Y2 L3 Introduction to gypsum products

Question 1

What is gypsum?

Answer 1

A calcium sulfate dihydrate material used to construct dental models/casts of a patient’s oral anatomy.

Question 2

What are the 4 gypsum products?

Answer 2

• Model plaster (type II)
• Dental stone (type III)
• Model stone (type III or IV)
• Die stone (type IV or V)

(Impression plaster is a historic material)

Question 3

What are the ideal properties of cast or die materials?

Answer 3

• Dimensionally accurate and stable
• Easy and quick to use, adequate setting time
• Accurate reproduction of fine detail
• No interaction between impression and cast or die material
• Good strength and hardness
• Resists abrasion
• Good colour contrast

Question 4

What are the 2 forms of gypsum?

Answer 4

Alpha-hemihydrate: smaller water to powder ratio, stronger and harder
Beta-hemihydrate: more water, weaker

Question 5

What is the difference between a dental cast and a die?

Answer 5

• The term cast or model describes a replica of a large proportion of a patient’s mouth
• The term die describes a small part of the cast upon which a restoration is made, dies are usually removable and a cast can contain multiple dies

Question 6

When is model plaster used? Describe the material.

Answer 6

• Used to mount a model to an articulator
• Sets in 5-10 minutes
• Plaster has a high water to powder ratio so is relatively weak and not suitable for all dental lab work

Question 7

When is dental stone used? Describe the material.

Answer 7

• Commonly used to make dentures and appliances
• Contains less water than plaster
• Much more resilient, better compressive and tensile strength
• Better abrasion resistance

Question 8

When is die stone used? Describe the material.

Answer 8

• Used to make a variety of restorations e.g. crowns, inlays, onlays
• Extremely hard, produces extremely accurate models
• Minimises errors in the fabrication of restorations
• Smooth surface