gypsum

Question 1

gypsum

Answer 1

Gypsum is a mineral found in nature. It has been used to make dental cast since 1756. It is mined as a solid mass.

Question 2

Chemical Composition of Gypsum:

Answer 2

Chemical Composition of Gypsum:

Ca SO4 * 2 H2O – Calcium Sulfate Dihydrate

Question 3

plaster dental and die stone chemical comp

Answer 3

Ca SO4 * 1/2 H2O  (Heat to 115°C in open vat) : Plaster of Paris (β Hemihydrate)

Ca SO4 * 1/2 H2O  (Heat to 130° - 200°C + increase Pressure)  Dental Stone (α Hemihydrate)

Ca SO4 * 1/2 H2O  (Heat to 130° - 200°C + increase Additional Pressure)  Die Stone

Question 4

β Hemihydrate – Plaster of Paris

Answer 4

β Hemihydrate – Plaster of Paris
• “Sponginess appearance”
• Irregular shaped, porous
• Uses: Study Models (Diagnostic Casts)

Question 5

α Hemihydrate – Stone

Answer 5

α Hemihydrate – Stone
• Crystals are denser and have a “prismatic shape”
• Uses: Working Casts (Master Casts)

Question 6

Die Stone

Answer 6

Die Stone
• More regular shaped particles with better packing ability
• Uses: Makes dies on which to wax patterns

Question 7

how do plaster and stone differ

Answer 7

• Plaster and stone are chemically identical but physically distinct forms of the compound calcium sulfate

Question 8

• Two factors contribute to the strength and abrasion resistance of the final product:

Answer 8

• Two factors contribute to the strength and abrasion resistance of the final product:

1. Shape of the particles
2. Porosity

Question 9

which is weaker , plater or stone and why ?

Answer 9

• Plaster particles are both porous and irregular, requiring more water to mix. The resulting product has more air space because of less densely packed particles, making plaster considerably weaker than the less porous and more more densely packed stone

Question 10

setting expansion

Answer 10

• Setting expansion is a result of the growth of crystals as they join

Question 11

setting expansion of plaster and stone

Answer 11

• Plaster has the greatest amount of expansion at .30% and high-strength stones the lowest at .10%

Question 12

types of gypsum : 5

Answer 12

type 1: impression plaster

2. model plaster
3. dental stone
4. dental stone : die stone
5. high strenght/ hish expansion stone

Question 13

impression plaster

Answer 13

rarely used

mouth must me edentulous

Question 14

model plaster

Answer 14

type 2
W/P =.45
durable but still weak compared to stone

Question 15

dental stone type 3

Answer 15

ideal for full or partial denture and ortho models
1/p ration.30
2,5 stronger than plaster

Question 16

dental stone type 4

Answer 16

a. Referred to as Die Stone, capable of fabricating wax patterns
b. Hard, abrasive-resistant surface
c. Pink or Green in color
d. W/P ratio = .23
e. 2 times stronger than Type III stone

Question 17

5) High-Strength, High-Expansion Dental Stone (Type V)

Answer 17

5) High-Strength, High-Expansion Dental Stone (Type V)
a. Recent addition – was developed in response to the need for even higher-strength and high-expansion dental stones
b. W/P ratio = .20
c. Blue or Green in color
d. Most costly of all gypsum products

Question 18

initial vs final setting time

Answer 18

Initial Setting Time
When a mix can resist penetration by a Gilmore needle, which has a tip 2.12 mm in diameter and weighs 113.4 g. 7 to 9 minutes from the beginning of the mix

Final Setting Time
A heavier Gilmore needle weighing 453.6 g with a tip 1.06 mm in diameter leaves barely a mark. 45 minutes to 1 hour before separating the cast from the impressio

Question 19

accelerators

Answer 19

• Accelerators

a. Potassium Sulfate (K2SO4)
b. Set gypsum particles obtained from slurry water (CaSO4)

Question 20

how do The dehydrate crystals in the slurry water accelerate the chemical reaction?

Answer 20

• The dehydrate crystals in the slurry water accelerate the chemical reaction by acting as established sites for crystallization

Question 21

retarders

Answer 21

•	Retarders
a.	Blood (organic substance)
b.	Saliva (organic substance)
c.	Alginate
sodium phosphte

Question 22

inorganic salts role

Answer 22

a. Accelerators at low concentrations

b. Retarders at high concentrations

Question 23

borax?

Answer 23

inorganic salt

b. Retarders at high concentrations (Example: Borax)

Question 24

when us Decreasing the amount of water in a mixture recommended?

Answer 24

 Decreasing the amount of water is only recommended when the mixture is being used to make a base to secure models on an articulator

Question 25

increasing/decreasing temp of water will have what effect ?

Answer 25

• Increase in temp of water to 100° F will accelerate the set
• Above 100° F will have a retard effect
• 212° F, no reaction takes place