Gypsum Flashcards
(35 cards)
How does a gypsum cast replicate the dentition?
Provides a positive replica of the dentition (as opposed to negative i.e impression)
What are the main functions of gypsum study casts?
- Records the position, shape and dimensions of teeth
- aids visualisation/assessment of dentition
- Enables manufacture of dental prostheses
- e.g denture framework, crowns etc
What are some uses of gypsum?
- Cast (plaster/stone)
- Die (stone/improved stone)
- Mould material (stone)
- Investment binder (stone)
How is gypsum manufactured?
- Calcium sulphate do hydrate is heated to produce calcium sulphate hemihydrate
What are the 3 main types of gypsum?
- Plaster (beta-hemihydrate)
- dental stone (alpha-hemihydrate)
- Densite (improves stone)
How is gypsum plaster produced?
What structure does it have?
- Heated in open vessel
- creates powder with large porous, irregular crystalline structure
How is dental stone produced?
What is it’s structure?
- Heated in an autoclave
- non-porous, regular crystalline structure (requires less water)
How is densite produced?
What is its structure?
- Heated in presence of calcium chloride and magnesium chloride
- compact smooth particles
How is gypsum study cast material produced?
- Reverse of manufacturing process
- water is added to the powder to produce the setting material
What are the mixing ratios of plaster and stone?
Water Powder
Plaster 50-60ml 100g
Stone 20-35ml 100g
The theoretical mixing ratio is 18.6ml of water to 100g of powder - why is this less than the values used?
- In practice excess water is needed for a workable mass and correct consistency
Summarise the setting process of gypsum
- Hemihydrate dissolves in water
- Dihydrate forms
- Dihydrate solubility los = supersaturated solution
- Impurities are present
- Dihydrate crystals precipitate on impurities as crystals
- More hemihydrate dissolved
- Continues until all hemihydrate is dissolved
Describe the initial set of gypsum and the properties of the material at this stage
- Dihydrate crystals come into contact - ie push apart
- Expansion starts
- Properties of weak solid and will not flow
- Can be carved
Describe the properties of gypsum material at the final setting stage
- Strong and hard enough to be worked
- strength continues to develop
What is used to measure initial and final setting times?
Gilmore needles
What happens to excess water during the setting reaction?
- During setting, excess water is trapped in the powder mass
- On completion of setting
- excess water evaporates
- voids are produced (porosity)
What properties are important to consider for gypsum?
- Reproduction of surface detail
- Strength
- Surface hardness
- Setting time
- Expansion
What width of groove can be reproduced by gypsum?
Is this suitable?
28-40microns
- yes, a suitable material must reproduce a 50micron wide groove
What is the compressive strength of gypsum stone and densite after:
1 hour
24 hours
- after 1 hour - 28-38 MPa
* after 24 hours - 75 MPa
What factors reduce setting time of gypsum ?
- Increased powder
- Increased spatulation
- Increased impurities
(These 3 also increase expansion) - Potassium sulphate
What factors increase setting time?
- Borax
Explain how spatulation reduces setting time and increases expansion
- Increased spatulation breaks down growing crystals
- Fragments act as nuclei of crystallisation
- More growing crystals - come into contact sooner
Therefore: decreased setting time and increased expansion
How does increased powder to water ratio reduce setting time and increase expansion?
- More nuclei of crystallisation per unit volume
- Crystals come into contact sooner
- Faster set and greater expansion
What are the expansion levels of gypsum plaster, stone and densite?
- Plaster: 0.2%-0.3%
- Stone: 0.08-0.1%
- Densite: 0.05-0.07%
