Gypsum

Question 1

what produces a positive replica of the dentition?

Answer 1

study model/cast

Question 2

what is the purpose of a study model?

Answer 2

to record position, shape and dimensions of teeth, aid visualisation/assessment of dentition, enables manufacture of dental prostheses

Question 3

what are the uses of gypsum?

Answer 3

cast, die, mould material, investment binder

Question 4

what are the 3 types of gypsum?

Answer 4

plaster, dental stone, densite (improved stone)

Question 5

what determines the properties of gypsum?

Answer 5

the crystalline structure

Question 6

where is plaster heated?

Answer 6

in an open vessel

Question 7

what does plaster look like?

Answer 7

large porous, irregular crystals

Question 8

where is dental stone heated?

Answer 8

in an autoclave

Question 9

what does dental stone look like?

Answer 9

non-porous, regular crystals, requires less water

Question 10

where is densite heated?

Answer 10

in presence of Ca and Mg chloride

Question 11

what does densite look like?

Answer 11

compact smoother particles

Question 12

how are the different types of gypsum produced?

Answer 12

dihydrate is heated in different environments to produce the different types of gypsum which is present in hemihydrate form

Question 13

what happens in the setting reaction of gypsum?

Answer 13

the reverse of the manufacture reaction - hemihydrate turns to dihydrate

Question 14

with 100g of powder how much water is needed to make plaster?

Answer 14

50-60ml

Question 15

with 100g of powder how much water is needed to make stone?

Answer 15

20-35ml

Question 16

what is the theoretical ratio for mixing powder and water?

Answer 16

18.6ml water and 100g powder

Question 17

what happens in the setting process of gypsum?

Answer 17

hemihydrate dissolved, dihydrate forms, dihydrate solubility is low-supersturated

Question 18

when the setting process occurs what do dihydrate crystals do?

Answer 18

precipitate on impurities as crystals

Question 19

what occurs in the initial set of gypsum?

Answer 19

dihydrate crystals come into contact, expansion starts, properties of weak solid and will not flow, can be carved

Question 20

what is the state of the gypsum when it has finally set?

Answer 20

strong and hard enough to be worked and strength continues to develop

Question 21

what happens to water during setting?

Answer 21

excess water is trapped in the powder mass

Question 22

what happens to water on completion of setting?

Answer 22

excess water evaporates, voids produced so porosity occurs

Question 23

what are the properties needed to consider with gypsum?

Answer 23

reproduction of surface detail, strength, surface hardness, setting time, expansion

Question 24

what results from gypsum being inherently porous?

Answer 24

a rough surface f about 28-40um

Question 25

what is the impression material standard?

Answer 25

material must reproduce a 50um wide groove

Question 26

what type of gypsum is the strongest?

Answer 26

improved stone

Question 27

what is the final compressive strength of gypsum?

Answer 27

75MPa

Question 28

what is the surface hardness?

Answer 28

low so easily abraded which is not ideal

Question 29

what should expansion on setting be?

Answer 29

low to minimise dimensional inaccuracy

Question 30

what does increased spatulation do?

Answer 30

breaks down growing crystals to nuclei of crystallisation allowing more growing crystals to come into contact sooner and decrease setting time with increased expansion

Question 31

what does increasing the powder do?

Answer 31

introduce more nuclei of crystallisation so the crystals come into contact sooner and have a faster set and greater expansion

Question 32

which type of gypsum has the smallest expansion on setting percentage?

Answer 32

densite

Question 33

what does expansion on setting allow?

Answer 33

for models to be a little big so crowns and bridges and dentures wont be too tight

Question 34

what happens to the rate of diffusion of ions with increasing temperature?

Answer 34

increases

Question 35

what happens to the solubility of hemihydrate with increasing temperature

Answer 35

decreases

Question 36

what are the chemical additives added to gypsum?

Answer 36

potassium sulphate and borax

Question 37

what does potassium sulphate do?

Answer 37

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

Question 38

what does borax do?

Answer 38

forms calcium borate and delays setting process

Question 39

what does dental stone model surface detail depend on?

Answer 39

type of impression material

Question 40

what is a weakness of gypsum?

Answer 40

it is brittle

Question 41

what does brittle mean?

Answer 41

it will readily fracture when subjected to a small amount of strain

Question 42

how is brittleness measured?

Answer 42

flexural strength

Question 43

what is the flexural strength of gypsum?

Answer 43

15 to 20MPa

Question 44

what are the advantages of gypsum?

Answer 44

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

Question 45

what are the disadvantages of gypsum?

Answer 45

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