gypsum

Question 1

classification of gypsum

Answer 1

Type 1—Dental plaster for impressions
Type 2—Dental plaster
-Class 1 - for mounting
-Class 2 - for models
Type 3—Dental stone for models
Type 4—Dental stone (high strength, low expansion) for dies
Type 5—Dental stone (high strength, high expansion) for dies

Question 2

manufacture of gypsum products

Answer 2

calcium sulphate dihydrate>110-130C>calcium sulphate hemihydrate>130-200C>hexagonal anhydrate (soluble anhydrite)>200-1000C>orthorhombic anhydrite (insoluble anhydrite)

Question 3

The amount of water that reacts chemically with 100 g of a dental gypsum product is which of the following?

Answer 3

Because all model materials require about 19 mL of water for every 100 g of powder for the conversion of the hemihydrate to the dihydrate, any water over 19 mL is excess.

Question 4

fate of excess water in gypsum chemical reaction.

Answer 4

The presence of excess water has significant consequences on the physical properties of gypsum, as discussed later. After the material is set completely, any excess water eventually evaporates and leaves voids in the mass. These voids reduce the overall density of the material. Therefore, set plaster has the lowest density (most porosity) because it had the most excess water and the most voids in the mass. Set high-strength dental stone has the highest density.

Question 5

normal setting expansion

Answer 5

the expansion that occurs when gypsum or a gypsum-bonded investment sets in ambient air.

Question 6

wet strength & dry strength

Answer 6

the wet strength (also known as green strength)
and the dry strength. The wet strength is the strength that is determined when water in excess of that required for hydration of the hemihydrate remains in the test specimen. When such excess water is removed by drying, the strength obtained is the dry strength. The dry strength may be two or more times as high as the wet strength.

Question 7

hygroscopic setting expansion

Answer 7

the expansion that occurs when gypsum or a gypsum-bonded investment sets while immersed in water (usually heated to approximately 38 °C [100 °F])

Question 8

applications of gypsum products

Answer 8

1. Impression plaster was used extensively in the past for impressions of the mouth and face.
2. Various types of plasters are used to make moulds, casts and dies over which dental
   prostheses and restorations are made.
3. To attach casts to an articulator
4. For bite registration (e.g., to record centric jaw relation).
5. Dental investments Plaster mixed with silica is known as dental investment. They are used to form refractory moulds into which molten metal is cast.