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Gypsum Materials

DA 122 Dental Materials

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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

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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

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Type I: Impression Plaster

original impression materialrigid, fractures and breaks easilyused for edentulous modelsRatio: 60 ml water : 100 gm powder

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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

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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

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Orthodontic Stone

• White• Used for: diagnosis and treatment of

orthodontic cases• Ratio: 30 ml of water : 100 gm powder

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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

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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

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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

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Armamentarium

• Flexible rubber bowl• Stiff narrow spatula• Water measuring

device• Scale• Model vibrator, with

protective covering• Room temperature

water

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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)

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Mixing Techniques

Using vibrator to remove air bubbles

Checking for proper thickness

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Caution re: mixing

• DO NOT WHIP: will add air and bubbles

• DO NOT OVERMIX: will add weak spots to model

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Setting Process (dihydration)

• Exothermic reaction (release of heat) drives off water to harden gypsum material

• Material will heat up and then cool down

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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

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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)

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Microbes

• Microbes can live within gypsum for 7 days–PPE very important!