Gypsum Flashcards
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%
Why is slight expansion of gypsum somewhat beneficial?
- Allows for the model to be a little too big
- meaning crowns, bridges and dentures won’t be too tight a fit when placed in the mouth
Why does water temperature have a variable effect on the setting time of gypsum?
- Rate of diffusion of ions increases with temperature
2. Solubility of hemihydrate decreases with increased temperature
What temperature ranges reduce setting time of gypsum?
10-40 degrees Celsius
What temperature range increases setting time of gypsum?
40-100 degrees Celsius
Explain why potassium sulphate decreases setting time
- Produces syngenite
- crystallises rapidly- encouraging growth of more crystals
- decreases setting time
Explain why borax increases setting time of gypsum
- Forms calcium borate - deposits on dihydrate crystals
- delays setting process (increases setting time)
Dental stone needs to be compatible with the impression material to provide adequate surface detail, what is meant when we say it must “wet” the impression material?
•no resistance to flow over surface; avoid bubble formation
What is a major weakness of gypsum?
It is brittle
- i.e. will readily fracture under a small amount of strain
Brittleness isn’t assessed directly, instead flexural strength is used. What is the flexural strength of gypsum?
15-20 MPa
Summarise the advantages of gypsum
- Dimensionally accurate and stable
- Low expansion (<0.1%) of stone/densite
- Good colour contrast
Summarise disadvantages of gypsum
- Low tensile strength
- Poor abrasion resistance
- Very brittle
- Surface detail less than elastomer impression (20micron groove)
- Poor “wetting” of some impression materials
