Chapter 9 Gypsum Flashcards
1
Q
What is calcination ?
A
- heating gypsum rock and driving off water
- different heating methods= different results in gypsums
1
Q
What is gypsum?
A
Made from gypsum rock
Calcium sulfate hemihydrate
2
Q
What are the three types of gypsum?
A
- Plaster
- Stone
- Improved Stone
3
Q
Plaster is the ?
A
- First gypsum product
- it is porous, irregular particles
- weakest
** BETA-hemihydrate, type ii
4
Q
Stone is ?
A
- uniform particles, less porous
- stronger than plaster
**ALPHA-hemihydrate, type iii stone/hydrocal
5
Q
Improved stone is?
A
- High strength stone
- very dense particle, cubodial in shape
- STRONGEST most expensive
- TYPE IV stone, die stone modified alpha-hemihydrate
6
Q
What are the setting times for stone?
A
initial= start of mix to semi-hard
can manipulate product during the initial set
Final= rigid
can be separated from impression
7
Q
Increasing setting time =
A
a slower setting product
* decreased mixing
* higher water/powder ratio (thin mix)
* Addition of chemical retarders
8
Q
decreasing setting time=
A
- ( a faster-setting product)
- increased mixing
- lower water/powder ratio (thick mix)
- addition of chemical accelerators (Potassium sulfate)
9
Q
What are the techniques for mixing gypsum stone?
A
- Measure water
- measure powder
- add powder to water
- mix
- fill the impression
