DMS- gypsum Flashcards
Compare the 3 types of gypsum?
- Plaster- produced in an open container- larger porous irregular crystals
- Dental stone- produced in an autoclave- non porous regular crystals.
- Improved stone- produced in presence of Ca and Mg chloride- produces compact smoother particles.
How does the change in the liquid/ powder ratio affect setting?
More powder= more crystalites so more likely to contact each other & cause expansion more quickly.
Describe the setting reaction of gypsum?
- Hemihydrate is dissolved
- Dihydrate is formed (supersaturated and low solubility)
- Dihydrate crystalites precipitate on impurities as crystals.
- Initial reaction- crystals come in contact with each other &push each other apart- causing expansion.
- Material is a weak solid- cannot flow but can be carved.
- Final reaction- material is strong and hard enough to be worked- strength continues to develop.
Why do porosities occur in the gypsum?
Because water is needed for the setting reaction.
However, once this setting reaction is complete the water evaporates leaving pores.
Why is setting of gypsum not affected by temperature?
The changes cancel each other out:
Increase in the crystalities
But decrease in the solubility (or vice versa)
Discuss the properties of Gypsum?
Low compressive strength
Low hardness
Very brittle.
why is improved stone the strongest?
It uses the least volume of water for the mix, so once the material is set, less pores are left.
How can we change the setting time of gypsum?
Spatulation- breaking up of the crystallites = more crystalites to come in contact= quicker setting
Decreased by pottasium sulphate (growth of more crystals)
Increased by BORAX- forms calcium borate deposists on the dihydrate crystals.
Discuss the accuracy of gypsum?
(40)It is more accurate than the impression material standard (Not as accurate as the elastomer impression (20)
It reproduces the fine details.
It is porous- causing a rough surface.
