gypsum

Question 1

uses

Answer 1

cast (plaster/stone)
die (stone/improved stone)
mould material (stone)
investment binder (stone)

Question 2

manufacture

Answer 2

calcium sulphate dihydrate

heat

calcium sulphate hemihydrate

Question 3

setting reaction

Answer 3

reverse of manufacture

hemihydrate + H2O - dihydrate

Question 4

types

Answer 4

plaster
stone
densite

Question 5

plaster

Answer 5

B-hemihydrate

heated in open vessel
large, porous, irregular crystals

Question 6

stone

Answer 6

a-hemihydrate

heated in an autoclave
non-porous, regular crystals
requires less H2O

Question 7

densite

Answer 7

improved stone

heated in presence of Ca and Mg chloride
compact smooth particles

Question 8

why is excess water needed and what is the consequence?

Answer 8

for a workable mass

affects properties

Question 9

plaster mixing ratios

Answer 9

50-60ml water

100g powder

Question 10

stone mixing ratios

Answer 10

20-35ml water

100g powder

Question 11

mixing theoretical ratios

Answer 11

18.6ml water

100g powder

Question 12

setting process

Answer 12

hemihydrate dissolves
dihydrate forms
 - solubility low - supersaturated solution
dihydrate crystals ppt on impurities
 - nuclei of crystallisation 

more hemihydrate dissolved, continues until all hemihydrate dissolved

initial set
 - contact each other - push apart
 - expansion
final set
 - strong and hard
 - Gilmore needles

excess water evaporates - voids (porosity) - weakened

Question 13

properties

Answer 13

compressive strength low
low hardness (develops 24hrs)
density strongest, plaster weakest - needs the most H2O for a workable mass (porous)
expansion
- plaster 0.2-0.3%
- densite 0.05-0.07%
want small % expansion
needs to be compatible with impression material
surface detail
- inherently porous so relatively rough surface 28-40um

Question 14

spatulation

Answer 14

breaks down growing crystals - act as nuclei of crystallisation
more growing crystals so come into contact sooner
reduced setting time, increased expansion

Question 15

more powder

Answer 15

more nuclei of crystallisation per unit vol
crystals into contact sooner
reduced setting time, increased expansion

Question 16

K2SO4

Answer 16

produces syngenite
crystallises rapidly - encourages growth of more crystals
reduced setting time

Question 17

BORAX

Answer 17

forms calcium borate - deposits on dihydrate crystals

increased setting time

Question 18

temp

Answer 18

conflicting mechanisms
rate of diffusion of ions increases with increased temp
solubility of hemihydrate decreases with increased temp
U graph

Question 19

advantages

Answer 19

dimensionally accurate and stable
low expansion of stone/density
good colour contrast

Question 20

disadvantages

Answer 20

low tensile strength
poor abrasion resistance
v brittle
surface detail less than elastomer impression
poor ‘wetting’ of some impression materials