Gypsum Products commercially available are hemihydrate form of calcium sulfate. Gypsum products used in Dentistry are broadly classified into 5 types Type 1- Impression Plaster Type 2- Model Plaster Type 3- Dental stone Type 4- Die Stone high strength low expansion Type 5- Die Stone high Strength High Expansion

1. BY- Dr. Prathamesh Fulsundar

2.  Introduction
 Production of Calcium Sulfate Hemihydrate
 Types of Gypsum Products
 Uses of different types of gypsum products
 Setting reaction
 Manipulation
 Physical Properties
 Infection Control
 References

3.  Gypsum- A mineral mined from the Earth. Occurs most
widely in a massive form known as rock-gypsum.
 It is the dihydrate form of calcium sulfate, usually appears
white to milky yellowish and is found as compact mass in
nature.
 It has commercial importance as a source of Plaster of
Paris and dental stone.

5.  Plaster and stone products are produced by
calcining calcium sulfate dihydrate or gypsum.
 CaSO4. 2H2O 1100-1300 C CaSO4. 1/2H2O
gypsum
(calcium sulfate dihydrate)
plaster or stone
( calcium sulfate hemihydrate)

6. Based upon the method of calcination
different forms of hemihydrate can be
obtained.
 Alpha –Hemihydrate (Stone)
 Beta - Hemihydrate (Plaster)
 Alpha modified Hemihydrate

7.  Gypsum is ground & heated at 110 to 130*C to obtain
Beta hemihydrate the process is called as dry
calcination.
 CaSO4. 2H2O 1100-1300 C CaSO4. 1/2H2O
gypsum
(calcium sulfate dihydrate)
Dental plaster
( Beta hemihydrate)

8.  Crystals are spongy & irregular in shape

9.  Gypsum is heated under steam pressure at 120 to 130*C
at 17lbs/sq.inch pressure for 5 to 7 hours to obtain
alpha hemihydrate the process is known as wet
calcination.
 CaSO4. 2H2O 1200-1300 C CaSO4. 1/2H2O
gypsum
(calcium sulfate dihydrate)
17lbs/sq.inch Dental stone
( Alpha hemihydrate)

10.  Crystals are Prismatic & more regular in shape

11.  Formed by boiling Gypsum in 30% aqueous solution
of Calcium chloride.
CaSO4. 2H2O + CaCl2 30% 1000 C CaSO4. 1/2H2O
gypsum
(calcium sulfate dihydrate)
Die stone
( Alpha Modified
hemihydrate)

12.  Crystals are Prismatic, more regular and more densely
packed.

14.  Crystals are regular, prismatic and more densely
packed.

17. • TypeI : Impression plaster
Non-elastic impressionmaterial
• TypeII Model plaster usedfor
1.Dignostic cast
2.Articulation of stonecast
3. Study models
4.Flasking procedure for acrylic
denture

18. • TypeIII Dental stone usedfor
making
1.final or Master cast
2. orthodontic cast
3.Flaskingprocedure for acrylic
denture
• TypeIVDie stone high strength used
for
1.fabricating diesforinlaysandonlays
• TypeVDie stone high strength high
expansion usedfor
1.Used as dies to compensate the casting
shrinkage seen in base metal alloys.

19. THE COLLOIDAL THEORY:
When powder is mixed with water, plaster
enters into the colloidal state through a
sol-gel mechanism.
In the sol state, hemihydrate particles are
hydrated to form dihydrate, there by
entering into an active state.
As the measured amount of water is
consumed, The mass converts to a solid
gel.

21. HYDRATION THEORY:
The rehydrated plaster particles join together
through H-bonding to the sulphate group to form
the set material.

22. THE DISSOLUTION- PRECIPITATION THEORY
(Most widely accepted mechanism)
Based on the dissolution of plaster and instant
recrystallization of gypsum, followed by
interlocking of the crystals to form the set
solid.
The hemihydrate is 4 times more soluble in
water than is the dihydrate near room
temperature (27⁰C).

23. SETTING REACTION:
Hemihydrate is mixed with water , a suspension is
formed that is fluid and workable
HEMIHYDRATE dissolves until it forms a saturated
Solution
saturated hemihydrate solution , supersaturated in
dihydrate, precipitates out dihydrate
New crystals form or growth occurs on the crystals
already present  the reaction continues

24. as the amount of gypsum formed increases, the
mass thickens and then hardens into needle like
crystals called SPERULITES.
Finally the intermeshing and entangling of crystals
of gypsum leads to strong solid structure.

26. • Forhand mixing aclean, scratch free
rubber or plastic bowl having a diameter
of about 130mm is normally
recommended.
• Should be clean becausethe gypsum
residues in the mixing bowl can
noticeably alter the workingand setting
characteristics
• A stiff spatula with around-edged
blade of around 20–25mm width and
100mm length isused.

27. • Considerable quantities of air maybe incorporated
during mixing and this maylead to porosity withinthe
set material. Air porosity may be reduced either by
vibratingthe mix of plaster or stone in order to bring
air bubbles to the surface or by mixing the material
mechanically under vacuum.

28.  The amount of water and hemihydrate should be
gauged accurately by weight.
 W/P ratio is an important factor in determining the
physical and chemical properties of the final
gypsum product.

29. • the amount of water needed is added
to amoist bowl and the powder
added slowly to thewater over about
10seconds.
• mixing/spatulation carried out for
around 60 seconds using a circular
motion.
• After the material has been mixed and
used, the mixing bowl should be
thoroughly cleaned before the next
mix isperformed.

30. • Thematerial should be usedassoonas possible after
mixing sinceits viscosity increasesto the stagewhere
thematerial isunworkable within afewminutes.
• Thesetting reaction isexothermic

32. Factorsunder the control of the operator are

33. 1.Temperature
• 0 to 50 degree C – No specific change seen in
setting time.
• 50 to 100 degree C- gradual retardation of setting
reaction can be seen
• At 100 degree C – complete retardation of setting
reaction.

34. 2. W/P ratio
• Increasing the W/P ratio retards setting by
decreasing the concentration ofcrystallization
nuclei

35. 3. Mixing time (hand mixing 1min/
mechanical 30sec)
• Increasing mixing time accelerates setting by
breakingup dihydrate crystals during the early
stagesof setting,thus producing more nuclei on
which crystallization canbe initiated.

37. • Loss of gloss method
• Exothermic reaction
• Penetration tests

38. “The set material when loses it surface shine and
smoothness it is considered as the end of working time
and initial set has taken place this method is known as
loss of gloss method.’’

39. “The temperature rise of the mass which leads the setting
of material.”

40.  Vicat needle.
Vicat needle : Weight - 300gm
:Diameter – 1mm
 Gillmore needle.
I)Small : Weight - ¼ lb or 113.4 gm
:Diameter - 1/12’’(2.12mm)
II)Large : Weight - 1lb or 453.6 gm
: Diameter – 1/24’’(1.06mm)

41. Vicat needle
Gillmore needle
 Initial gillmore test for initial set:
It is measured in terms of their ability to resist
penetration by needle. Heavier needles have a
smaller tip diameter than lighter one and hence
applies greater pressure.

42. •The mixture is spread out and the needle is lowered on
to the surface.
•The time at which it no longer leaves an impression is
called the initial set. Noted as ‘initial gillmore’.
•It indicates partial progress of a setting reaction.

43. VICAT TEST FOR SETTING TIME
The rod holding the needle weighs 300 gms, the
needle is 1 mm in diameter and 5cm long.
Next stage in the reaction is determined by the vicat
penetrometer .

44. The needle with a weighted plunger rod is
supported and held just in contact with the
mix.
The time elapse from the start of the mix until
the needle no longer penetrates to the bottom
of the mix is known as the setting time.

45. GILLMORE TEST FOR FINAL SETTING TIME:
Use of the heavier gillmore needle for a 453.6 gms
(1pound) in weight and 1.06mm in diameter(1/24th
inch)
The elapse time at which this needle leaves only a
barely precipitable mark on the surface is called the
final setting time
This is the time at which chemical reaction is
practically completed.
The compressive strength is at least 80% at this stage
Most products reach ready for use state in 30 min.

46. SETTING EXPANSION:
Expansion is the result of a thrusting action of the dihydrate
crystals during the setting reaction.
An expansion of the mass can be detected during the change
from the hemihydrate to dihydrate
Depending on the composition of the gypsum product this linear
expansion may be as low as 0.06% or as high as 0.5%

47. Since the higher water powder ratio
Fewer nuclei of crystallization/unit vol.
Space between the nuclei will be greater
Less growth interaction of the dihydrate crystals
Resulting into Setting Expansion

48. UNDER NORMAL CONDITIONS:
SETTING EXPANSION
1. MODEL PLASTER - 0.2 – 0.3%
2. DENTAL STONE - 0.08 - 0.1%
3. HIGH STRENGTH DENTAL STONE - 0.05 – 0.07%
ABOUT 70% OF THE EXPANSION OBSEREVED AT 24
HOURS OCCURS DURING THE FIRST HOUR.

49. HYGROSCOPIC SETTING EXPANSION
If setting process is allowed to occur under water, the
setting expansion may be more than double in
magnitude.
Reason for increased expansion is the additional
crystal growth permitted by allowing crystals to grow
freely.
The basic mechanism of crystal growth is the same in
both.
Both phenomenon are true setting expansion

52. • Gypsummodel and die materials havethe
advantages of
1.being inexpensive
2.easyto use.
3.Theaccuracy and dimensional stability aregood
4. reproduce fine detail from theimpression
Disadvantages
1.the brittle nature ofgypsum occasionally leads to
fracture – particularly through the teeth, which
form the weakest part ofany model.

53. • Problems occasionally arise when gypsumproducts
are used in conjunction with alginateimpression.
• Thesurface of the model may remain relativelysoft
due to an apparent retarding effect which
hydrocolloids haveon the setting of gypsum
products. It is not certain whether the retarding
effect is due to borax in the hydrocolloid or to the
absorption of hydrocolloid onto thegypsum crystals
which act asnuclei of crystallization.
• Despite these observations it cannot be saidthat
gypsumproducts are incompatible with alginate
impression materials since problems arise very
infrequently.

54. • Alternative materials for the productionof
models and dies exist but are hardly ever
used.
• Theseinclude various resins, cementsand
dental amalgam.
• Thealternatives may be stronger but are
generally lessstable, difficult to useandmore
expensive.

55. Impressions
1)Rinse the impression under tap water for 1 minute
2) 0.5% Chlorhexidine (Spray)
3) 0.525% sodium Hypochlorite (immersion- 10 min)
4) 2% Glutaraldehyde (Immersion- 10 min)
Cast
1) Immersion for 10 min in disinfection solution
post 24 hrs

56.  ANUSAVICE K.J – PHILLIPS 12TH EDITION.
 CRAIGS – RESTORATIVE DENTAL MATERIALS 12TH
EDITION.
 ANUSAVICE,SHEN,RAWLS-PHILLIPS SOUTHEAST
ASIA EDITION.