Gypsum Products used in Dentistry

2. Good Morning. . .

3. Presented By:
Dr.VAIBHAV BUDAKOTI
PG 1st yr
Dept. of Prosthodontics,
Crown , Bridge &
Implantology
GYPSUM
PRODUCTS

4. CONTENTS
• Introduction
• Production of Gypsum Products
• Types of Gypsum Products
• Setting of Gypsum Products
• Setting Expansion
• Strength of Set Gypsum Products
• Manipulation of Gypsum Products
• Infection Control
• Specialized Gypsum Products

5. INTRODUCTION
• Gypsum (CaSO4•2H2O; calcium sulfate
dihydrate) is a mineral mined in various parts
of the world
• ADA Specification no. 25
• ISO Specification no. 6873:1998

6. TYPES
 Albaster- pure white,
fine grained and
translucent
Satin spar - fibrous
needle like with silky
lustre.
Selenite - colourless,
crystalline and
transparent.

7. USES
• Building construction
• Soil conditioning
• Food additives
• Pharmaceuticals
• Medical devices

8. King Solomon’s Temple

9. APPLICATION IN DENTISTRY
Phillips Pfaff introduced
gypsum to dentistry.
• For cast preparation.
• Models and dies.
• Impression Material
• As Investment Material
• Mounting of Casts
• As a mould material for
processing of complete
dentures

10. PRODUCTION OF GYPSUM PRODUCTS
Calcination: The process of heating gypsum to
manufacture plaster and stone products is
known as calcination.
i) Dry Calcination
ii) wet Calcination
iii) Dehydration by boiling with chemicals
iv) Synthetic method

11. Dry Calcination:
• Commercially gypsum is finely ground and
subjected to high temperatures of 110-130 °C
in open kettle or kiln to eliminate the water of
crystallization
• Beta hemihydrates (Type I & II Gypsum
products) are made with this method

12. Wet Calcination:
• Gypsum is ground and steam heated to a
temperature of 110-130 °C in a closed kiln,
kettle or autoclave
• Alpha hemihydrates (Type III) are made
through this method

13. 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.

14. 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
• when product is properly made its properties
are equal or exceed to natural gypsum
products.

17. 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

19. Type I: Impression Plaster
• 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.
Composition:
• Dental plaster
• K2SO4(Potassium sulphate)
• Borax
• Colouring and flavouring agent

20. Type II: Model Plaster/
Laboratory Plaster
• Used for
Diagnostic cast
Articulation of stone cast
Art portion of working cast
Flasking procedure for acrylic
dentures
Composition
Βeta hemihydrate + Modifiers

21. Type III: Dental Stone
• Also known as Class I stone or
Hydrocal. Used for
• complete or partial denture models
• Orthodontic models
• Flasking procedure for acrylic
dentures
• It requires less water and is stronger
than plaster
• Composition:
Alpha hemihydrate + 2 to 3%
coloring agent+ K2SO4 (accelerator) +
borax (retarder)

22. Type IV: Dental Stone high strength
Also called as Class II Stone
or die 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.

23. • Type V: Dental Stone high strength high
Expansion
• E.g: Hard rock, Jade rock, Resinrock XL5,
Denflo HX
• 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.

24. SETTING OF GYPSUM PRODUCTS
• The reaction between gypsum products and water
produces solid gypsum, and the heat evolved in
the exothermic reaction is equivalent to the heat
used originally for calcination.
• Set gypsum products probably never attain 100%
conversion unless they are exposed to high
humidity for a long time. Therefore, there are
unreacted hemihydrates remaining in the set
materials.

25. SETTING REACTIONS
There are three theories of gypsum setting.
Colloidal Theory: Mahaelis in 1893
The gypsum product when mixed with water
enters into a colloidal stage through a sol-gel
mechanism.
When in sol stage, the hemihydrate undergoes
hydration to form active dihydrate, which then
undergoes gelation to form a solid set mass.

26. • Hydration Theory:
• Suggest that the hemihydrate undergoes
hydration.
• These rehydrated crystals are joined together
through hydrogen bond to the sulfate groups
forming a set solid mass.

27. • Dissolution-precipitation theory:
• French chemist Henry Louis Le Chatelier, 1887
• Widely accepted theory, based on the principle
of gradual dissolution of hemihydrate and
instant crystallization of dihydrate with inter
locking of crystals forming a set mass.

28. Stages of Crystallization
• Dissolution
• Suspension
• Saturation
• Super saturation
• Nuclear formation
• Growth of nuclei
• Crystallization
• Spherulite formation

29. Physical stages of gypsum
• Fluid
• Plastic
• Firable
• Carvable

30. Water-Powder Ratio and effect of low and high
water powder ratio on Gypsum Products
• Proportion of water and powder to make a
workable mix. Water required per 100 gram
powder.

31. 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:

32. 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°C to
50°C
• If temperature of water exceeds 50°C,
there is gradual retardation of the
reaction
• If the temperature approaches 100°C, no
reaction takes place
• Between 50 to 100 °C, reaction will get
reversed.

33. 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

34. 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.

35. Test for Initial set
• 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

36. • 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.

37. 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.

38. 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.

39. Weight And Diameter Of Vicat And
Gillmore Needle
• Vicat needle : Weight - 300gm
• Diameter – 1mm
• Gillmore needle :
I)Small : Weight - ¼ lb
Diameter - 1/12’’(2.12mm)
II)Large : Weight - 1lb
Diameter – 1/24’’(1.06mm)

40. 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.

41. 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.

42. 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.

43. 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.

44. Modifiers: Retarders and Accelerators
RETARDER ACCELERATOR
Chemical added to increase setting time Chemical added to decrease 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
Materials like Potassium sulfate (>2%)
or NaCl (2%), Sodium sulfate ( 3-
4%), slurry water (gypsum <20%),
tera alba act as sites for crystallization
Salts: Borax, Potassium Citrate, NaCl
(20%). Form a layer of calcium salt
that
is less soluble than sulphate
Balanced Stone: A stone with a setting
time established by addition of proper
quantities of both accelerators and
retarders
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.

45. Tera-alba
• It is finely powdered gypsum /white earth like
substances used to control setting time of
plaster of Paris. It acts by providing additional
nuclei of crystallization.

46. 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.

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

49. • 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.

50. • 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.

51. Properties of Gypsum Products
%Setting
expansion at
hrs
Comp str.(1hr)
(MPa)
Hardness (dry)
(RHN)
( μm) Detail
reproduction
Type 1 0.15 max 4 min
8max
75±8
Type2 0.05 9 75±8
Type2 0.06min,
0.30 max
9 75±8
Type3 0.20 max 20 82 50±8
Type4 0.15 max 35 92 50±8
Type5 0.16min
0.30max
35 50±8

52. 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

53. 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

54.  Effect of W/P Ratio:
• As W/P ratio increases,
compressive strength decreases
 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.

55. 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.

56. 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.

57. 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

58. 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:

59. • 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?

60. 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

61. 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.

62. 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 tested both for drying
and disinfection.

63. 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

64. 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

65. 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

66. Soluble plaster
• When impression plaster contain potato starch
to make the impression soluble it is termed as
‘soluble plaster’.
• MODE OF ACTION:

67. 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.

68. References
• Philips’ Science of Dental Materials, 11th ed.
• S. Mahalaxmi Materials Used in Dentistry, 1st
ed.
• John J Manappallil Basic Dental Materials, 4th
ed.

69. Thank You. . .