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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!