Gypsum

Question 1

what is gypsum used for?

Answer 1

to provide a positive replica of a patients dentition from the impression

Question 2

why are positive replicas of dentition required?

Answer 2

• records the position, shape & dimensions of teeth
• aids visualisation/assessment of dentition
• enables manufacture of dental prostheses

Question 3

what is a cast?

Answer 3

a replica of the patients full dentition (made from plaster/stone)

Question 4

what is a die?

Answer 4

a replica of an individual tooth (made from stone/improved stone)

Question 5

what are the different types of gypsum?

Answer 5

• plaster (ß-hemihydrate)
• dental stone (α-hemihydrate)
• densite (improved stone)

Question 6

what do the different gypsum types depend on?

Answer 6

dependant on the heating process

Question 7

what does the crystalline structure of gypsum determine?

Answer 7

the properties of the material

Question 8

what type of gypsum is heated in an open vessel?

Answer 8

plaster (beta-hemihydrate)

Question 9

what is the structure of plaster?

Answer 9

• large porous

- irregular crystals

Question 10

what type of gypsum is heated in an autoclave?

Answer 10

dental stone (alpha-hemihydrate)

Question 11

what is the structure of dental stone?

Answer 11

• non-porous
• regular crystals
• requires less water

Question 12

what is an autoclave?

Answer 12

closed vessel

Question 13

how is densite (improved stone) heated?

Answer 13

in the presence of calcium chloride & magnesium chloride

Question 14

what is the structure of densite?

Answer 14

• compact smoother particles

Question 15

what is the simplified setting reaction of gypsum?

Answer 15

powder + water = gypsum (study cast)

Question 16

what is the mixing ratio for plaster?

Answer 16

50-60ml water : 100g powder

Question 17

what is the mixing ratio for stone?

Answer 17

20-35ml water : 100g powder

Question 18

what occurs during the overall setting process of gypsum?

Answer 18

• hemihydrate dissolves in water
• dihydrate forms & are pulled towards impurities
• the dihydrate precipitate on impurities as crystals
• more hemihydrate is dissolved
• continues until all hemihydrate is dissolved

Question 19

what do the impurities act as in the setting process of gypsum?

Answer 19

they act as nucleating agents (centres of crystal growth)

Question 20

explain the initial set of the gypsum:

Answer 20

• dihydrate crystals continue to grow and come into contact with each other
• expansion of material starts

Question 21

what are the properties of gypsum like during the initial set?

Answer 21

• weak solid
• will not flow
• can be carved

Question 22

explain the properties of final set gypsum:

Answer 22

• strong and hard enough to be worked

- strength continues to develop

Question 23

what are initial & final setting times of gypsum measured with?

Answer 23

Gilmore needles

Question 24

water is present during the setting stage of gypsum, how does this differ on completion of setting?

Answer 24

• excess water evaporates

- voids produced (porosity)

Question 25

does gypsum reproduce fine detail on the impression material?

Answer 25

• gypsum is porous, resulting in a rough surface (28-49um)

- however this is good enough

Question 26

what is the compressive strength of set gypsum after 1 hour?

Answer 26

28 MPa - 38 MPa

Question 27

what is the compressive strength of gypsum after 24 hours?

Answer 27

approx. 75 MPa

Question 28

what is the strongest type of gypsum?

Answer 28

improved stone (densite)

Question 29

what is the weakest type of stone?

Answer 29

plaster

Question 30

how can the surface hardness of gypsum be described?

Answer 30

• surface hardness is low (easy to abrade)

Question 31

what does an increase in powder (during the mix) result in?

Answer 31

• reduced setting time

- increased expansion

Question 32

what methods can be used to decrease setting time of gypsum?

Answer 32

• increase powder
• increase spatulation
• increase impurities

Question 33

what does spatulation do to gypsum?

Answer 33

breaks down growing crystals

• decreased setting time
• increased expansion

Question 34

how does increasing the temperature affect the solubility of hemihydrate?

Answer 34

lower solubility

Question 35

what are the purpose of chemical additives in gypsum?

Answer 35

used as antiexpansion agents

Question 36

what chemical additives are used in gypsum?

Answer 36

• potassium sulphate

- borax

Question 37

what is the function of potassium sulphate in gypsum?

Answer 37

• produces syngenite
• crystallises rapidly & encourages growth of more crystals
• DECREASES SETTING TIME

Question 38

what is the function of borax in gypsum?

Answer 38

• forms calcium borate
• deposits on dihydrate crystals (restricting their growth)
• delays setting process (INCREASES SETTING TIME)

Question 39

why must you ‘wet’ the impression surface before casting?

Answer 39

• so there is no resistance to flow over surface

- avoids bubble formation

Question 40

what is a weakness of gypsum?

Answer 40

• gypsum is brittle

- has a flexural strength of 15 to 20 MPa

Question 41

what is meant by a brittle material?

Answer 41

a brittle material will readily fracture when subjected to a small amount of strain

Question 42

compare the flexural strength of gypsum to that of composite resin? what does this indicate?

Answer 42

• gypsum = 15 to 20MPa
• composite resin = 150MPa

therefore gypsum is much more brittle than composite resin

Question 43

what are the advantages of gypsum?

Answer 43

• dimensionally accurate and stable
• low expansion (<0.1%) of stone/densite
• good colour contrast

Question 44

what are the disadvantages of gypsum?

Answer 44

• VERY BRITTLE
• low tensile strength
• poor abrasion resistance
• surface detail less than elastomer impression
• poor ‘wetting’ of some impression materials