Dental Materials Gypsum

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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.