85. Gypsum

Question 1

What is gypsum in layman’s terms?

Answer 1

Dental stone/ cast

Question 2

What type of replica does a study model provide of a dentition?

Answer 2

A positive replica

Question 3

What type of replica does an impression provide of a dentition?

Answer 3

A negative replica

Question 4

What are the three main properties of gypsum that are important for its use?

Answer 4

• Accuracy;
• Ease of use;
• Limitations.

Question 5

What is calcium sulphate dihydrate converted to by manufacturers? For use by dental technicians.

Answer 5

Calcium hemihydrate + water

Question 6

What is the reaction equation for the conversion of calcium sulphate dihydrate to hemihydrate?

Answer 6

2CaSO4.2H2O > (CaSO4)2.H2O + 3 H2O

Question 7

What determines the type of calcium sulphate hemihydrate produced by manufacturers?

Answer 7

Conditions, e.g. heat

Question 8

What chemical property determines the material properties of gypsum?

Answer 8

The crystalline structure

Question 9

What type of hemihydrate is plaster and under what conditions is this manufactured?

Answer 9

• Beta-hemihydrate;
• Large, porous, irregular crystals;
• Heated in open vessel.

Question 10

What type of hemihydrate is stone and under what conditions is this manufactured?

Answer 10

• Alpha-hemihydrate;
• Non-porous, regular crystals;
• Heated in autoclave.

Question 11

What type of hemihydrate is densite (improved stone) and under what conditions is this manufactured?

Answer 11

• Compact, smoother crystals;

- Heated in presence of Ca and Mg chloride.

Question 12

What is the setting reaction of gypsum?

Answer 12

(CaSO4)2.H2O + 3 H2O > 2CaSO4.2H2O

i.e. calcium sulphate hemihydrate to calcium sulphate dihydrate

Question 13

Describe the reaction process of gypsum mixed with water.

Answer 13

• Hemihydrate dissolves;
• Dihydrate forms;
• Dihydrate crystals precipitate on impurities as crystals;
• More hemihydrate dissolved;
• Continues until all hemihydrate has dissolved.

Question 14

Describe the setting process of gypsum.

Answer 14

• 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.

Question 15

What causes the differences between initial and final setting of gypsum?

Answer 15

During initial setting the excess water is trapped in the mass but upon final setting the water has evaporated (voids produced - porosity).

Question 16

What are the three main properties to consider of gypsum types?

Answer 16

• Strength (compressive forces and hardness);
• Setting time;
• Expansion.

Question 17

What is the typical compressive force range of gypsum?

Answer 17

~ 20-35 MPa

Question 18

What is the relative hardness of gypsum to other DMs?

Answer 18

Low, but hard enough at the surface for required practical use

Question 19

How do expansion values differ between plaster-stone-densite?

Answer 19

Expansion decreases from plaster-stone-densite

Question 20

Which is the strongest gypsum typically used?

Answer 20

Densite (improved stone)

Question 21

How does increased spatulation effect the setting time and expansion, and why?

Answer 21

• Reduces setting time;
• Spatulation breaks down growing crystals;
• This produces more nuclei of crystallisation;
• Therefore, more crystals come into contact sooner;
• Increased expansion.

Question 22

How does the powder: water ratio effect the setting time and expansion, and why?

Answer 22

• 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)

Question 23

What do most impurities (chemical additives) of gypsum effect?

Answer 23

Setting time, can increase (potassium sulphate) or decrease (borax) it.

Question 24

Why must gypsum be compatible with impression material?

Answer 24

• Must be ‘wet’ to be able to flow over the impression;
• No resistance to flow;
• Avoid bubble formation.

Question 25

What level of detail must gypsum be able to capture and how does this compare to impression material? (um)

Answer 25

• 28-40 um;

- Slightly better than impression material at 50um.

Question 26

State an advantage of gypsum.

Answer 26

• Dimensionally stable and accurate;
• Low expansion (of stone/ densite);
• Good colour contrast.

Question 27

State a disadvantage of gypsum.

Answer 27

• Low tensile strength;
• Poor abrasion resistance;
• Brittle;
• Surface detail less than elastomer impression (~20um);
• Poor ‘wetting’ of some impression materials.