gypsum

Question 1

die

Answer 1

replica of an individual tooth

Question 2

how is gypsum produced

Answer 2

occurs naturally as CaSO4 dihydrate (CaSO4.2H2O), during manufacture this is heated to produce CaSO4 hemihydrate (CaSO4.H2O)

Question 3

how are different types of gypsum produced

Answer 3

by using different heating processes

Question 4

how is dental plaster produced (beta-hemihydrate)

Answer 4

gypsum heated in an open vessel with air readily available. this results in large porous and irregular crystals

Question 5

how is dental stone produced (alpha-hemihydrate)

Answer 5

heated in an autoclave (closed vessel) and produces non porous regular crystals

Question 6

how is improved dental stone produced

Answer 6

same as dental stone i.e in autoclave but in presence of of calcium or magnesium chloride which reduces porosity and sees more compact and smoother particles

Question 7

setting reaction of gypsum products

Answer 7

reverse of manufacture
hemihydrate powder + water - dihydrate cast
excess water trapped in powder mass may produce voids

Question 8

what depth of surface detail can gypsum reproduce

Answer 8

28-40 micrometres

Question 9

describe the process of gypsum setting

Answer 9

hemihydrate dissolves and dihydrate forms
dihydrate precipitates on impurities as crystals
continues until all hemihydrate is dissolved

Question 10

what effect does adding borax to gypsum have

Answer 10

deposits on dihydrate crystals restricting their growth therefore increasing setting time
also decreasesexpansion

Question 11

what effect does adding K2SO4 to gypsum have

Answer 11

encourages crystal growth so decreases setting time

also decreases expansion

Question 12

spatulation effect on setting time

Answer 12

breaks down growing crystals therefore more nuclei of crystallisation so they come into contact faster therefore decreasing setting time

Question 13

what are the downsides to gypsum

Answer 13

poor abrasion resistance, low tensile strength and very brittle