Gypsum
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what is gypsum used for?
to provide a positive replica of a patients dentition from the impression
why are positive replicas of dentition required?
• records the position, shape & dimensions of teeth • aids visualisation/assessment of dentition • enables manufacture of dental prostheses
what is a cast?
a replica of the patients full dentition (made from plaster/stone)
what is a die?
a replica of an individual tooth (made from stone/improved stone)
what are the different types of gypsum?
• plaster (ß-hemihydrate) • dental stone (α-hemihydrate) • densite (improved stone)
what do the different gypsum types depend on?
dependant on the heating process
what does the crystalline structure of gypsum determine?
the properties of the material
what type of gypsum is heated in an open vessel?
plaster (beta-hemihydrate)
what is the structure of plaster?
• large porous - irregular crystals
what type of gypsum is heated in an autoclave?
dental stone (alpha-hemihydrate)
what is the structure of dental stone?
• non-porous • regular crystals • requires less water
what is an autoclave?
closed vessel
how is densite (improved stone) heated?
in the presence of calcium chloride & magnesium chloride
what is the structure of densite?
• compact smoother particles
what is the simplified setting reaction of gypsum?
powder + water = gypsum (study cast)
what is the mixing ratio for plaster?
50-60ml water : 100g powder
what is the mixing ratio for stone?
20-35ml water : 100g powder
what occurs during the overall setting process of gypsum?
• hemihydrate dissolves in water • dihydrate forms & are pulled towards impurities • the dihydrate precipitate on impurities as crystals • more hemihydrate is dissolved • continues until all hemihydrate is dissolved
what do the impurities act as in the setting process of gypsum?
they act as nucleating agents (centres of crystal growth)
explain the initial set of the gypsum:
• dihydrate crystals continue to grow and come into contact with each other • expansion of material starts
what are the properties of gypsum like during the initial set?
• weak solid • will not flow • can be carved
explain the properties of final set gypsum:
• strong and hard enough to be worked - strength continues to develop
what are initial & final setting times of gypsum measured with?
Gilmore needles
water is present during the setting stage of gypsum, how does this differ on completion of setting?
• excess water evaporates - voids produced (porosity)