gypsum Flashcards
what is a study model?
positive replica of dentition
produced from the impression
what is the purpose of a study model?
records position, shape and dimensions of teeth
aids visualisation/assessment of dentition
enables manufacture of dental prostheses
-partial denture framework, crowns
what are the uses of gypsum?
cast (plaster/stone)
die (stone/improved stone)
mould material (stone)
investment binder (stone)
describe the manufacture of gypsum
calcium sulphate dihydrate heated to become calcium sulphate hemihydrate and release water
what are the 3 types of gypsum?
plaster
dental stone
densite (improved stone)
what determines gypsum properties?
crystalline structure
describe plaster
heated in open vessel
large porous, irregular crystals
describe stone
heated in autoclave
non-porous, regular crystals, requires less water
describe densite
heated in presence of Ca and Mg chloride
compact smoother particles
how does gypsum become the different types?
processed in different ways influences structure
describe the setting reaction (reverse of manufacture)
hemihydrate powder + water = gypsum
what is the water to powder ratio for plaster?
50-60ml to 100g
what is the water to powder ratio for stone?
20-35 ml to 100g
what happens during the setting process?
hemihydrate dissolves
dihydrate forms
dihydrate solubility low-supersaturated solution
dihydrate crystals precipitate on impurities as crystals
more hemihydrate dissolved
continues until all hemihydrate dissolved
describe the initial set
dihydrate crystals come together -grow until contact each other expansion starts properties of weak solid will not flow can be carved can be carved