Gypsum Materials - mPortfolios.net
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Gypsum Materials
DA 122 Dental Materials
Applications for Gypsum
Used to make a model from an impression
Use of model determines type of gypsum used
Gypsums vary in
Strength
Dimensional accuracy
Resistance
Reproduction detail
Water/powder ratio
Setting times
Calcination
Manufacturing process used for
gypsum production
Gypsum is mined rock; then
ground into a fine powder
Powder is heated until specific
amount of water is driven out
of the gypsum
= CALCINATION
Types of Gypsum Materials
See pages 859-860 in Dental Assisting text
Type I: Impression Plaster
original impression material
rigid, fractures and breaks easily
used for edentulous models
Ratio: 60 ml water : 100 gm powder
Type II: Lab Plaster/Model Plaster
Slightly stronger than Type I
Used for: diagnostic casts or study models
Used to fabricate bleach trays, night guards, sports
mouth guards
Used in treatment planning, measuring dentition, legal
record of, comparison of before and after treatment
Ratio: 50 ml water : 100 gm powder
Type III: Lab Stone
Stronger than plaster
Normally yellow (“buff ”)
Used for: study models, working casts, models for
partial and full dentures
Ratio: 30 ml water : 100 gm powder
Orthodontic Stone
White
Used for: diagnosis and treatment of orthodontic
cases
Ratio: 30 ml of water : 100 gm powder
Type IV: Die Stone
Positive replica of prepared tooth
Strong and resistant to abrasion
Most accuracy and detail
Used for: dies and where strong model is needed in casting
metals or making crowns
Ratio:
24 ml water :
100 grams powder
Type V: High-strength Die Stone
High strength and high expansion
Recently added by the ADA
Strongest accepted gypsum product
Ratio: 18-22 ml water : 100 gms powder
Water : Powder Ratio
Less water
Shortens setting time; dry and crumbly
More water
Thin mixture; weaker product with air bubbles
After mix starts: adding water to thin or powder to
thicken =weakens final product
Increasing water temperature = faster set with no affect
on quality of final product
Armamentarium
Flexible rubber bowl
Stiff narrow spatula
Water measuring device
Scale
Model vibrator, with
protective covering
Room temperature water
Mixing Process (hydration)
Add powder to water; let it dissolve into water
Slowly mix particles with spatula (20 seconds)
Add powder to desired creamy, putty-like
thickness (peanut butter) (1 minute)
Material will stick to spatula when lifted from
bowl, when it is proper thickness
Vibrate to remove air bubbles (1 minute)
Mixing Techniques
Using vibrator to remove air
bubbles
Checking for proper
thickness
Caution re: mixing
DO NOT WHIP: will add air and bubbles
DO NOT OVERMIX: will add weak spots to
model
Setting Process (dihydration)
Exothermic reaction (release of heat)
drives off water to harden gypsum
material
Material will heat up and then cool
down
Setting Time (40 – 60 minutes)
Initial Set:
Time between start of spatulation until material loses gloss
Firm and solid enough to handle
Still moist and pliable (cold and wet)
Final Set:
After all heat is driven away
Cool, hard and dry
Factors Affecting Setting Time
Type of gypsum powder
Water and air temperature
Water : powder ratio
Addition of
Retardants (slow down = cold water)
Accelerators (speed up) = warm water OR
SLURRY (mix of watery trimmed plaster residue)
Microbes
Microbes can live within gypsum for 7
days
PPE very important!