Gypsum

Question 1

what is a die?

Answer 1

removable section of a model

Question 2

ISO Standard for Dental Gypsum Products
Type 1

Answer 2

impression plaster

Question 3

ISO Standard for Dental Gypsum Products
Type 2

Answer 3

dental plaster for models

Question 4

ISO Standard for Dental Gypsum Products
Type 3

Answer 4

dental stone for die/models

Question 5

ISO Standard for Dental Gypsum Products
Type 4

Answer 5

dental stone, die high strength, low expansion

Question 6

ISO Standard for Dental Gypsum Products
Type 5

Answer 6

dental stone, die high strength, high expansion

Question 7

gypsum products for models and dies 3

Answer 7

dental plaster
dental stone
densite

Question 8

what colour is dental plaster?

Answer 8

white

Question 9

colour of dental stone

Answer 9

yellow

Question 10

colour of densite

Answer 10

blue/pink

Question 11

chemical equation for the manufacture of gypsum product

Answer 11

Calcium sulphate dihydrate -> calcium sulphate hemihydrate + water

Question 12

how to convert gypsum to dental plaster

Answer 12

Heat gypsum at 120*c

Question 13

how to convert gypsum to dental stone

Answer 13

Apply steam pressure in an autoclave to 125*c

Question 14

how to convert gypsum to densite

Answer 14

Boil in CaCl2

Question 15

porosity decreases from which gypsum product

Answer 15

plaster -> stone -> densite

Question 16

particle size increases from which gypsum product

Answer 16

density -> stone -> plaster

Question 17

more water is needed to mix plaster in reality than theoretically because?

Answer 17

lots of air is present in the material increasing porosity

Question 18

air in the plaster mixture leads to 2

Answer 18

loss of detail
weakness

Question 19

air is expelled from plaster by 2

Answer 19

vibrating it
mechanically mixing under a vacuum

Question 20

what are the 3 stages of the setting reaction?

Answer 20

water becomes saturated with hemihydrate

hemihydrate converted to dihydrate form

dihydrate crystallises out, initiated by exposure to nuclei of crystalisation

Question 21

2 forms of nuclei crystallisation

Answer 21

impurities
stray dihydrate crystal in the mix

Question 22

what is used to determine the initial and intermediate set point of a material

Answer 22

Gilmore needles

Question 23

setting is endothermic or exothermic?

Answer 23

exothermic - gives off heat

Question 24

how can the manufacturer decrease setting time? 2

Answer 24

increase concentration of nucleating agents

accelerators e.g. K2SO4 (increase solubility of hemihydrate)

Question 25

how can the manufacturer increase the setting time 1

Answer 25

retarder e.g. borax

Question 26

how can the operator decrease the setting time 1

Answer 26

increase mixing time - breaks up dihydrate crystals so more crystallisation nuclei

Question 27

how can the operator increase the setting time 2

Answer 27

increase water:powder ratio - decreases concentration of crystallisation nuclei borax retards the mix

Question 28

what effect does temperature have on setting time

Answer 28

little affect as increasing water temperature accelerates solution process but retards crystallisation

Question 29

Is dental plaster or stone the strongest and why?

Answer 29

stone as lower porosity

Question 30

the strength of set material increases with time due to

Answer 30

progressive evaporation of water

Question 31

advantages of gypsum products 2

Answer 31

inexpensive easy to use

Question 32

disadvantages of gypsum products 2

Answer 32

May interact with borax in alginate (Rare) Hypochlorite may interact if not rinsed off impression before casting (Rare)