Gypsum
No description yet. This curated deck is ready for daily study.
Card preview (24)
what produces a positive replica of the dentition?
study model/cast
what is the purpose of a study model?
to record position, shape and dimensions of teeth, aid visualisation/assessment of dentition, enables manufacture of dental prostheses
what are the uses of gypsum?
cast, die, mould material, investment binder
what are the 3 types of gypsum?
plaster, dental stone, densite (improved stone)
what determines the properties of gypsum?
the crystalline structure
where is plaster heated?
in an open vessel
what does plaster look like?
large porous, irregular crystals
where is dental stone heated?
in an autoclave
what does dental stone look like?
non-porous, regular crystals, requires less water
where is densite heated?
in presence of Ca and Mg chloride
what does densite look like?
compact smoother particles
how are the different types of gypsum produced?
dihydrate is heated in different environments to produce the different types of gypsum which is present in hemihydrate form
what happens in the setting reaction of gypsum?
the reverse of the manufacture reaction - hemihydrate turns to dihydrate
with 100g of powder how much water is needed to make plaster?
50-60ml
with 100g of powder how much water is needed to make stone?
20-35ml
what is the theoretical ratio for mixing powder and water?
18.6ml water and 100g powder
what happens in the setting process of gypsum?
hemihydrate dissolved, dihydrate forms, dihydrate solubility is low-supersturated
when the setting process occurs what do dihydrate crystals do?
precipitate on impurities as crystals
what occurs in the initial set of gypsum?
dihydrate crystals come into contact, expansion starts, properties of weak solid and will not flow, can be carved
what is the state of the gypsum when it has finally set?
strong and hard enough to be worked and strength continues to develop
what happens to water during setting?
excess water is trapped in the powder mass
what happens to water on completion of setting?
excess water evaporates, voids produced so porosity occurs
what are the properties needed to consider with gypsum?
reproduction of surface detail, strength, surface hardness, setting time, expansion
what results from gypsum being inherently porous?
a rough surface f about 28-40um