Gypsum Flashcards
gypsum
soft sulphate, made of calcium sulphate, casts
gypsum uses
study model/cast - positive replica of dentition
cast, die, mould material
types of gypsum
plaster - heated in open vessel, large porous
dental stone - heated in autoclave, regular crystals
densite - heated in presence of chloride, compact particles
crystalline structure determines properties
setting reaction
dihydrate crystals form, expansion occurs
properties of gypsum
reproduction of surface detail, strength, surface hardness, setting time, expansion, porous [rough surface], low surface hardness
develops over 24hr period, dendrite the stronges
adv and disadv of gypsum
adv - dimensionally accurate and stable, low expansion of stone/densite, good colour contrast
disadv - low tensile strength, poor abrasion resistanc,e very brittle, surface detail less than elastomer impression, poor wetting of some impression materials