Definition:
Impression plaster is a rigid, non-elastic material made from gypsum (calcium sulfate hemihydrate). When mixed with water, it undergoes an irreversible chemical setting reaction forming a hard, dimensionally stable cast. It is too brittle for undercuts and is mainly used where rigidity is required, such as edentulous impressions.
Clinical Case:
A 60-year-old edentulous patient requires a maxillary complete denture. You select impression plaster for the primary impression due to its high surface detail and rigidity. A stock tray is loaded with the mixed plaster and seated carefully over the edentulous ridge. Once set (3-5 minutes), the tray is removed as a single rigid unit without deformation, allowing accurate mounting for the denture fabrication.