Gypsum

Question 1

what’s the difference between a model and a die

Answer 1

• model: representation of the entire dental arch

- die: removable section of a model used to make crowns

Question 2

model and die products are made from gypsum, state the name of these model/die products

Answer 2

• dental plaster
• dental stone
• densite

Question 3

how are the model/die products made from gypsum

Answer 3

• dental plaster: gypsum in 120C
• dental stone: gypsum in steam pressure, autoclaved at 125C
• densite: boil in CaCl2

Question 4

what is porosity?

Answer 4

the measure of void space in a material, in this case, linked with the amount of water and air uptake by the materials. the higher the porosity, the weaker the material. Impression materials with high porosity suffer from loss of fine detail, however, can be overcome by mixing the material under vacuum.

Question 5

rank the die/model material in order of increased porosity

Answer 5

1- densite (lowest porosity)
2- dental stone
3- dental plaster (highest porosity)

hence dental stone is stronger than plaster.

Question 6

the model/die products are differentiated using dyes. what colour is each product dyed?

Answer 6

plaster - white
stone - yellow
densite - blue/pink

Question 7

what are the properties of set materials

Answer 7

• their strength increases w time
• brittle
• low flexural strength
• good dimensional stability
• good detail reproduction
• water-soluble (issue within labs as it surfaces can soften)