85. Gypsum Flashcards
(27 cards)
What is gypsum in layman’s terms?
Dental stone/ cast
What type of replica does a study model provide of a dentition?
A positive replica
What type of replica does an impression provide of a dentition?
A negative replica
What are the three main properties of gypsum that are important for its use?
- Accuracy;
- Ease of use;
- Limitations.
What is calcium sulphate dihydrate converted to by manufacturers? For use by dental technicians.
Calcium hemihydrate + water
What is the reaction equation for the conversion of calcium sulphate dihydrate to hemihydrate?
2CaSO4.2H2O > (CaSO4)2.H2O + 3 H2O
What determines the type of calcium sulphate hemihydrate produced by manufacturers?
Conditions, e.g. heat
What chemical property determines the material properties of gypsum?
The crystalline structure
What type of hemihydrate is plaster and under what conditions is this manufactured?
- Beta-hemihydrate;
- Large, porous, irregular crystals;
- Heated in open vessel.
What type of hemihydrate is stone and under what conditions is this manufactured?
- Alpha-hemihydrate;
- Non-porous, regular crystals;
- Heated in autoclave.
What type of hemihydrate is densite (improved stone) and under what conditions is this manufactured?
- Compact, smoother crystals;
- Heated in presence of Ca and Mg chloride.
What is the setting reaction of gypsum?
(CaSO4)2.H2O + 3 H2O > 2CaSO4.2H2O
i.e. calcium sulphate hemihydrate to calcium sulphate dihydrate
Describe the reaction process of gypsum mixed with water.
- Hemihydrate dissolves;
- Dihydrate forms;
- Dihydrate crystals precipitate on impurities as crystals;
- More hemihydrate dissolved;
- Continues until all hemihydrate has dissolved.
Describe the setting process of gypsum.
- Initial set: dihydrate crystals come into contact;
- Push apart and expand;
- Properties of weak solid, will not flow but can be carved.
- Final set: strong and hard enough to be worked;
- This strength continues to develop.
What causes the differences between initial and final setting of gypsum?
During initial setting the excess water is trapped in the mass but upon final setting the water has evaporated (voids produced - porosity).
What are the three main properties to consider of gypsum types?
- Strength (compressive forces and hardness);
- Setting time;
- Expansion.
What is the typical compressive force range of gypsum?
~ 20-35 MPa
What is the relative hardness of gypsum to other DMs?
Low, but hard enough at the surface for required practical use
How do expansion values differ between plaster-stone-densite?
Expansion decreases from plaster-stone-densite
Which is the strongest gypsum typically used?
Densite (improved stone)
How does increased spatulation effect the setting time and expansion, and why?
- Reduces setting time;
- Spatulation breaks down growing crystals;
- This produces more nuclei of crystallisation;
- Therefore, more crystals come into contact sooner;
- Increased expansion.
How does the powder: water ratio effect the setting time and expansion, and why?
- More powder, quicker setting time, more expansion;
- More nuclei of crystallisation per unit volume;
- Crystals come into contact sooner and greater expansion.
(vice versa w/ less powder)
What do most impurities (chemical additives) of gypsum effect?
Setting time, can increase (potassium sulphate) or decrease (borax) it.
Why must gypsum be compatible with impression material?
- Must be ‘wet’ to be able to flow over the impression;
- No resistance to flow;
- Avoid bubble formation.
