Type IV Gypsum
A high-strength, low-expansion alpha-hemihydrate dental stone used for die fabrication in fixed prosthodontics, as classified by ADA Specification No. 25 and ISO 6873.
Full Definition
Type IV gypsum, designated 'high-strength dental stone' or 'die stone' under ADA Specification No. 25 and ISO 6873, is manufactured from alpha-hemihydrate (α-CaSO₄·½H₂O) produced by autoclaving or boiling gypsum in calcium chloride solution. Its dense, regular crystal morphology requires less water for spatulation, yielding a set product with higher compressive strength (≥35 MPa) and lower setting expansion than Type III stone or Type II plaster. It is the standard material for working dies when fabricating cast metal or ceramic restorations. Editors should note that 'die stone' and 'Type IV gypsum' are synonymous in most contexts; 'high-strength stone' is also acceptable but should be used consistently. Type V (high-strength, high-expansion) is a distinct category and should not be conflated with Type IV.
Usage
Usage note: Use the Roman numeral form 'Type IV', not 'Type 4'. Always specify the ADA or ISO type when reporting gypsum product properties to avoid ambiguity between stone grades.
In Context
- "Dies were fabricated from Type IV gypsum to ensure dimensional accuracy during wax pattern carving." — Fixed prosthodontics laboratory procedure manual
- "The author used 'stone' generically throughout; the editor queried whether Type III or Type IV gypsum was employed, as the distinction affects reported expansion values." — Manuscript copyediting note