Download Gypsum Products: DENTAL PLASTER AND STONE Gypsum

Key Terms
Gypsum Products:
DENTAL PLASTER
AND STONE
Gypsum
A mineral mined from the Earth. Occurs most
widely in a massive form known as rock-gypsum.
Also used in homes and industry. 3 types
discussed:
• Plaster
• Stone
• Die stone
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Gypsum
Calcination
Plaster
Dental Stone
Die stone
Dihydrate
Calcium Sulfate
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Hemihydrate
Alpha hemihydrate
Beta hemihydrate
Exothermic heat
Accelerator
Retarder
Making Plaster or Stone
• 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 Gypsum Products
• 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
Gypsum in
hemihydrate
form
Water is added to
Amount of exothermic
hemihydrate to
heat given off during
change it back to
Crystallization = amount
dihydrate form
added during calcination
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Plaster
• If 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
Dental Stone
Label on box of plaster
Die Stone
• Die stone is regular
stone that has been
modified so it is
stronger and harder
• 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)
• If calcination occurs
under steam pressure,
an alpha hemihydrate
is formed (Type III
Stone)
• Plaster is usually
white in color, stone is
usually yellow
• Stone is harder and
stronger (and more
expensive) than plaster
•Is used for products
that require strength
such as Removable
Prosthodontics
Crystallization
Die
Water/Powder Ratio
• Proportion of water to powder used to make
a workable mix of a particular gypsum
• Depends on the physical characteristics of
the powder particles
• Has direct result on the properties of the
gypsum and must be controlled for
optimum results
Plaster crystals
are large so more
porous, not strong,
takes the most water
to mix
(Mix 45-50 ml H20 per
100 g. powder)
Stone crystals are
smaller, less porous,
strong, takes less water
to mix (Mix 28-30 ml H20
per 100 g. powder)
Die stone crystals
smallest, strongest,
takes least water
to mix (Mix 19-24 ml
H20 per 100 g.
powder)
Altering/Controlling Setting
Time
• Accelerators speed
setting
• Best accelerator is
potassium sulfate
• Can also accelerate
by
-Decreasing H20
• Retarders slow setting
• Best retarder is borax
or sodium citrate
• Can also retard by
- Cooling H2O
- Increasing amount of H2O
-Warming H20
-Mixing vigorously
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Mixing Gypsum
• Measure the water
• Measure the powder
• Add powder to water
to avoid incorporation
of air
• Mix/wipe against
side of bowl (don’t whip)
-hand mixing/vibrator
-Vacuum mixing
• Filling tray/pouring model
Care of Gypsum
• Plaster and stone should be stored in airtight
containers due to sensitivity to humidity
• Exposure to moisture or humidity can cause
setting reaction to occur
• Gypsum is soluble in water; fine
detail/dimension can be lost
Questions?
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