A material composed of fatty acids, shellac, glycerine, and filler used as a primary impression material. When heated in a water bath at about 65°C (149°F) it becomes plastic and can be moulded in an impression tray and inserted in the mouth. The material becomes fairly rigid on cooling to mouth temperature and has low material flow at room temperature. It has a low thermal-expansion coefficient so that there is minimal dimensional change as the impression is cooled from mouth temperature and it does not adhere to moist oral tissue. It may be modified by the addition of an alginate wash. Impression compound is usually used for taking primary impressions of the edentulous ridge prior to the construction of a custom-made impression tray for complete dentures. Impression compound is also available in sticks of various colours indicating different softening temperature ranges e.g. greenstick compound (composition); these are used for impression correction and moulding the borders of custom impression trays, particularly around muscle attachments.