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34 Cards in this Set
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Gypsum
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Calcium Sulfate Dihydrate
Chemistry: CaSO4·2(H2O) Uses: plaster, wall board, some cements, fertilizer, paint filler, ornamental stone, etc.. Where found: Mexico; Sicily; Utah and Colorado, USA; & other places throughout the world. |
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Dihydrate
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Material consisting of two parts water to one part of compound.
Calcium sulfate dihydrate, is 1 part calcium sulfate & 2 parts water. |
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Calcining a process of converting gypsum into plaster or stone
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gypsum is ground to fine powder particles.
Plaster is produces when the gypsum is heated in an open vat (~115°C). Artificial stone is produced when gypsum is processed by steam under pressure. (even more heat will produce die stone) **With both products, the reaction converts calcium sulfate dihydrate into calcium sulfate hemihydrate by removal of 75% of the water molecules. |
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Storage
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Gypsum products can absorb water from the environment.
Humidity & close proximity to water sources will adversely affect the powder. Gypsum should be stored in airtight, moisture-proof containers. |
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plaster & stone are identical in which way
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chemically
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Plaster particles are
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rough, irregular, & porous, called model plaster or β hemihydrate
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Stone particles are
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prismatic, regular in size, & dense called Α hemihydrate
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Larger, irregular particles make:
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a weaker stone
Break easily Abrade easily |
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Smaller, more uniform particles are:
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stronger
Do not break easily Do not abrade |
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What two factors contribute to strength & abrasion resistance of the final product:
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shape of the particles & porosity
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strength of gypsum products is related to
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the amount of water used to produce study model or working cast.
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Factors that affect strength of gypsum products also affect
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their hardness
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Increased porosity of particles makes it necessary to use
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more water to convert hemihydrate particles back to dihydrate particles.
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Setting expansion occurs with
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all gypsum products
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Plaster has highest rate
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of expansion
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Stone has lowest rate
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expansion
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Setting expansion is the result of
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growth of crystals as they join together
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Classification of Gypsum Products
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Impression Plaster (type I)
Model Plaster (type II) (white) Dental Stone (type III) (yellow) Dental Stone (type IV) (pink/green) High-Strength, High-Expansion Dental Stone (type V) (blue/green) |
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Impression Plaster (type I)
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rarely used today
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Model Plaster (type II) (white)
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used for diagnostic casts
Durable but relatively weak material |
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Dental Stone (type III) (yellow)
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Ideal for making full or partial denture models, orthodontic models, & casts
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Dental Stone (type IV) (pink/green)
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Often referred to as die stone
Used to fabricate crowns & bridges |
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High-Strength, High-Expansion Dental Stone (type V) (blue/green)
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Good to use with the newer base metals because of casting shrinkage
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the calcining process is reversed
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When plaster or stone is mixed with water, a hard substance is formed
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set material is strong & rigid when
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the setting reaction, crystals of gypsum intermesh & become entangled with one another
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When a hemihydrate is mixed with water
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it becomes a flowable mixture.
An exothermic chemical reaction occurs, & the material hardens. |
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The greater the porosity of the final gypsum product
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the less surface detail it has
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Set gypsum products are not
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highly soluble in water
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Solubility is directly related to the porosity of the material; therefore
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plaster is more soluble than stone.
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Powder-to-Liquid Ratio
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the mix needs sufficient flow to reproduce details of impression.
Water should be measured with a graduated cylinder, & powder should be weighed on a scale. Too much water makes mix weak. Too little water, & the mix will not flow. |
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Uses for Plaster
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Pouring casts, making matrices for prosthodontic restorations, attaching casts to articulators, & general use in the dental laboratory where strength is not important.
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Mixing Plaster
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Follow water-powder ratios
Use a clean, dry rubber bowl & spatula to manipulate materials. Add measured room termperature water to bowl first. Powder is weighed & sifted into water to avoid trapping air. spatulate 60 seconds in a knifing or stirring motion, making sure to include all powder from sides of bowl. (Whipping the mix will entrap air & should be avoided.) Before mixture is poured, it should be vibrated a few seconds to remove any trapped air bubbles. After mixing, ~ 5 minutes working time |
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Initial Setting Time–Working Time For Plaster
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After mixing for 1 minute, working time begins. During this time, the semifluid mixture is poured into impression on a mechanical vibrator.
As viscosity of mixture increases, flow characteristics will be decreased & product loses its glossy appearance. loss of gloss indicates that gypsum has reached its initial set. Model will begin to give off exothermic heat of setting reaction |
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Final Set For Plaster
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final set- once material is hard & exothermic reaction cools completely.
manufacturers recommend waiting about 1 hour before cast is separated from impression or mold. If model is removed too soon teeth will break. If left too long, impression will dry out & possibly break off the teeth when removing. |