2. Uses and Desirable Qualities of
Gypsum Products
Gypsum:
Mineral that is widely found in nature
Ideal for making replicas of the patient’s mouth
The replicas are called:
Diagnostic casts (also called study models)
Working casts (also called working models)
Dies
The production of gypsum casts requires meticulous attention to
detail.
Inaccurate or incomplete casts are of little use to the dentist.
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3. Properties and Behaviors of
Gypsum Products
Chemical properties
Mineral gypsum = dihydrate of calcium sulfate
Ground to produce a powered hemihydrate
Model plaster: Commonly used for diagnostic casts (study
models)
Dental stone: Used for working casts (master casts)
Die stone
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4. The figure shows diagnostic casts made from
gypsum: a working cast with a stone base and a high-
strength stone anatomic portion, and a die made
from high-strength stone with metal plating.
5. Properties and Behaviors of
Gypsum Products
Even though gypsum is always basically the same, different dental plasters
are formed according to how the material is processed.
If the material is heated in open vats at approximately 115° C, the resulting
hemihydrate is porous and irregular in shape.
This process will form model plaster.
If the heating process is done under pressure, in the presence of steam
and at a higher temperature, a more uniformly shaped and less porous
form of hemihydrate is produced.
This is referred to as dental stone.
A further increase in pressure and heat will produce a material called die
stone.
6. Properties and Behaviors of
Gypsum Products
Physical properties
Main differences in physical forms dependent on
variations in size, shape, and porosity of the powders
Strength, hardness, and resistance to abrasion
Strength of gypsum products related to the amount of water
used to produce the study or working cast
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7. Properties and Behaviors of
Gypsum Products
Factors that affect the strength of gypsum products also
affect their hardness.
Increased porosity of the particles makes it necessary to use
more water to convert the hemihydrate particles back to
dihydrate particles.
8. Properties and Behaviors of
Gypsum Products
Dimensional accuracy
Control setting expansion
Setting expansion occurs with all gypsum products.
Plaster has the highest rate of expansion.
Stones have the lowest rate of expansion.
Setting expansion is the result of growth of crystals as they join
together.
Some expansion is acceptable, but large amounts of expansion
will keep the end result from fitting.
It is important to control setting expansion.
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9. Properties and Behaviors of Gypsum Products
Reproduction of detail
The greater the porosity of the final gypsum product, the less surface detail is
produced.
Even products that have the least amount of porosity have surface
irregularities visible at the microscopic level.
Solubility
Set gypsum products are not highly soluble in water.
Solubility is directly related to the porosity of the material; therefore plaster is
much more soluble than stone.
Exposing models to water for prolonged periods should be avoided.
10. Classification of Gypsum Products
The desired physical properties and the behavior necessary for a
particular use determine the criteria for selection of a gypsum
product.
If you are pouring up diagnostic models, plaster is typically used.
Impression plaster (type I)
Rarely used today
Model plaster (type II)
Frequently used for diagnostic casts
Durable but relatively weak material
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11. Classification of Gypsum Products
Dental stone (type III)
Buff stone
Ideal for making full or partial denture models, orthodontic models, and
casts
usually used to make an appliance, such as orthodontic retainers,
whitening trays, sports guards, and partials.
Dental stone, high-strength/low-expansion (type IV)
Often referred to as die stones or densite
Used to fabricate crowns and bridges
Dental stone, high-strength/high-expansion (type V)
Good to use with the newer base metals because of casting shrinkage
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12. Manipulation of Gypsum Products
Material selection
Selection of a gypsum product should be based on the desired properties of the material.
It is important that the mix have sufficient flow to reproduce details of the impression.
Proportioning (water-to-powder ratio)
Water should be measured with a graduated cylinder, and powder should be weighed on a
scale.
Too much water will make the mix weak.
Too little water, and the mix will not flow. Detail will be lost
Mixing: Spatulation
Most commonly, plaster and stone are mixed in a flexible rubber bowl with a broad metal
spatula.
Mechanical devices are used when control of spatulation is critical.
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13. Initial setting time and working time
The first step is to add the water to the bowl.
Sift the powder into the water, and mix with a broad spatula and spatulate the material until smooth.
It takes about 1 minute to mix the material thoroughly.
After mixing for 1 minute, the working time begins.
During this time, the semifluid mixture is poured into the impression with the help of a mechanical vibrator.
After mixing, about 5 minutes of working time is available.
As the viscosity of the mixture increases, the flow characteristics will be decreased and the product loses its
glossy appearance.
This loss of gloss indicates that the gypsum has reached its initial set (8-16 minutes)
Final setting time
The final set is reached once the material is hard and the exothermic reaction cools completely.
Most manufacturers recommend that about 1 hour should pass before the impression is separated from the cast.
Gypsum products continue to harden and are two to three times harder after 24 hours
If the impression is removed too soon, the model or teeth will break.
If left too long, the impression will dry out (moisture absorbed by alginate) and will possibly snap off the teeth.
14. Manipulation of Gypsum Products
Control of setting times:
Altering the water-to-powder ratio
Decrease water; decrease working time; more difficult to manipulate; air bubbles (only
when pouring base)
Increase water; increase working time; weaker by 50%
Spatulation
Longer and more rapid; will also increase setting expansion
Temperature
Increase temp; decrease working time
Accelerators and retarders
It is impossible to accelerate the final set of a mixture without also accelerating
the initial set, thereby reducing the working time.
15. Manipulation of Gypsum Products
Fabricating and trimming
diagnostic/working casts
Diagnostic and working casts have two
parts:
Anatomic portion
replicates the hard and soft structures
Art portion
aids in handling and articulating the casts.
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16. Manipulation of Gypsum Products
The anatomic portion is poured by vibrating small increments of
flowable gypsum into the duplicated oral imprints.
The mixture should be poured slowly in small increments under
vibration and allowed to flow from one tooth impression to the next,
pushing out air ahead of itself as it fills the entire impression, thus
eliminating air voids.
17. Double-Pour Technique
Involves two separate mixes and two separate setting times.
The anatomic portion of one or both arches is poured and is left in
the upright position.
Approximately 10 minutes after the loss of gloss, a second mix is
produced for the art portion(s).
This mixture is placed on a glass tile or in a base former.
The filled impression is inverted onto the base, and the peripheries of
the two portions are joined.
18. Single-Step Method
One mix of gypsum is produced to pour the anatomic and
art portions of the cast.
After the impression is poured, the remaining material is
used for the base.
This material is placed on a glass tile or in a base former, the
impression is inverted onto it, and the peripheries of the two
portions are joined.
19. Boxing Method
A strip of wax is used to surround the impression,
forming a wall into which the gypsum is poured.
The wax should not distort the impression.
It should extend at least 0.5 inch higher than the
highest point of the impression and create a base that
is parallel to the occlusal plane.
The figure shows pouring the art portion of a cast by
boxing; model former and inversion on patty base.
20. Manipulation of Gypsum Products
Storage
Gypsum products can absorb water from the environment
Gypsum should be stored in airtight, moisture-proof containers
Humidity and close proximity to water sources will adversely affect
the powder.
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21. Manipulation of Gypsum Products
Cleanup
Bowls, spatulas, mechanical vibrators, and mixing devices should be
cleaned of all traces of gypsum as soon as possible after manipulation
Gypsum mixing and handling equipment must be kept
meticulously clean.
Set gypsum will accelerate the setting times of a mixture.
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22. Manipulation of Gypsum Products
On setting of the gypsum, the impression, tray, and the cast
must be separated.
When the impression is poured, care should be taken to
make sure the gypsum does not flow onto the tray, locking it
into the set gypsum.
Separating the impression from the cast
Gently ease a lab knife under the impression tray and lift the tray
slightly in several areas
Care should be taken to not tilt too far in one direction or force the
impression and cast apart
Broken teeth may result
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23. Manipulation of Gypsum Products
Infection control and safety issues
Routine disinfection of impressions should be done
in the dental office.
Disinfection of impressions is the best way to
prevent the introduction of contaminants into the
laboratory area.
24. Manipulation of Gypsum Products
Trimming
Cast or study model
Models are trimmed with a model trimmer to produce an attractive,
symmetric model with easy access to all anatomic portions of the model and
a base of sufficient bulk for stability.
Bases made from dental stones should be soaked in water for 5 to 10
minutes before trimming to saturate the stone, making it easier to trim.
The cast should be trimmed so that proportionately the base makes up one
third and the anatomic portion two thirds of the total depth.
Model for whitening trays
Casts used to make whitening trays are trimmed down much further than
traditional casts and symmetry is not of concern with casts used to make
whitening trays.
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25. Manipulation of Gypsum Products
Metal-plated and epoxy dies and resin-reinforced die
stone
Type IV and V gypsum products are commonly used die
materials
Silver or copper plating can create metal-plated dies that are
highly resistant to abrasion
Type IV and V gypsum are very hard, but they are susceptible to
abrasion during carving of wax patterns.
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26. Manipulation of Gypsum Products
Investment materials:
Used to form metal castings through the lost wax
technique
These materials, which combine gypsum and silica, are
able to produce models sufficiently strong to allow
molten metal to be poured into them.
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Notas do Editor
Refer students to Procedures 16-2 and 16-3 (pp. 293-295 in the textbook).
Refer the students to Procedure 16-4 (pp. 295-296 in the textbook).
Refer students to Procedure 16-6 (pp. 298-299 in the textbook).
