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

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 can live within gypsum for 7 days PPE very important!