Dental Gypsum Products: Types, Ratios, and Setting Times

Study model

Is used to plan treatment and to observe treatment progress

Cast

Is a replica on which a restoration or appliance is fabricated

Die

Is a working replica of a single tooth

Dental plaster

The first gypsum product available for dentistry

Stone

Made from gypsum by carefully controlled calcination under steam pressure in a closed container

Improved stone

Gypsum is calcinated in a calcium chloride solution

Water : powder ratio for plaster

45-50ml/100g

Water : powder ratio for stone

28-30ml/100g

Water : powder ratio for improved stone

19-24ml/100g

Initial setting time

The length of time from the start of the mix until the setting mass reaches a semi-hard stage

Final set time

Represents the length of time from the start of the mix until the setting mass becomes rigid

Increased setting time

Decreased mixing, higher water/powder ratio, addition of chemical retarders (e.g., borax)

Decreased setting time

Increased mixing, lower water/powder ratio, addition of chemical accelerators (e.g., potassium sulfate)

Wet strength

Strength dependent on w/p ratio and porosity of the material

Dry strength

The strength that is measured when the excess water is not present in the sample

Plaster expansion

Plaster expands the most 0.2% to 0.3%

Stone expansion

Stone expands 0.08%- 0.10%

Improved stone expansion

Diestone expands the least -0.05% to 0.07%

Initial setting time for gypsum products

30-45 mins

Chemical reaction water ratio

Chemically -18.6ml : 100g