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BDS2 CLINICAL Dental Materials > Gypsum > Flashcards

Flashcards in Gypsum Deck (49)
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1

What is a study cast

a positive replica of the dentition

2

What are study casts produced from

impressions

3

What is the purpose of a study cast

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

4

What is the use of gypsum

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

5

How is gypsum made

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

6

What determines the type of gypsum

manufacturing conditions

7

What are the types of gypsum

plaster
dental stone
densite

8

What determines the properties of the different types of gypsum

crystalline structure

9

What is beta-hemihydrate

plaster

10

What is alpha-hemihydrate

dental stone

11

What is improved stone

densite

12

What is plaster heated in

an open vessel

13

What is the crystallite structure of plaster

large, porous, irregular crystals

14

What is the crystallite structure of dental stone

non-porous, regular crystals, requires less water

15

How is dental stone heated

in an autoclave

16

How is densite heated

in presence of calcium and magnesium chloride

17

What is the structure of densite

compact smoother particles

18

What is the setting reaction

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

19

Why is excess water used in the mixing ratios

it is needed for a workable mass
affects properties

20

What is the mixing ratio for plaster

50-60ml
100g

21

What is the mixing ratio for stone

20-35ml
100g

22

What is the theoretical ratio

18.6ml
100g

23

Describe the setting process

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

24

Describe the initial set of gypsum in the setting process

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

25

Describe the final set of gypsum

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

26

What happens to water during setting

excess water is trapped in the powder mass

27

What happens to water on completion of setting

excess evaporates
voids produced - not good

28

What is the compressive strength of gypsum

20-35 MPa

29

What gypsum has the strongest compressive strength

densite with around 38 MPa

30

Why is stone stronger than plaster

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