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24 Cards in this Set
- Front
- Back
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Gypsum:
What is made from? How is it found in nature? What color is it? What is made from it? |
Made from calcium sulfate
In nature found as compact mass White or milky yellowish Plaster of paris is made from it |
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What are gypsums used for in dentistry?
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Creation of study models, casts or dies.
Replicas of hard and soft tissues. Used in diagnosis and treatment. |
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What are the qualities of gypsum?
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1. Accuracy
2. Dimensional stability 3. Fine details 4. Strength/resistance to abrasion 5. Easily adapts to the impression 6. Color (variety) 7. Safety |
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What are some examples of gypsum?
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Mounting plaster
Microstone Snap-Stone |
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What is the chemical reaction of gypsum formation?
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CaSO4 * 1/2 H2O * 11/2 H2O yields CaSO4 * 2 H20 + heat
Hemihydrate to Dihydrate (gypsum) |
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How does the preparation for type III (stone) gypsum differ from types IV & V gypsum (high strength stone)?
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Type III (stone)-High pressure steam
Types IV & V (high-strength stone)-Boil, CaCl2 |
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What is an accelerator?
Examples? |
Chemicals that increase the rate of setting.
Potassium Sulfate Terra alba |
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What is a retarder?
Example |
A chemical that decreases the rate of setting.
Borax |
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What does a 45/100 water powder ratio mean?
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45 mL of water added to 100 g of powder.
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What does an excess of water cause?
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1. Increased setting time
2. Reduced strength 3. Reduced expansion 4. Reduced hardness |
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How does water temperature affect gypsum's setting time?
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The warmer the water, the longer the setting time.
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Gypsum materials are hydroscopic. What does that mean?
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They absorb water from the air.
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What does hardening solutions do when using gypsum?
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Replace water to increase hardness, increase resistance to abrasion, etc.
Colloidal Silica is main ingredient. |
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What is the difference between initial setting time and final setting time?
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Initial-Working time
Final-When conversion to dihydrate is complete. |
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What are the important properties to know of with gypsum?
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1. Setting time
2. Reproduction of detail 3. Strength 4. Hardness & Abrasion Resistance 5. Dimensional Accuracy |
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What factor needs to be considered in gypsum's dimensional accuracy?
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Slight expansion
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What is investment?
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A stone like material that can resist high temperatures and forces experienced during burnout and casting.
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What is the composition of investment?
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Binders (Gypsum)-holds the investment together
Refractory material (form of Silica SiO2)-heat resistance |
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Investment expansion is designed...why & how?
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To compensate for casted metal shrinkage.
Expansion happens through thermal expansion (primarily) and hygroscopic means. |
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What does a wax's 'flow' refer to?
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A change in shape or dimension in response to an applied force.
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What is notable about wax's coefficient of thermal expansion? What does this mean?
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They have the highest thermal expansion of any dental material...they expand significantly when heated.
323 x 10 -6/degree C |
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What is a wax's residual stress?
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The stress remaining in a wax as a result of manipulation (heating, cooling, bending, etc.)
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What are the two types of investment? Which is stronger?
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Gypsum bonded
Phosphate bonded (stronger) |
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How much will casted metals shrink when they cool from the solid state at a high temperature to room temperature?
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1-2.5%
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