1. Gypsum Materials
DA 122 Dental Materials

2. 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

3. 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

4. Types of Gypsum Materials
See pages in Dental Assisting text

5. Type I: Impression Plaster
original impression material rigid, fractures and breaks easily used for edentulous models Ratio: 60 ml water : 100 gm powder

6. 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

7. 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

8. Orthodontic Stone White
Used for: diagnosis and treatment of orthodontic cases
Ratio: 30 ml of water : 100 gm powder

9. 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

10. Type V: High-strength Die Stone
High strength and high expansion
Recently added by the ADA
Strongest accepted gypsum product
Ratio: ml water : 100 gms powder

11. 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

12. Armamentarium Flexible rubber bowl Stiff narrow spatula
Water measuring device
Scale
Model vibrator, with protective covering
Room temperature water

13. 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)

14. Mixing Techniques Using vibrator to remove air bubbles
Checking for proper thickness

15. Caution re: mixing DO NOT WHIP: will add air and bubbles
DO NOT OVERMIX: will add weak spots to model

16. Setting Process (dihydration)
Exothermic reaction (release of heat) drives off water to harden gypsum material
Material will heat up and then cool down

17. 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

18. 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)

19. Microbes can live within gypsum for 7 days
PPE very important!