Impression plaster is:

IMPRESSION PLASTER Impression plaster is a β-calcium sulfate hemihydrate used at a water/powder ratio of approximately 0.5 to 0.6. Its fluidity makes it suitable for making impressions of soft tissues in the uncompressed state, a characteristic of mucostatic impression material.  Key concept: Impression plaster is less compressive than Alginate. Ref: Phillips 12 ed page no 177