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Key Terms Gypsum Products: DENTAL PLASTER AND STONE Gypsum A mineral mined from the Earth. Occurs most widely in a massive form known as rock-gypsum. Also used in homes and industry. 3 types discussed: • Plaster • Stone • Die stone • • • • • • • Gypsum Calcination Plaster Dental Stone Die stone Dihydrate Calcium Sulfate • • • • • • Hemihydrate Alpha hemihydrate Beta hemihydrate Exothermic heat Accelerator Retarder Making Plaster or Stone • Gypsum (calcium sulfate) is mined, ground to a powder and heated (calcined) • Calcination drives off water and changes the original dihydrate form (2 water molecules) to a hemihydrate form( ½ of one water molecule) Making Gypsum Products • Gypsum product stays in its hemihydrate form until water is added back • When water is added, hemihydrate returns to a dihydrate • The new dihydrate forms as a series of interlocking crystals (Crystallization) • As the crystals form, they give off exothermic heat, which equals the amount of heat added during calcination Gypsum in hemihydrate form Water is added to Amount of exothermic hemihydrate to heat given off during change it back to Crystallization = amount dihydrate form added during calcination 1 Plaster • If calcination occurs without pressure, plaster is formed • Plaster is called a beta hemihydrate (Type II) • Plaster is white • Plaster is used for study models and working models on which splints and orthodontic appliances are made Dental Stone Label on box of plaster Die Stone • Die stone is regular stone that has been modified so it is stronger and harder • Die stone can be any color except white (plaster) or yellow (regular stone) • Restorations are made on a die (a working replica of a single tooth) • If calcination occurs under steam pressure, an alpha hemihydrate is formed (Type III Stone) • Plaster is usually white in color, stone is usually yellow • Stone is harder and stronger (and more expensive) than plaster •Is used for products that require strength such as Removable Prosthodontics Crystallization Die Water/Powder Ratio • Proportion of water to powder used to make a workable mix of a particular gypsum • Depends on the physical characteristics of the powder particles • Has direct result on the properties of the gypsum and must be controlled for optimum results Plaster crystals are large so more porous, not strong, takes the most water to mix (Mix 45-50 ml H20 per 100 g. powder) Stone crystals are smaller, less porous, strong, takes less water to mix (Mix 28-30 ml H20 per 100 g. powder) Die stone crystals smallest, strongest, takes least water to mix (Mix 19-24 ml H20 per 100 g. powder) Altering/Controlling Setting Time • Accelerators speed setting • Best accelerator is potassium sulfate • Can also accelerate by -Decreasing H20 • Retarders slow setting • Best retarder is borax or sodium citrate • Can also retard by - Cooling H2O - Increasing amount of H2O -Warming H20 -Mixing vigorously 2 Mixing Gypsum • Measure the water • Measure the powder • Add powder to water to avoid incorporation of air • Mix/wipe against side of bowl (don’t whip) -hand mixing/vibrator -Vacuum mixing • Filling tray/pouring model Care of Gypsum • Plaster and stone should be stored in airtight containers due to sensitivity to humidity • Exposure to moisture or humidity can cause setting reaction to occur • Gypsum is soluble in water; fine detail/dimension can be lost Questions? 3