Gypsum

Question 1

What is gypsum used for?

what is the purpose of a study case?

Answer 1

Cast (plaster/stone, positive replica)
Die (stone/improved stone)
Mould material (stone)
Investment binder (stone)

• records the position, shape & dimensions of teeth
• aids visualisation/assessment of dentition
• enables manufacture of dental prostheses
• shows changes in dentition over time

Question 2

What is gypsum called before and after it is heated?

Answer 2

before: calcium sulphate dihydrate

after: calcium sulphate hemihydrate

Question 3

How are plaster, stone and densite made?

Answer 3

Plaster (β-hemihydrate) - Heated in open vessel, large porous irregular crystals

Dental stone (α-hemihydrate) - Heated in autoclave, non-porous regular crystals

Densite (improved stone) - Heated with Ca & Mg chlorides, compact smooth particles

Question 4

How does gypsum set?

Answer 4

Reverse of manufacture - hemihydrate + water → dihydrate (gypsum cast)

Hemihydrate dissolves, dihydrate crystals precipitate on impurities as crystals

Initial set when crystals interconnect, expansion starts (can be carved)

Final set when hard and can be worked on

Question 5

What are setting times measured by?

Answer 5

gilmore needles

Question 6

Why is excess water needed and what happens to the excess trapped water after setting?

Answer 6

for a workable mass

evaporates leaving pores and voids

Question 7

Does gypsum reproduce surface detail?

Answer 7

gypsum is inherently porous, resulting in a relatively rough surface – about 28 to 40um * (good enough for most applications but less than impression material)

Question 8

What value is the compressive strength of gypsum and what form of gypsum is strongest?

Answer 8

28 MPa – 38 MPa

stone stronger as it requires less water
improved stone (DENSITE) is the strongest

Question 9

What is the surface hardness of gypsum?

Answer 9

is low (surface easy to abrade) - not ideal
must be careful when handling

Question 10

What does an increase in powder, impurities and spatulation cause?

Answer 10

decrease in setting time

increase in expansion

Question 11

Why does spatulation affect setting time and expansion?

Answer 11

increased spatulation breaks down growing crystals

fragments act as nuclei of crystallisation

more growing crystals -come into contact sooner

(faster set, greater expansion)

Question 12

Why does increasing powder affect setting time and expansion?

Answer 12

more nuclei of crystallisation per unit volume
crystals come into contact sooner
(faster set & greater expansion)

Question 13

What does expansion on setting allow in practise?

Answer 13

Allows for model to be a little big
…..so crowns, bridges and dentures won’t be too tight a fit when placed in mouth

Question 14

What two chemicals can be added to gypsum?

Answer 14

potassium sulphate
borax

Question 15

What does potassium sulphate do?

Answer 15

decreases setting time

Question 16

What does borax do?

Answer 16

delays setting process (increases setting time)

Question 17

What must be done to impression material before pouring cast?

Answer 17

wet the impression material so
no resistance to flow over surface; avoid bubble formation

Question 18

Can gypsum be stored?

Answer 18

may be stored for a considerable time while maintaining its dimensional stability

Question 19

What is a proxy measurement used to assess brittleness?

Answer 19

flexural strength

Question 20

What happens to gypsum under a small amount of strain?

Answer 20

it is brittle
ie it will readily fracture when subjected to a small amount of strain.

Question 21

What are the advantages of gypsum?

Answer 21

• dimensionally accurate and stable
• low expansion (<0.1%) of stone/densite
• good colour contrast

Question 22

What are the disadvantages of gypsum?

Answer 22

• low tensile strength
• poor abrasion resistance
• very brittle
• surface detail less than elastomer impression (~20um groove)
• poor “wetting” of some impression materials