11.1 dental plaster and stones

What are dental plasters and stones used for in dentistry

They are used to make models of teeth from impressions

Why are study casts made from dental plaster

Dentists use them to monitor changes in teeth over time

Why are working casts made from dental stone

Dental technicians use them to make dentures crowns and bridges

What newer technology is replacing plaster models in some cases

Three dimensional digital models

What is gypsum

Gypsum is calcium sulfate dihydrate with the chemical formula CaSO4·2H2O

What is plaster of Paris

Plaster of Paris is calcium sulfate hemihydrate with the chemical formula CaSO4·½H2O

Where does the name plaster of Paris come from

It was historically mined in Paris

What ancient structure is plaster of Paris believed to have been used on

The Temple of Solomon

When was plaster of Paris historically used

Between the 10th and 6th century BCE

What chemical reaction occurs when plaster of Paris is mixed with water

Calcium sulfate hemihydrate reacts with water to form calcium sulfate dihydrate

Write the chemical equation for plaster setting

2CaSO4·½H2O plus 3H2O forms 2CaSO4·2H2O plus heat

Is the plaster setting reaction exothermic or endothermic

It is exothermic meaning heat is released

What happens when gypsum is heated

It loses water and changes phase

At what temperature does gypsum convert to hemihydrate

Between 110 and 130 degrees Celsius

At what temperature does hemihydrate convert to anhydrite

Between 130 and 200 degrees Celsius

What happens to calcium sulfate above 200 degrees Celsius

It forms anhydrite in different crystal structures

What is the difference between alpha and beta hemihydrate

They have different crystal shapes and densities not different phases

Which hemihydrate requires less water for mixing

Alpha hemihydrate

Which hemihydrate has cubic crystals and lower surface area

Alpha hemihydrate

Which hemihydrate gives stronger material

Beta hemihydrate

Which hemihydrate has needle like acicular crystals

Beta hemihydrate

Which hemihydrate releases more heat during setting

Beta hemihydrate

How much heat does alpha hemihydrate release

17.2 kilojoules per mole

How much heat does beta hemihydrate release

19.3 kilojoules per mole

What happens when water is added to hemihydrate powder

It forms a slurry and some dissolves

What happens after hemihydrate dissolves

It reprecipitates as dihydrate

Why does a supersaturated solution form

Because dihydrate has lower solubility

What forms after supersaturation

Nuclei form and crystals precipitate

What is impression plaster

A low strength quick setting porous plaster

Why is impression plaster rarely used today

It has been replaced by hydrocolloids and elastomers

What is model plaster

A medium strength plaster with some setting contraction

What material is model plaster made from

Hydrocal

What is dental stone

A high strength low contraction gypsum material

What material is dental stone made from

Densite

Why is crystal interlocking important

It increases strength

Why do cement lorries rotate

To prevent crystals growing large enough to entangle and cause setting

How are hydrocal crystals produced

By dehydrating gypsum under pressure at 125 degrees Celsius with water vapour

What are the properties of hydrocal crystals

High density uniform needle like crystals

How are densite crystals produced

By boiling gypsum in calcium chloride solution then washing drying and grinding

What shape are densite crystals

Dense cubic crystals

What effect does drying time have on compressive strength

Longer drying increases strength

What happens to plaster weight during drying

Weight decreases as water is lost

What is water to powder ratio

The weight or volume of water divided by weight of powder

How does lower water to powder ratio affect crystals

Crystals become broader and stronger

How does lower water to powder ratio affect mechanical properties

It increases strength

How many gypsum types does the ADA define

Five

What is Type I gypsum

Impression plaster

What is Type II gypsum

Model plaster

What is Type III gypsum

Dental stone

What is Type IV gypsum

High strength low expansion dental stone

What is Type V gypsum

High strength high expansion dental stone

Why does water increase setting expansion

It allows more space between crystals

What is an accelerator in plaster

A substance that increases setting rate

What is potassium sulfate used for

Accelerating setting by increasing nuclei

What is terra alba

Set calcium sulfate dihydrate used as accelerator

What is a retarder

A substance that slows the setting reaction

What is sodium citrate used for

Retarding plaster setting

What is borax used for

Retarding plaster setting

How does borax work

It forms an insoluble layer preventing dissolution

What compound does borax form

Colemanite

How does sodium chloride affect setting

Low concentrations accelerate high concentrations retard

What concentration of sodium chloride accelerates

Less than 2 percent

What concentration of sodium chloride retards

More than 2 percent

What is sodium sulfate used for

Accelerating plaster setting

What tool measures plaster setting time

Gillmore or Vicat needles

How do Gillmore needles work

They indent the surface under known load

What indicates initial set with Gillmore needle

The material resists the initial needle

What indicates final set with Gillmore needle

The material resists the final needle

What does the Vicat needle measure

Intermediate setting time

How is compressive strength tested

Using cylindrical samples loaded in compression

How do plaster samples usually fail

By longitudinal fracture due to tensile stress

How is surface hardness measured

By indentation tests

What hardness tests are used

Knoop and Vickers

What is abrasion resistance

Resistance to scratching

How is tensile strength measured

Using Brazilian disc test

How is reproduction of detail tested

Using metal blocks with fine grooves

What groove sizes are commonly used

50 or 75 micrometers

What causes poor final stone moulds

Air bubbles and poor mixing

What standards apply to dental plasters

ISO 9917