Gypsum, commonly known as plaster of Paris, has many uses in dentistry including for dental impressions, casts, dies, and replicas due to its properties like strength, accuracy, and low cost. Different types of gypsum products include plaster, dental stone, die stone, and high-strength stones used for various dental applications like diagnostic casts, working models, and fabrication of crowns and bridges. Gypsum sets through a chemical reaction when mixed with water and can be manipulated in terms of water-powder ratio, mixing, and setting times to achieve the desired properties for its intended dental use.

1. 1

2. Introduction
 Gypsum is a mineral naturally found in nature.
 Dental applications:
 Impression materials
 Constructing casts and dies
 Making replicas
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3. Ideal properties of die materials
 Dimensional accuracy.
 Ease of use.
 Accurate reproduction of fine detail.
 Inert.
 Good strength.
 Inexpensive.
 Abrasion resistant.
 Good color contrast
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4. Properties and behaviour

Chemical properties: (exothermic reaction)
Ca sulfate dihydrate heat water loss ground Ca sulfate
hemihydrate
 The production of various types of gypsum is basically the same:
 If the heating process occurs in atmospheric pressure at 115°C,
the powder particles are porous and irregular (b hemihydrate,
plaster).
 if heating process is under pressure, powder particles are
regular and less porous (a hemihydrate, or dental stone).
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5.  A further increase in pressure and refining of the
powder by grinding results in denser stone, high-
strength or die stone
 When stone is mixed with silica: dental
investment, a material that can withstand high heat
and stress produced when molten metal is forced
into molds to form indirect restorations
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6. Gypsum products
Plaster of Paris Dental stone
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Die stone

7. Physical properties
Type W/P Porosity Compressive Abrasion Setting
strength resistance expansion
plaster 0.45 High 8.8 MPa Low High
Stone 0.30 moderate 20.6 MPa Moderate Moderate
High strength 0.23 Low 34.3 MPa High Low
stone
High strength 0.20 Low 48.0 MPa High High
High-expansion
stone
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8. 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.
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9.  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.
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10.  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
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11. 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 Impression (negative replica), poured
available is fast and regular into gypsum to make a cast (positive
sets. replica
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12. Plaster
Plaster
Flask
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13.  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
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14.  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
Die stone used in the fabrication
type III stone of cast crown restoration
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15.  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
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16. Manipulation
 Selection: based on the desired properties and
dental application. e.g.:
 For a diagnostic cast, plaster can be used.
 For a working cast, strength is required and
accuracy, dental stone is the gypsum of choice
 Working models for cast restorations require die
stone
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17. Manipulation (cont.)
 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 the mix will be runny, the model will
be weaker.
 Too little water difficult to mix, chance to get bubbles
inside the mixture.
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18. Manipulation (cont.)
 Mixing:
 Manual: Rubber
bowl and spatula.
Powder is sifted into
water to ensure good
wetting and avoid
clumps, and avoid air
bubbles 18

19. Mechanical (also used with vacuum)
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20. Manipulation (cont.)
 Initial setting time-working time
 Working time start after mixing for 1 minute
 Initial setting time: time elapsed from the start of
mix until loss of gloss (8-10 minutes)
 6-10 minutes of working time are available to pour
the gypsum.
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21.  Final setting time: is reached when the materials can be
safely handled, the gypsum is cool (exothermic reaction
is over).
 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
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22. 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 =
initial setting time.
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23. A
 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.
B
 Gillmore test (B) for final setting B
time: a heavier Gillmore needle
to determine final setting time.
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25. 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.
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26.  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.
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27. 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
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28. 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.
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29. 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.
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30.  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:
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31. 1. Double-pour method
1 2
3
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32. 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?
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33. 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.
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34. Storage and clean up
 Gypsum should be stored in airtight 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.
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35. 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
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36. Trimming
 Plaster bases are recommended since trimming
them is easier than dental stone.
 If base is made from stone, it should be soaked
in water for 5-10 minutes to soften it before
trimming. Important considerations when
trimming?
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37. Trimming considerations
 Proportion of base to
anatomical part
 Parallelism
 Use of wax bite registration
 Outer border of cast
 Shaping of anterior part of
upper and lower arches
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38. Other types of Die stone
 Metal plated die stone: silver or copper plated
to increase abrasion resistance
 Epoxy die stone: resin and a hardener are used
to increase abrasion resistance.
 Resin reinforced die stone: resin is
incorporated into the gypsum material to
increase abrasion resistance. 38

39. Investment materials
 Used to form metal
casting through the lost
wax technique
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40. References
 Dental materials, clinical applications for
dental assistants and dental hygienists.
Chapter 12
 Phillips’ science of dental materials.
Chapter10
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