Gypsum

Question 1

What is a study cast

Answer 1

a positive replica of the dentition

Question 2

What are study casts produced from

Answer 2

impressions

Question 3

What is the purpose of a study cast

Answer 3

records the position, shape of teeth
aids visualisation/assessment of dentition
enables manufacture of dental prostheses (e.g partial dentures, crowns etc)

Question 4

What is the use of gypsum

Answer 4

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

Question 5

How is gypsum made

Answer 5

calcium sulphate dihydrate is heated until calcium sulphate hemihydrate is produced and it is surrounded by water

Question 6

What determines the type of gypsum

Answer 6

manufacturing conditions

Question 7

What are the types of gypsum

Answer 7

plaster
dental stone
densite

Question 8

What determines the properties of the different types of gypsum

Answer 8

crystalline structure

Question 9

What is beta-hemihydrate

Answer 9

plaster

Question 10

What is alpha-hemihydrate

Answer 10

dental stone

Question 11

What is improved stone

Answer 11

densite

Question 12

What is plaster heated in

Answer 12

an open vessel

Question 13

What is the crystallite structure of plaster

Answer 13

large, porous, irregular crystals

Question 14

What is the crystallite structure of dental stone

Answer 14

non-porous, regular crystals, requires less water

Question 15

How is dental stone heated

Answer 15

in an autoclave

Question 16

How is densite heated

Answer 16

in presence of calcium and magnesium chloride

Question 17

What is the structure of densite

Answer 17

compact smoother particles

Question 18

What is the setting reaction

Answer 18

reverse of the manufacturer
hemihydrate + 3H2O –> dihydrate.2H2O
powder + water –> gypsum (study cast)

Question 19

Why is excess water used in the mixing ratios

Answer 19

it is needed for a workable mass

affects properties

Question 20

What is the mixing ratio for plaster

Answer 20

50-60ml

100g

Question 21

What is the mixing ratio for stone

Answer 21

20-35ml

100g

Question 22

What is the theoretical ratio

Answer 22

18.6ml

100g

Question 23

Describe the setting process

Answer 23

hemihydrate dissolves
dihydrate forms but its solubility is low resulting in a super saturated solution
the dihydrate crystals precipitate on impurities as crystals
more hemihydrate dissolves and this continues until all hemihydrate is dissolved

Question 24

Describe the initial set of gypsum in the setting process

Answer 24

dihydrate crystals come into contact and start to push each other apart
expansion starts
properties of weak solid and will not flow
can be carved

Question 25

Describe the final set of gypsum

Answer 25

strong and hard enough to be worked strength continues to develop initial and final setting times measured using gilmore needles

Question 26

What happens to water during setting

Answer 26

excess water is trapped in the powder mass

Question 27

What happens to water on completion of setting

Answer 27

excess evaporates | voids produced - not good

Question 28

What is the compressive strength of gypsum

Answer 28

20-35 MPa

Question 29

What gypsum has the strongest compressive strength

Answer 29

densite with around 38 MPa

Question 30

Why is stone stronger than plaster

Answer 30

stone stronger as it requires less water for workable mix making it less porous

Question 31

Which has the lowest expansion

Answer 31

densite

Question 32

Which has the greatest expansion

Answer 32

plaster

Question 33

How does increased powder effect setting time and expansion

Answer 33

decreased setting time | increased expansion

Question 34

How does increased spatulation effect setting time and expansion

Answer 34

decreased setting time and increased expansion

Question 35

How does increased impurities effect the setting time and expansion

Answer 35

decreased setting time | increased expansion

Question 36

How does an increase in temperature effect setting time

Answer 36

not very predictable | different temperatures have different effects

Question 37

What do chemicals such as borax and NCl effect

Answer 37

increase/decrease setting time | decrease expansion

Question 38

What does increased spatulation do

Answer 38

increased spatulation breaks down growing crystals fragments act as nuclei of crystallization more growing crystals - come into contact sooner

Question 39

What does increasing powder do

Answer 39

more nuclei of crystallization per unit volume crystals come into contact sooner faster set and greater expansion

Question 40

What is the effect of expansion

Answer 40

can result in a bigger model | can allow for slightly bigger so crowns bridges etc won't be too tight

Question 41

What is the effect of an increase temperature on the rate of diffusion of ions

Answer 41

increase in rate of diffusion

Question 42

What is the effect of an increase in temperature on solubility of hemihydrate

Answer 42

decrease in solubility

Question 43

What are the chemical additives

Answer 43

potassium sulphate | borax

Question 44

What does potassium sulphate do

Answer 44

``` produces syngenite (K2(CaSO4)2.H2) crystallites rapidly and encourages growth of more crystals so decreases setting time ```

Question 45

What does borax do

Answer 45

forms calcium borate which deposits on dihydrate crystals and this delays the setting process which increases the setting time

Question 46

Why is compatibility of impression material important

Answer 46

dental stone model surface detail depends on type of impression material needs to be chemically compatible must 'wet' the impression material with no resistance to flow

Question 47

What should gypsum ideally reproduce

Answer 47

fine detail on the impression material | but gypsum is inherently porous resulting in a relatively rough surface

Question 48

What are the advantages of gypsum

Answer 48

dimensionally accurate and stable low expansion of stone/densite good color contrast

Question 49

What are the disadvantages of gypsum

Answer 49

low tensile strength poor abrasion resistance very brittle surface detail less than elastomer impression poor 'wetting' of some impression materials