1. DENT 1200 Dental Materials I
Lesson 3 Gypsum

2. 1. Identify the Source and Color of pure raw gypsum.
The raw form of gypsum is obtained from gypsum rock called alabaster. The pure rock is white and translucent. It may be tinged with gray or yellow due to impurities.
The composition is crystalline in structure (a solid with regular arrangement of atoms). There are three main forms of gypsum products used in dentistry: Type II plaster, Type III dental (lab) stone and Type IV improved (die) stone. Chemically, plaster and dental stone are identical. Physically the shape and density of the powder particles differ.

3. 2. Chemical formula for raw and calcined gypsum
Calcium Sulfate Dihydrate
CaSO4 2H2O
Calcined: a process to remove water from raw gypsum and make into powder form
Calcium Sulfate Hemihydrate
CaSO4 ½ H2O

4. 3.W/P ratio for 3 types Type II Plaster 50/100 Type III Stone 33/100
Type IV Improved or Die Stone 22/100
Different amount of water is needed because particle size and shapes are different due to the calcination process

5. 4. Uses of three types
Type II study or diagnostic model, to articulate
Type III cast for fabrication of removables
Type IV cast of removable dies for fixed
and final impression denture pour

6. Articulated Model May need a bite registration to achieve this.
Maxillary and Mandibular arches attached to simulate patient bite.
1 or more teeth can be made removable dies.

7. 5.Precaution for Use and Storage
Wear mask and eyewear as this material is an irritant to the lungs.
Can be very drying to the skin (hands)
Keep storage bins closed- or water content changes and material does not react as it should

8. 6. Mixing techniques 1. Rubber bowl and lab spatula
2. Mechanical spatulation
3. Vacuum investor
All techniques require a vibrator to remove air voids and increase flow of the material.

9. Mechanical Vacuum

10. 7. Initial Set and Final Set
Time between the spatulation and the “loss of gloss”
Several minutes
Final Set
Material is hard( can not indent a fingernail) , dry and cool (heat has been given off in the EXOTHERMIC reaction)
45 minutes – 1 hour

11. 8. Methods to Accelerate and Inhibit the set
ACCELERATE_ speed
Potassium sulfate
Sodium chloride(salt)
Slurry- from unclear bowl or spatula
HEAT
INHIBIT slow
Borax
Hydrocolloid gels
Sodium citrate
BLOOD
COLD

12. 9. Strong and Accurate
Follow manufacturers directions, including measuring water and powder, place water in bowl first, do not add extra water or powder once the mix has begun.
A thin mix is weaker.
Add the material in increments and vibrate!

13. IMPRESSION MODEL/CAST
IMPRESSION- negative reproduction
CAST/MODEL
positive reproduction
fabricate/study