Gypsum products

Gypsum Products
Dr. Md. Arifur Rahman
Assistant Professor & Head of the Dental Unit
North East Medical College
arahman@neub.edu.bd
1Dr. Md. Arifur Rahman, NEMC

CAST / MODEL: A replica of several teeth and their
associated soft tissues or an edentulous arch.
DIE: Replica of a single prepared tooth.
Ordinary/ MODEL PLASTER: Weakest and most porous
form of gypsum product used in dentistry.
Hard Plaster/ DENTAL STONE: A stronger and less
porous form of gypsum product used in dentistry.
DIE STONE: The densest form of gypsum product used in
dentistry.
DIAGNOSTIC CASTS: The casts generally used for
patient education, treatment planning or tracking the
progress of treatment. (e.g. orthodontic models)
2
Important Terms
Dr. Md. Arifur Rahman, NEMC

WORKING CAST: The cast that is strong enough to
resist the stresses of fabricating an indirect
restoration or prosthesis.
POURING: The process of vibrating the flowable
gypsum product into the impression.
HYGROSCOPIC SETTING EXPANSION: The amount
of setting expansion that occurs when the gypsum
bonded casting investment is immersed in water.
NORMAL SETTING EXPANSION: The amount of
setting expansion that occurs when a gypsum
bonded casting investment is allowed to set in air.
3
Important Terms (cont.)

4
Introduction
 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 in compact mass in nature.
 It has commercial importance as a source of
Plaster of Paris.

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

6
Types of gypsum or gypsum products
 Type 1- Plaster, impression
 Type 2- Plaster, model
 Type 3- Dental stones
 Type 4- Dental stone high strength
 Type 5- Dental stone , High strength / high
expansion
(Identical chemical formulae- CaSo4. 1/2H2O)

7
Dental Uses of gypsum products
 Dental applications:
 Impression
materials
 Constructing casts
and dies
 As an investment
material

8
Requirements of model and die materials
1. Dimensional accuracy.
2. Adequate mechanical properties.
3. Should ideally be fluid at the time it is poured into the
impression.
4. Set material should be sufficiently strong to resist
accidental fracture.
5. Set material must be hard enough to resist the abrasion
during carving of wax pattern.
6. Should be compatible with all the materials it comes in
contact.
7. Should have good color contrast with various waxes.

9
 Gypsum (calcium sulfate) is mined, ground
to a powder and heated (calcined)
• Calcination drives off water and changes the
original dihydrate form (2 water molecules) to
a hemihydrate form( ½ of one water
molecule)
Making Plaster or Stone

10
Making Gypsum Products (cont.)
 Gypsum product stays in its hemihydrate form
until water is added back
• When water is added, hemihydrate returns to a
dihydrate
• The new dihydrate forms as a series of interlocking
crystals (Crystallization)
 As the crystals form, they give off exothermic
heat, which equals the amount of heat added
during calcination

11
Gypsum in
hemihydrate
form
Water is added to
hemihydrate to
change it back to
dihydrate form
Amount of exothermic
heat given off during
Crystallization = amount
added during calcination

12
Dental Plaster
• Calcination occurs without pressure, plaster
is formed.
• Plaster is called a beta hemihydrate (Type II)
• Plaster is white
• Plaster is used for study models and working
models on which splints and orthodontic
appliances are made

13
Dental Stone
 If calcination occurs under steam pressure,
an alpha hemihydrate is formed (Type III
Stone)
 stone is usually yellow in colour.
 Stone is harder, stronger and more
expensive than plaster.
 It is used for products that require strength
such as Removable Prosthodontics.

14
 A further increase in
pressure and refining of
the powder by grinding
results in denser stone,
high-strength or die
stone.
 Die stone can be any
color except white
(plaster) or yellow
(regular stone)
 Restorations are made
on a die (a working
replica of a single tooth)
Die stone

15
 Stone is mixed with silica dental
investment
 Dental investment is a material that can
withstand high heat and stress produced when
molten metal is forced into molds to form
indirect restorations
Dental Investment

16
Gypsum products – Microscopic view
Plaster of Paris Dental stone
Die stone

17
Physical properties
Type W/P Porosity Compressiv
e strength
Abrasion
resistance
Setting
expansion
Plaster,
impression
0.40-
0.70
High 8.8 MPa Low High
Stone 0.30 moderate 20.6 MPa Moderate Moderate
High strength
stone
0.22 Low 34.3 MPa High Low
High strength
High-
expansion
stone
0.20 Low 48.0 MPa High High

18
Physical properties (cont.)
 Strength and hardness: Affected by
 Porosity
 Shape of particles (large irregular particles don’t
condense well leading to decreased density).
 Amount of water mixed with the powder. Higher
porosity requires more water to be mixed to
produce a cast which will be weaker.

19
 Dimensional accuracy:
 The higher the setting expansion, the lower
the accuracy.
 Setting expansion results from the growth of
crystals as they join.
 Solubility:
 Set gypsum is not highly soluble
 The greater the porosity the greater the
solubility.

20
 Reproduction of detail:
 Greater porosity decreases surface detail
production
 The impression material should be compatible
with the gypsum to improve detail reproduction.
The best compatibility is between silicon and
polyethers and gypsum

21
Classification of gypsum
 Impression plaster (Type I)
*seldom used*
 Model plaster (Type II): 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.
Impression (negative replica), poured
into gypsum to make a cast (positive
replica

22
Plaster
Plaster
Flask

23
 Dental stone (Type III):
used for making
 Full or partial denture
models
 Orthodontic models
 Flasking procedure for
acrylic dentures (teeth
end)
It requires less water,
stronger than plaster
Dental stone casts

24
 Dental stone, high strength
(Type IV):
 AKA die stone used in
fabricating wax patterns of
cast restorations (crown
&bridge)
 Implants
 Precision attachments work
 Often colored pink or green
 Almost 2 times stronger than
type III stone
Die stone used in the fabrication
of cast crown restoration

25
 High strength, high expansion dental stone (Type V)
 Colored blue or green
 Most costly of all gypsum materials
 Lowest W:P ratio, higher compressive strength
 The need for higher expansion (0.1%) was to
compensate for the solidification shrinkage of
some alloys used as base metals used for dental
casting

26
 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.
Manipulation

27
Proportioning (W:P ratio)
Golden rule: Manufacturer instructions should always be followed.
Variations in W:P ratio affect the set materials’ properties such as
strength and accuracy
 Too much water?
( increased water powder
ratio)
 Increases setting time
 Increased porosity
 Decreases strength
 Too little water?
( decreased water powder
ratio)
 Decreases setting time
 decreased porosity
 high strength
 .

28
Manipulation (cont.)
 Mixing:
 1.Manual &
2.Mechanical
 Manual: Rubber bowl
and spatula.
 Powder is sifted into
water to ensure good
wetting and avoid
clumps, and avoid air
bubbles

29
Mechanical

30
Setting reactions: Theories
 1.Colloidal theory
 2.Hydration theory
 3. Dissolution – Precipitation theory

31
Colloidal theory
 Plaster enters into the colloidal state through a sol-
gel mechanism.
 In sol state hemihydrate particles are hydrated to
form dihydrate.
 As the measured amount of water is consumed the
mass coverts to solid gel.
 CaSO4. 1/2 + H2O CaSO4. 2H2O
plaster or stone gypsum
( calcium sulfate hemihydrate) (calcium sulfate dihydrate)

32
Hydration theory
Rehydrated plaster particles join together
through hydrogen bonding to the sulfate
groups to form the set material.

33
Dissolution – precipitation theory
 Based on dissolution of plaster and instant
recrystallization of gypsum, followed by
interlocking of the crystals to form the solid
gel.
 ( most widely accepted mechanism)

34
 Initial setting time-working time
 Working time is measured from the start of mixing
to the point where the consistency is no longer
acceptable for the product’s intended purpose.
 6-10 min.
 Initial setting time: time elapsed from the start of
mix until loss of gloss (8-10 minutes)

 Final setting time: It is the elapsed time at which the
Gillmore needle leaves only a barely perceptible mark
on the surface.
 It is reached when the materials can be safely
handled, the gypsum is cool (exothermic reaction is
over).
35Dr. Md. Arifur Rahman, NEMC

36
 Setting expansion: Results from crystal growth during
setting. Can be decreased by the addition of
potassium sulfate, sodium chloride, borax.
 Hygroscopic expansion-
If gypsum soaked during setting, water fills pores
and increases volume
 It’s recommended to separate the cast from
impression after 1 hour.
 Strength increases 2-3 times after 24 hours

37
Tests for initial setting, and final
setting times
 Loss of gloss test for initial setting time: loss
of gloss occurs as water is taken up by
gypsum to form the dihydrate. The materials
does not have measurable compressive
strength.
 Initial Gillmore test for initial set: needles are
used to indent the material until no
indentation can be seen.

38
 Vicat penetrometer (A) for setting
time: used for the next stage of
setting (refer to figure) after loss
of gloss, the plunger rod is
released onto the mix. Time
elapsed until the rod no longer
penetrates is the setting time.
 Gillmore test (B) for final setting
time: a heavier Gillmore needle
to determine final setting time.
B
B

40
Properties of gypsum products
type w/p
ratio
Setting time
Min.
2 hr setting
expansion(1%)
Min. Max.
1-hr compressive
strength
Mpa
I.Plaster,
impression
0.40-
0.75
4+,- 1 0.00 0.15 4
II.Plaster,
model
0.45-
0.50
12+,- 4 0.00 0.30 9
III.Dental stone 0.28-
0.30
12+,- 4 0.00 0.20 20.7
IV. Dental
stone, high
strength
0.22-
0.24
12+,- 4 0.00 0.10 34.5
V. Dental stone,
high strength,
high expansion
0.18-
0.22
12+,- 4 0.10 0.30 48.3

41
Ready for use criteria
 The ability to judge readiness of gypsum to
be handled improves with experience.
 Technically, the material is considered ready
when compressive strength reaches 80% of
the strength attained after 1 hour.
 Most products are ready to use in 30
minutes.

42
 Clinical tip: before separating cast from
impression, ensure that no part of the tray is
connected to the gypsum
 Clinical tip: if alginate impression dried before
cast separation, soak in water for 15 minutes.

43
How to control setting time
1. Changing water : powder ratio
Increasing water Decreasing water
Retarded setting Shorter setting time
Weaker model or cast Mix difficult to manipulate
Inaccurate model Bubbles inclusion in mix
-------- Inaccurate model

44
2. Spatulation: rapid and prolonged
spatulation accelerates setting and also
increases setting expansion.
3. Temperature: increasing water temperature
to a certain level will accelerate setting.

45
4. Accelerators and retarders:
 Manufacturers add accelerators or retarders to
gypsum. These chemicals increase or decrease
gypsum solubility respectively which will alter
setting time accordingly.
 Clinicians can add accelerators such as
potassium sulfate or set gypsum (slurry water),
they act as sites for crystallization.
 Setting reaction retarders: blood, saliva,
alginate. If left on impression, can affect surface
details of impression. Impression surface need
to be properly rinsed before being poured.

46
 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:

47
1. Double-pour method
1 2
3

48
2. Single step
 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?

49
3. 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.

50
Storage and clean up
 Gypsum should be stored in air tight dry area.
Prolonged exposure to moisture can retard
setting due to decreased solubility of powder.
 Relevant equipment should be kept clean to
avoid unwanted acceleration of setting by set
gypsum.

51
Infection control
 Casts should have set for 24 hours before
being disinfected if necessary.
 Spray rather than immerse.
 Disinfectants commonly used:
 Sodium hypochlorite
 iodophors
 Chlorine dioxide

53
Investment
materials
 Used to form metal
casting through the lost
wax technique

54
References
 Craig’s Restorative Dental Materials
13th edition by Ronald L. Sakaguchi and
John M. Powers.
 Dental Materials and Their Selection
- 3rd edition by William J. O'Brien
 Philip’s science of dental materials, 11th edition by
Kenneth J. Anusavice.

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