Gypsum

Question 1

What are study casts and what are their uses

Answer 1

Positive replica of the dentition
Records the position, shape and dimensions of all the teeth present and allows visualisation of the patients dentition
Enables manufacture of dental prostheses

Question 2

What are the uses of gypsum

Answer 2

Cast - plaster/stone
Die - stone/improved stone
Mould material - stone
Investment binder - stone

Question 3

How does gypsum occur naturally

Answer 3

As calcium sulphate dihydrate

Question 4

What happens chemically during the manufacturing stage of gypsum powder

Answer 4

The dihydrate form of calcium sulphate is heated to produce calcium sulphate hemihydrate and the release of water

Question 5

What are the different types of gypsum that can be produced on heating

Answer 5

Plaster - B-hemihydrate
Dental stone - a-hemihydrate
Densite - improved stone

Question 6

What determines the properties of gypsum

Answer 6

The crystalline structure

Question 7

How is plaster (B-hemihydrate) produced

Answer 7

Gypsum is heated in an open vessel (with air readily available)
This creates a powder with a large porous and irregular structure

Question 8

How is dental stone (a-hemihydrate) produced

Answer 8

Heated in an autoclave (a closed vessel)

This produces non-porous and regular shaped crystals

Question 9

How is densite (improved stone) produced

Answer 9

Heating process is performed with CaCl and MgCl present

This produces a more compact, less porous material with smoother particles

Question 10

Describe the setting reaction of gypsum

Answer 10

Gypsum powder in the lake is the hemihydrate form which is mixed with water to produce the gypsum material used to create study casts or die
Reverse of manufacture

Question 11

What are the water/powder ratios of plaster and stone, and what is the theoretical ratio

Answer 11

Plaster - 50-60 ml to 100g
Stone - 20-35 ml to 200g
Theoretical ratio - 18.6 ml to 100g

Question 12

Why is excess water used in the mixing process

Answer 12

To allow the powder and water to be mixed easily and achieve the right consistency

Question 13

What happens in the setting process of gypsum

Answer 13

Hemihydrate dissolves and dihydrate forms
Dihydrate precipitate on impurities as crystals
More hemihydrate is dissolved
The crystal growth process continues until all the hemihydrate is consumed

Question 14

What role do impurities play in the setting process of gypsum

Answer 14

They are nucleating agents and the centres of crystal growth

Dihydrate is pulled towards them

Question 15

Describe the initial set of gypsum

Answer 15

Dihydrate crystals come into contact and push apart causing expansion
Properties of a weak solid and will not flow

Question 16

Describe the final set of gypsum

Answer 16

Crystals that are quite large, of different sizes and shape CaSO4s coming into direct contact with one another
Now strong and hard but strength will continue to develop over the next few hours

Question 17

What happens to the excess water after setting of gypsum

Answer 17

During the setting the water is trapped in the powder
When setting is over the water evaporates leaving voids
Evaporation may cause any dissolved dihydrate crystals to cement together

Question 18

What sizes of grooves can be replicated by gypsum

Answer 18

Grooves between 28 and 40 um

Question 19

Rank the different types of gypsum from lowest to highest on compressive strength

Answer 19

Plaster
Stone
Densite

Question 20

What is the average compressive strength of gypsum

Answer 20

After 1 hour is between 28 and 38 MPa

Develops over 24 hours to around 75MPa

Question 21

Why do different types of gypsum have a higher compressive strength than others

Answer 21

Some types require less water (stone) for a workable mix and is less porous

Question 22

Describe the surface hardness of gypsum

Answer 22

Is low so the surface is easily abrasive which isn’t ideal

Question 23

What is the ideal expansion on setting of gypsum

Answer 23

Should be low to minimise dimensional inaccuracy

Question 24

What can be done to reduce the setting time of gypsum

Answer 24

Increasing powder quantity
Increase spatulation
Adding impurities
Raising the temperature above 40*C

Question 25

Describe how spatulation affects mixing time

Answer 25

Increased spatulation breaks down growing crystals meaning more nuclei of crystallisation are formed and subsequently more crystals are created These crystals come into contact with one another at an earlier stage causing greater expansion and a decreased setting time

Question 26

What happens when increasing powder quantity is used in the powder/water ratio

Answer 26

More nuclei of crystallisation are created | Crystals contact one another sooner leading to a faster setting reaction and greater expansion

Question 27

What is typical for expansion on setting for different types of gypsum

Answer 27

Plaster - 0.3-0.3% Stone - 0.08-0.1% Densite - 0.05-0.07%

Question 28

What will expansion on setting cause

Answer 28

Study casts will be slightly too large so crowns, bridges and dentures produced from the casts will be slightly too large and so not too tight a fit when located in the mouth

Question 29

What mechanisms cause temperature to have an effect on setting

Answer 29

Rate of diffusion of ions increases with increasing temperature Solubility of hemihydrate decreases with increasing temperature

Question 30

What effects does changing temperature have on setting

Answer 30

Changing water temperature from 10-40*C will reduce setting time Raising the temperature from 40-80*C will increase the setting time Raising the temperature above 80*C will rapidly increase setting time

Question 31

What effect does adding potassium sulphate have on gypsum

Answer 31

Produces syngenite which encourages growth of more crystals | This decreases setting time

Question 32

What chemical additives are commonly used in gypsum

Answer 32

Potassium sulphate | Borax

Question 33

What effect does adding borax have on gypsum

Answer 33

Forms calcium borate which deposits on dihydrate crystals | This delays the setting process increasing setting time

Question 34

Describe gypsums compatibility with impression material

Answer 34

Must be compatible with the selected impression material so that it wets the surface, making close contact and avoiding the formation of voids and bubbles

Question 35

Describe the storage of gypsum

Answer 35

Once study casts are made they can be stored for a considerable time Long term dimensional stability is good so it can be used as a positive replica for a long time

Question 36

How brittle is gypsum

Answer 36

Flexural strength is between 15 and 20 MPa which is very low so it is very brittle

Question 37

What are the advantages of gypsum

Answer 37

``` Dimensionally accurate Low expansion Can replicate most grooves well Has sufficient compressive strength Maintains dimensions within storage ```

Question 38

What are the disadvantages of gypsum

Answer 38

Doesn’t withstand large tensile stresses Surface can be abraded quite easily Brittle so may fracture easily Cannot reproduce very fine surface features