Gypsum

Question 1

Function of study cast

Answer 1

• accurately records the position, shape and dimension of teeth
• assessment of dentition
• manufacture of dental prostheses

Question 2

3 Gypsum types

Answer 2

• plaster
• dental stone
• densite

Question 3

which gypsum has Beta hemihydrate

Answer 3

plaster

Question 4

which gypsum has alpha hemihydrate

Answer 4

dental stone

Question 5

what is densite

Answer 5

(improved stone)

Question 6

How is gypsum manufactured? (chemical equation)

Answer 6

calcium sulphate + dihydrate -> calcium sulphate x2 + hemihydrate

Question 7

how is plaster manufactured

Answer 7

Heated in open vessel

50-60ml water
100g powder

Question 8

how is dental stone manufactured

Answer 8

Heated in autoclave

20-35ml water
100g powder

Question 9

how is densite manufactured

Answer 9

Heated with calcium and magnesium chloride

Question 10

what is the crystalline structure of plaster?

Answer 10

Large
Porous
Irregular crystals

Question 11

what is the crystalline structure of dental stone?

Answer 11

Non porous
Regular crystals

Question 12

what is the crystalline structure of densite?

Answer 12

Compact
Smoother crystals

Question 13

How does gypsum set?

Answer 13

reverse of manufacturing process

hemiihydrate powder + water -> dihydrate (gypsum study cast)

 hemihydrate powder dissolves in water
 dihydrate crystals form/precipitates on impurities
 continues until all hemihydrate dissolved
 dihydrate crystals come into contact and push apart, expanding
 will have properties of a weak solid, will not flow
 final set strong and hard, strength continues developing

Question 14

why does porosity of gypsum increase after setting?

Answer 14

• during setting : excess water trapped in the powder mass
• after setting : excess water evaporates, leaving voids/ porosity increases and strength is compromised, must handle with care after setting

Question 15

can gypsum capture fine details?

Answer 15

yes. can match the standard of 50um

gypsum porosity 28-40um which is within the standard

Question 16

strongest gypsum

Answer 16

densite. plaster weakest.

Question 17

why is dental stone stronger than plaster

Answer 17

 stone has less water so less porous thus more strong
 strength develops over 24h to 75MPa
 generally has low tensile strength

Question 18

is the surface hardness ideal?

Answer 18

no, poor abrasion resisatnce

Question 19

is gypsum dimensionally accurate after setting

Answer 19

yes, low expansion

plaster expands the most and densite the least but all are acceptable

Question 20

does increasing temp increase set time?

Answer 20

yes, makes it slower

Question 21

does solubility of hemihydrate decrease with increasing temperature?

Answer 21

yes

Question 22

what does potassium sulphate do to setting time

Answer 22

potassium sulphate -> speeds up setting time by encouraging growth of more crystals

Question 23

what does borax do to setting time

Answer 23

borax -> slows down setting time by delaying setting process and forming calcium borate deposits on dihydrate crystals

Question 24

is gypsum brittle

Answer 24

 very brittle
 fractures easily
 flexural strength 15-20MPa
 CR IN COMPARISON IS 150MPa

Question 25

what does adding more powder do to the setting time and the expansion

Answer 25

• faster set but greater expansion
• more nuclei of crystallization per unit volume
• crystals come into contact sooner

Question 26

what does increase spatulation do to the setting time and the expansion

Answer 26

• faster setting time
• but increases expansion which is bad
• breaks down crystals
• more fragments for nuclei of crystallization
• more growing crystals
• faster contact of crystals

Question 27

what does increase impurities do to the setting time and the expansion

Answer 27

decreases setting time but increases expansion which is bad

Question 28

more crystals deposition/ nuclei of crystallization = faster or slower set ??

Answer 28

faster