
clumps and air bubbles
 If the mixing has to be done by hand , the bowl must be
parabolic , smooth and abrasion resistant
 Spatula should have a stiff blade and a convenient handle
 Stirring of the water powder during mixing has to be done more
vigorously in order to avoid air entrapment
 Wetting of all powder particles with water must be ensured
Improper Mix
Proper Mix

45. Pouring of the cast
The cast is composed of two parts which are prepared separately
 The anatomical part (hard and soft tissue), impression poured using a
vibrator
 Art portion or base, which is important to aid in handling and articulating
the casts. Can be poured in different ways:

46. Double Pour Method
Single Step Method
Both anatomical and art portions of the cast are prepared at the same time.
This method requires skill and accurate timing. Difficulty encountered:
 If mix is too runny?
 If mix started to initially set?

47. Boxing Method
A strip of wax is fitted around the impression then gypsum is poured. The wax
border should extend at least 0.5 inch above the highest point of the impression

48. Care of the cast
 If the cast surface is not hard and smooth when removed from the mould , then
its accuracy is questionable
 Cast once set will maintain the dimensions relatively stable with changes in
humidity and room temp.
 But in few cases the cast has to be immersed in water , negligible expansion may
occur if the water is saturated with calcium sulphate. If not saturated, gypsum
will dissolve.
 The safest method of soaking the cast in water is to immerse the cast in water
bath that contain plaster debris at the bottom of the container.

49. Infection Control
 There are chances of cross infection of HBV, HIV etc on clinicians due to
impression which has lead to the idea of disinfection sprays and immersion
techniques to disinfect the impressions and its effect on the surface of the cast.
 If the impression is not disinfected then the cast should be disinfected using
disinfectant solutions . Alternatively dental stone containing disinfectant can be
used.
 Commonly used stone disinfectants are spray disinfectants, hypochlorites and
iodophores and glutaraldehyde.
 Microwave irradiation has been test both for drying and disinfection.

50. Specialized Gypsum Products
• Dental Casting Investments:
Silica + dental plaster/stone.
Capable of withstanding high temperatures.
Used to prepare refractory molds for casting.
• Divestment:
 Die-stone + Gypsum bonded investment mixed with colloidal silica
 Used to make refractory dies

51. • Orthodontic Stone
 Orthodontists prefer to use white stone or plaster
 Have longer working time for pouring multiple models
 Treated with “model glow”- model soap to produce glossy surface
• Resin modified stones
 They are resin fortified die-stones. Synthetic resin + alpha Gypsum
 Less brittle, improved surface smoothness, increased resistance to abrasion
 Compressive strength as high as 79MPa

52. • Mounting Plasters
 Used for attaching casts to the articulators
 Low setting expansion of 0 to .05(important for accuracy of mounting),
low strength of 12 MPa (allows easy separation from the cast) and fast
setting time (3 minutes)
• Fast setting stone
 Exceptionally fast setting stones (2 minutes)
 Early high compressive strength (41 MPa at 1 Hr.), which allows separation
of cast from impression in 5 minutes

53. Conclusion
A wide choice of Gypsum Products is available so we should choose
the one or a combination which is most suitable for our desired
requirements.
If we self manipulate gypsum to improve one feature, others may be
sacrificed. So we must follow the manufacturer instructions for better
results and avoid ad-hoc approaches at dealing with Gypsum products.