DM4 - Gypsum

Question 1

what is type 1 ISO standard dental gypsum products?

Answer 1

impression plaster

Question 2

what is type 2 ISO standard dental gypsum products?

Answer 2

dental plaster for models

Question 3

what is type 3 ISO standard dental gypsum products?

Answer 3

dental stone for die/models

Question 4

what is type 4 ISO standard dental gypsum products?

Answer 4

• dental stone
• die high strength
• low expansion

Question 5

what is type 5 ISO standard dental gypsum products?

Answer 5

• dental stone
• die high strength
• high expansion

Question 6

What is the name for how easily a substance spreads?

Answer 6

good wetting

Question 7

What makes a good wetting?

Answer 7

small contact angle

Question 8

what products are used models/dies?

Answer 8

dental plaster
dental ston e
densite

Question 9

how does gypsum turn to plaster?

Answer 9

120 degrees

Question 10

how does gypsum turn to stone?

Answer 10

By steam pressure in an autoclave to 125 degrees

Question 11

how does gypsum turn to densite?

Answer 11

boil in CaCl2

Question 12

What accounts for the uptake of water in gypsum?

Answer 12

porosity §

Question 13

On mixing what does air incorporation lead to? (3)

Answer 13

• loss os detail
• weakness
• may overcome by vibrating/mechanical mix under vacuum

Question 14

what is porosity?

Answer 14

measure of the void spaces in a material

Question 15

what is the first stage of setting?

Answer 15

water becomes saturated with hemihydrate

Question 16

what is the 2nd stage of setting?

Answer 16

conversion of hemihydrate to dihydrate

Question 17

what is the 3rd stage of setting?

Answer 17

dihydrate crystallises out initiated by exposure to nuclei of crystallisation

Question 18

what is setting accompanied wit?

Answer 18

• increased viscosity
• exothermic
• expansion

Question 19

What kind of set does a Gilmore needle give you?

Answer 19

The intermediate set but NOT the final set

Question 20

How would the manufacturer control setting time? (3)

Answer 20

• increase concentration of nucleating agents to speed up
• use K2SO4 as an accelerator which increases solubility of hemihydrate
• can use borax as a retarder

Question 21

How would the operator control setting time? (3)

Answer 21

• Increase the water to powder ratio. This decreases setting as decreases concentration of crystallisation nuclei
• increase mixing time. This increases set rate due to formation of more nuclei of crystallisation as breaks up dihydrate crystals
• increase temperature. accelerates solution process and retards crystallisation

Question 22

what colour is plaster?

Answer 22

white

Question 23

what colour is stone?

Answer 23

yellow

Question 24

what colour is die stone?

Answer 24

blue/pink

Question 25

What are the set material properties?

Answer 25

• brittle
• low flexural strength
• good dimensional stability
• good detail reproduction
• soluble in water

Question 26

why is stone stronger than plaster?

Answer 26

less porosity

Question 27

What are the advantages of setting materials? (2)

Answer 27

• inexpensive

- easy to use

Question 28

What are the disadvantages of setting materials? (2)

Answer 28

• may interact with borax in alginate

- hypochlorite if not rinsed off impression before casting