Gypsum products-a topic of dental materials for dental students.... lots of knowledge...includes classification,uses,manufacturing processes etc. COURTESY: My college friends....

1. GYPSUM PRODUCTS
Presented by:
I. Prianka Dodiya,
II. Noori Dalvadi,
III. Meera Dobariya.
Guided by:
I. Dr.Deval Shah,
II. Dr.Mansi Gohil,
III. Dr.Apoorva Raval,
IV. Dr.Uzma Pathan.

2. What is gypsum ? Name the varieties of
gypsum.
It is a natural mineral mined around the various parts of the world having a
chemical formula, CaSO4·s2H2O(calcium sulphate dihydrate).
TYPES:·
 Albaster:-pure white ,fine grained and translucent.

3.  Satin spar :-fibrous needle like with silky lustre.
 Selenite:-colourless, crystalline and transparent.

4. What are different uses of ‘plaster of
Paris’?
 Impression of mouth and face(impression plaster)
 For making moulds, study casts, diagnostic cast,prliminary cast, master cast,
and dies over which dental prostheses and restorations are made.

5.  To mount cast on articulator.
 For bite registration

6.  In dental investments.

7. Define 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.

8. Classify the gypsum products according to
ADA specification no.
According to ADA specification number 25:
 TYPE I:Dental plaster,impression
 TYPE II:Dental plaster,model
 TYPE III:Dental stone, model
 TYPE IV:Dental stone,die,high strength,low expansion
 TYPE V:Dental stone,die,high strength,high expansion

9. What do you mean by calcination?
 The process of heating gypsum for manufacturing plaster is known as
calcination.
OR
Removing 3/4th of water of crystallization by burning of gypsum

10. Write the process for manufacturing of
Dental plaster(β hemihydrate).
OR
What do you mean by “dry calcination” ? Give
its reaction.
 Gypsum is ground and heated in an open kettle on kiln at a temperature of
to for manufacturing of beta type crystals or dental plaster. This process
is known as “dry calcination”.
(beta hemihydrate)
C
110
C
130
OHCaSOOHCaSO
Heat
24
130110
24
2
1
.2.    

11. Write the process for manufacturing of
dental stone(α hemihydrate).
OHCaSOCaSO 24
130120
4
2
1
.   
17 lbs/sq. inch
pressure (alpha hemihydrate)

12. Write the process of manufacturing for
alpha modified stone or improved stone.
 Gypsum is calcined by boiling it in 30% calcium chloride solution . The
chlorides are then washed away or autoclaved in presence of sodium
succinate 0.5%.Than grinded to obtain alpha(α) modified stone.

13. Give the reaction for manufacturing of
orthorhombic anhydrite.
anhydrite)anhydrite)e)hemihydratdihydrate)
ic(orthohomb(hexagonalsulphate(calicumsulphate(Calcium
2
1
.2. 4
1000200
4
200130
24
130110
24 CaSOCaSOOHCaSOOHCaSO      

14. Write the setting reaction of plaster.
HeatOHCaSOunreactedOHCaSOOHOHCaSO  224242224
2
1
.)(2.23)(
Hemihydrate + water Dihydrate + unreacted hemihydrate + Heat
EXOTHERMIC REACTION

15. What are modifiers?
 They are the chemicals added in order to alter some of the properties like
expansion , setting time etc. or to enhance the other properties like working
time , dimensional stability etc.
 EX.:accelerator & retarder.

16. What are accelerator and retarder?
 ACCELERATORS:-The substances which decreases the setting time of gypsum
products by increasing the rate of reaction are called accelerator.
E.g., Tera alba, sodium sulphate, potassium sulphate and sodium chloride(low
concentration)
 RETARDERS:-They are the substances which increases the setting time of
gypsum products by decreasing the rate of reaction.
E.g., Borax, sodium chloride, sodium sulphate(high concentration) ,glue ,agar,
coagulated blood.

17. What is 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.

18. What are the theories of setting of
gypsum product?
 Colloidal theory
 Hydration theory
 Dissolution precipitation theory

19. Define crystallization.
 Crystallization is the slow precipitation of crystals from a solution of a
substance. Crystallization can also refer to the solid-liquid separation and
purification technique in which mass transfer occurs from the liquid solution
to a pure solid crystalline phase

20. What are the stages(chemical) of
crystallization?
 Dissolution
 Suspension
 Saturation
 Super saturation
 Nuclear formation
 Growth of nuclei
 Crystallization
 Spherulite formation

21. Name physical stages of gypsum
product.
 Fluid
 Plastic
 Firable
 Carvable

22. What do you mean by spherulites?
 “As the reaction proceeds gypsum is formed in the form of needle like
clusters called spherulites”

23. What do you mean by water of
crystallization?
 The invisible water present in the gypsum is known as water of
crystallization.

24. What is impression plaster? Give its
uses.
 Impression plaster is one of earliest rigid impression materials in dentistry.
USES:
 For making impressions in complete denture and maxillofacial prosthesis.
 For bite registration material.

25. What is soluble plaster? Give its mode of
action.
 When impression plaster contain potato starch to make the impression soluble
it is termed as ‘soluble plaster’.
MODE OF ACTION:
Impression Starch swells
Poured with in hot water and impression Cast get separated
the cast(after disintegrates
Hardening)

26. What is the chemical formula for dental
plaster/β hemihydrate?
OHCaSO 24
2
1
.

27. Write the composition of impression
plaster?
 Dental plaster
 (Potassium sulphate)
 Borax
 Colouring and flavouring agent
42SOK

28. Write various uses of TYPE II gypsum product
or model plaster and give its composition.
 MODEL PLASTER = LABORATERY PLASTER.
USES:
 For making study cast and model.
 To make moulds of curing dentures
 For mounting cast on articulator
COMPOSITION:
Βeta hemihydrate + Modifiers

29. What do you mean by ‘hydrocal’?Give its
uses.
 It is the type III gypsum product called DENTAL STONE used for model
preparation.
USES:
 For preparing master casts and to make moulds.

30. Write the composition of dental stone(α
hemihydrate).
 Alpha hemihydrate + 2 to 3% coloring agent+ (accelerator) + borax (retarder)42SOK

31. Write the various properties of dental
stone,model?
 COMPRESSIVE STRENGTH:- 3000 to 5000 psi
 SETTING EXPANSION:- 0.06% to 0.12%
 HARDNESS:- 82 KHN

32. What is the difference between the crystal
structure of dental plaster and dental stone?
DENTAL PLASTER DENTAL STONE
Needle like crystals Rods and prism like crystals
Irregular in shape with capillary pores Regular in shape
Loosely packed Closely packed

33. What is excess water?
 The actual amount of water necessary to mix the calcium sulphate
hemihydrate is greater than the amount required for the chemical reaction,
this is called excess water.

34. What is a balanced stone?
 A stone with a setting time established by the addition of proper quantities of
both accelerator and retarder is called “balanced stone”.

35. Write the synonyms for TYPE IV gypsum
products.
 Class II stone
 Die stone
 Densite
 Improved stone

36. Write the uses of die stone.
 It is used when high strength and surface hardness is required
E.g., dies used for inlays , crown and bridge wax pattern.

37. Give the examples of TYPE V stone.
 Hard rock
 Jade rock
 Resinrock XL5
 Denflo HX

38. How type V is better than type IV stone?
 It has higher compressive strength.
 Setting expansion is increased from a maximum of 0.10 to 0.30% to
compensate for the shrinkage of base metal alloys during solidification.

39. What do you mean by w/p ratio?
 It is defined as water required for 100gm of powder.

40. What is the w/p ratio of the gypsum
products?
 Impression plaster : 0.50 to 0.75
 Dental plaster : 0.45 to 0.50
 Dental stone : 0.28 to 0.30
 Die stone,Type 4 : 0.22 to 0.24
 Die stone,Type 5 : 0.18 to 0.22

41. What is the of low and high water
powder ratio on gypsum products?
LOW W/P RATIO(thick mix) HIGH W/P RATIO (thin mix)
Quick setting Slow setting
High expansion Low expansion
More strength Less strength
More surface hardness Poor surface hardness
Less porosity More porosity

42. Define mixing time and Give its value
for gypsum products.
 “It is the time from the addition of the powder to the water until mixing is
complete.”
 1 minute

43. Define working time and give its value
for gypsum products,
 “It is the time available to work with the mix for the intended purpose.’’
 3 minutes.

44. What do you mean by setting time? Give
its two types.
 “ It is the time elapsing from the beginning of mixing until the material
hardens.”
 TYPES:-
1. Initial setting time
2.Final setting time

45. Define initial setting time.
 “As the reaction proceeds,more hemihydrate crystals react to form dihydrate
crystals.The viscosity of the mass is increased and it can no longer be
poured.The material become rigid(but not hard).It can be carved but not
moulded.This is known as initial setting time’’

46. What is final setting time?
 “The time at which the material can be separated from the impression
without distortion or fracture.’’

47. What are the methods to measure the
setting time?
 Loss of gloss method
 Exothermic reaction
 Penetration tests

48. What is induction period?
 “Initial time during setting before release of exothermic heat”

49. What is loss of gloss method?
 “The set plaster 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.’’

50. What do you mean by exothermic
reaction?
 “The temperature rise of the mass which leads the setting of material.”

51. Which are the types of penetrometers?
 Vicat needle.
 Gillmore needle.
(Vicat needle) (Gillmore needle)

52. Give 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)

53. Which are the factors affecting setting
time?
 Manufacturing process
 Mixing and spatulation time(time and rate)
 Water/powder ratio
 Temperature
 Modifiers

54. What is nuclei poisoning?
 Presence of more than 20% of NaCl , colloids like coagulated blood etc. act as
retarder to setting of plaster by wetting of the nuclei and their growth.

55. What is normal setting expansion?
 It is a linear expansion during setting due to outward thrust of the growing
crystals during setting which intermesh and intercept each other during
growth.

56. What is hygroscopic expansion and how
much it is?
 It is increased expansion of setting plaster in presence of water .It can be
double the amount of normal setting expansion.
USE:-
It is used to expand some gypsum bonded investments.

57. Write the factors controlling expansion.
 Water powder ratio :- setting expansion
 Modifiers :- setting expansion
 :- 4% solution decreases the setting expansion from 0.5 to 0.06 %.
 NaCl and borax :- setting expansion
42SOK

58. Write the factors affecting strength and
give its two types.
 Water powder ratio :- water , porosity , strength
 Spatulation :- strength (within limit)
 Addition of accelerators and retarders :- strength
TYPES:-
1.Dry strength
2.Wet strength

59. What is wet strength ? How much it is for
model plaster, dental stone and die stone?
 It is strength when the set mass is still contains excess water and specimen is
wet to touch.
 E.g.,
Model plaster – 12.5 Mpa
Dental stone - 31 Mpa
Die stone – 45 Mpa

60. What is dry strength? How much it is ?
 It is strength when the set mass has no excess water and specimen is dry to
touch. It is 2- 3 times more than wet strength.

61. What care should be taken with temperature
to avoid shrinkage and reduction in strength
of gypsum products?
 Gypsum is stable only below about . Drying at higher temperature
rapidly at or higher ,lose of water of crystallisation occurs which
causes shrinkage and reduction in strength.
C
40
C
100

62. What are divestment ?
 It is mixture of die stone and anyone of the dental investments materials used
to make die or master model.

63. What are model duplicating materials?
 These are used to make study model during metal casting e.g., hydrocolloid
impression materials.

64. What care should be taken during storage of
powder?
 As plaster is hygroscopic it should be kept in air tight containers. Otherwise
when humidity is more than 70% plaster starts setting reaction and small
crystals are produced. As hygroscopic expansion continues the entire
hemihydrate mass is covered with more crystals of gypsum. Thus water
penetrates the mass with difficulty, thereby delaying setting.

65. THE END