Gypsum

Question 1

when poured what is gypsum? what does this allow?

Answer 1

fluid

captures fine detail

Question 2

once set what is gypsum like?

Answer 2

hard and easily fractured

Question 3

what is gypsum compatible with?

Answer 3

all materials

Question 4

ISO defines gypsum into what types?

Answer 4

1 - impression plaster
2 - dental plaster for models
3 - dental stone for die/model
4 - dental stone, die high strength, low expansion
5 - dental stone, die high strength, high expansion

Question 5

making model/die use what?

Answer 5

dental plaster
dental stone
densite

Question 6

what happens to gypsum in the manufacturing process?

Answer 6

gypsum = gypsum product and water

calcium sulphate dihydrate -> calcium sulphate hemihydrate

Question 7

how to make dental plaster form gypsum?

Answer 7

heat to 120 degrees

Question 8

how to make dental stone from gypsum?

Answer 8

steam pressure in autoclave to 125 degrees

Question 9

how to make densite from gypsum?

Answer 9

boil in calcium chloride

Question 10

dental plaster -> dental stone -> densite as move to each what happens?

Answer 10

porosity decreases and particle size decreases

Question 11

what does the porosity of materials account for?

Answer 11

the uptake of water

Question 12

on mixing and air incorporation what happens?

Answer 12

loss of detail
weakness
overcome by vibrating/mechanical mix under vacuum

Question 13

stages of the rapid setting reaction?

Answer 13

1st - water becomes saturated with hemihydrate 0.8%
2nd - conversion of hemihydrate to dihydrate 0.2% sol.
3rd - dihydrate crystallises out initiated by exposure to nuclei of crystallisation

Question 14

what is the setting reaction accompanied by?

Answer 14

increased viscosity - initial/final set. gilmore needles.
exothermic up to 30 degrees
expansion 0.15-0.3% because outward thrust of crystals, may enhance in investments, to good effects by hygroscopic expansion

Question 15

how can manufacturers control the setting time?

Answer 15

increase conc of nucleation agents to speed it up
accelerators - potassium sulphate which increases solubility of hemihydrate
reatrder - borax

Question 16

what else can be used to control expansion?

Answer 16

accelerators and retardars

Question 17

how can the operator control the setting time?

Answer 17

temp - accelerates solution process and retards crystallisation
W/P - increase retards setting as decreases conc of crystallisation nuclei
mixing time - increased = increase in set rate because formation of more nuclei of crystallisation as breaks up dihydrate crystals

Question 18

how are products differentiated?

Answer 18

colours
plaster - white
stone - yellow
die stone - blue/pink

Question 19

why is stone stronger than plaster?

Answer 19

less porosities

Question 20

why does strength increase with time?

Answer 20

progressive evaporation of water

process of precipitation of dissolved dihydrate cements crystals of gypsum together

Question 21

properties of set material?

Answer 21

brittle
low flexural strength
good dimensional stability 
good detail reproduction
soluble in water

Question 22

advantages of gypsum?

Answer 22

inexpensive
easy to use
may electroplate surface to strengthen

Question 23

disadvatges of gypsum?

Answer 23

may interact with borax alginate

hypochlorite may interact if not washed off impression before casting