Gypsum products

What is gypsum?

Calcium sulfate dihydrate

What happens when gypsum is heated?

Looses some of its water of crystallization and becomes calcium sulfate hemihydrate

Type I gypsum

Impression plaster - impression and bite registration

Type II gypsum?

Dental plaster - study casts , denture curing mounds, articulator mounting

Type Ill gypsum

Dental stone - stronger master casts and moulds

Type IV gypsum

High strength , low expansion die stone - crown, bridge, and inlay dies

Type V gypsum?

High strength , high expansion die stone - dies and moulds where increased expansion is needed

Distinction of type II to type III

Type II contains beta hemihydrate

Type III contains alpha hemihydrate

What is calcination?

Process of heating gypsum to remove water of crystallization

How is beta hemihydrate in type I and II produced? What is the process called?

Heating gypsum in an open kettle at 110° -130° C

Process is called dry calcination

How are the crystals described as in beta hemihydrate?

Needle-like

How is alpha hemihydrate in type lll produced?

Produced in an autoclave under steam pressure at 120- 130°C at a pressure of 17 lb/in2 for 5-7 hours

How do the crystals in alpha hemihydrate shaped?

Rod or prism shaped and denser than beta crystals

How are modified alpha hemihydrate found in types IV and V produced?

Boiling gypsum in 30% calcium chloride solution, followed by washing away chlorides

How are the crystals in modified alpha hemihydrate?

Dense and hard, described as cuboidal

What type of reaction is the setting reaction?

Exothermic - releases heat

How long is the mixing time?

60 seconds

How long is the working time?

1 minute

How long is the manipulation, tray filling and cast pouring?

3 minutes

What occurs during the initial setting?

Surface gloss disappears - mass reaches semi-hard stage - can still be carved with a knife

What occurs during final setting?

Conversion from hemihydrate to dihydrate is complete - model can be separated from impression without fracture

What are the two penetration tests used to test for setting time?

vicat and gillmore tests

Longer and faster mixing

Decreases setting time

Increased water / powder ratio

Increases setting time

Temperature rising from 20°C to 37°C

Decreases setting time

Temperature above 31°C

Increases setting time

Temperature at 100°C

No setting occurs

Increased humidity

Increases setting time

What do accelerators do?

Decrease setting time by providing additional nuclei of crystallization

What do retarders do

Increase setting time by reducing solubility and inhibit crystal growth

Example of accelerators

Sodium or potassium sulfate 2-3%

Examples of retarders

Borax 1-2%, sodium chloride 3.4-20%

What happens in normal setting expansion?

Crystals grow and push outward against one another during setting range of 0.05-0.5%

What happens in hygroscopic setting expansion?

Happens when gypsum is placed under water before initial setting. Greater than normal setting expansion

With impression plaster , dental plaster, and stone what type of setting is undesirable?

Normal setting expansion

More excess water in gypsum does what to strength?

Lower strength

How much stronger is dry strength than wet strength?

More than twice

4 - 9.0 - 20.7 - 34.5 - 48.3

When can a cast be used?

When it reaches at least 80% of its 1 hour strength

What does higher compressive strength of the hardening mass mean?

Higher surface strength

Is gypsum stronger in compression or tension?

Stronger in compression