Gypsum

Question 1

Purpose of study casts (3)

Answer 1

1. Records the position + shape of teeth
2. Aids assessment of dentition
3. Enables manufacture of dental prosthesis

Question 2

List uses of gypsum (4)

Answer 2

1. Cast (plaster/stone)
2. Die (stone)
3. Mould material
4. Investment binder

Question 3

List different types of gypsum (3)

Answer 3

1. Plaster (B-hemihydrate)
2. Dental stone (a-hemihydrate)
3. Densite

Question 4

Function of the gypsum crystalline structure

Answer 4

It determines properties

Question 5

How is plaster (b-hemihydrate) heated and describe the particles (2)

Answer 5

1. Heated in open vessel

2. Large porous irregular crystals

Question 6

How is dental stone (a-hemihydrate) heated and describe the particles (2)

Answer 6

1. Heated in an autoclave in the presence of Ca and Mg chloride
2. Non-porous smooth crystals

Question 7

How does improved stone particles look?

Answer 7

Compact smooth particles

Question 8

Heating Calcium Sulphate Dihydrate leads to the production of what 3 things?

Answer 8

1. Plaster
2. Stone
3. Improved stone

Question 9

Mixing ratio for plaster

Answer 9

WATER: 50-60ml
POWDER: 100g

Question 10

Mixing ratio for stone

Answer 10

WATER: 20-35ml
POWDER: 100g

Question 11

Theoretical ratio

Answer 11

WATER: 18.6ml
POWDER: 100g

Question 12

How is dihydrate formed?

Answer 12

Hemihydrate dissolves

Dihydrate forms

Question 13

Dihydrate solubility

Answer 13

Solubility low - supersaturated solution

Question 14

Setting process of gypsum (3)

Answer 14

1. Dihydrate crystals precipitate on impurities as crystals
2. More hemihydrate dissolved
3. Continues until all hemihydrate dissolved

Question 15

Describe the initial set process

Answer 15

1. Dihydrate crystals come into contact (push apart)
2. Expansion starts
3. As the set is fairly weak solid at this stage, it will not flow
4. Can be carved as its easy to manipulate

Question 16

List gypsum advantages (3)

Answer 16

1. Dimensionally accurate and stable
2. Low expansion (<0.1%) of stone/densite
3. Good colour contrast

Question 17

List gypsum disadvantages (4)

Answer 17

1. Low tensile strength + very brittle
2. Poor abrasion resistance
3. Surface detail less than elastomer impression
4. Poor wetting of some impression materials

Question 18

What is used to measure initial and final setting times

Answer 18

Gilmore needles

Question 19

What happens to excess water during setting?

Answer 19

1. Excess water is trapped in the powder mass
2. Excess water now has the opportunity to evaporate or else bubbles are formed and will become porous (making the material v weak)

Question 20

What happens on completion of setting?

Answer 20

1. Excess water evaporates

2. Voids are produced (porosity)

Question 21

Typical compressive strength of gypsum

Answer 21

Compressive (~20~35MPa)

hardness - low

Question 22

Compare gypsum strength to composite

Answer 22

1/10th of composite strength

But roughly the same bonding strength

Question 23

State the typical expansion value of plaster?

Answer 23

0.2-0.3%

Question 24

State the typical expansion value of stone?

Answer 24

0.08-0.1%

Question 25

State the typical expansion value of densite?

Answer 25

0.05-0.07%

Question 26

Why is improved stone stronger?

Answer 26

Requires less water for a workable mix (less porous)

Question 27

List the effects of increasing: 1. Powder 2. Spatulation 3. Impurities On the setting time of gypsum products

Answer 27

Decrease setting time

Question 28

List the effects of increasing 1. Powder 2. Spatulation 3. Impurities On the expansion of gypsum products

Answer 28

Increase expansion rate

Question 29

How does spatulation lead to: Decreased setting time Increased expansion

Answer 29

1. Increased spatulation breaks down growing crystals 2. Fragments act as nuclei of crystallisation 3. More growing crystals come into contact sooner

Question 30

Effect of increasing powder on the powder/water ratio (3)

Answer 30

1. More nuclei of crystallisation per unit volume 2. Crystals come into contact sooner 3. Faster set and greater expansion

Question 31

Why should expansion only be a small %? (2)

Answer 31

- Small % otherwise its problematic - Allows for model to be a little big so it means crowns, bridges and dentures won't be too tight a fit when placed in the mouth

Question 32

How does temperature affect the rate of diffusions of ions?

Answer 32

Rate of diffusions of ions increases with increased temperature

Question 33

What chemical additives can be added? (2)

Answer 33

1. Potassium sulphate | 2. Borax

Question 34

Function of Potassium sulphate (3)

Answer 34

- Produces syngenite - Crystallises rapidly encourages growth of more crystals - Decreases setting time

Question 35

Function of borax (2)

Answer 35

- Deposits on dihydrate crystals | - Delays setting process (increases setting time)

Question 36

Name a disadvantage of gypsum

Answer 36

Porous - so creates a rough surface about 28 to 40um (but good enough for most applications)