AbstractConditions for production of plasters are surveyed in relation to variations in phase composition, surface area and crystallite size.The commercially known α‐ and β‐hemihydrate plasters prepared by autoclaving and dry calcination are not two distinct phases—they differ in crystal habit, surface area, crystallite size and lattice perfection. The terminal β‐hemihydrate and γ‐CaSO4 produced in vacuo have greater surface areas than samples produced in air.The most active γ‐CaSO4 (Class C plaster) samples are finely porous and rapidly rehydrate in atmospheric water vapour with considerable loss of surface area and porosity. ‘Wet’ rehydration (liquid water) or γ‐CaSO4 causes more rapid development of the 6.01 Å (10.10) spacing, in accord with the c‐direction elongation of the ‘α’‐hemihydrate crystals prepared by autoclaving or precipitation from 50% HNO3.The porosity of the γ‐CaSO4 diminishes considerably between 200° and 300°, but remains to a smaller extent up to 500° when the γ‐CaSO4 has changed to β‐CaSO4, anhydrite (Class D plaster). Anhydrite sintered extensively between 500° and 900° before decomposing to lime at temperatures up to 1400° (Estrich Gips plaster). Removal of the SO2 and O2 is facilitated by streams of nitrogen or carbon dioxide, which increases the decomposition rate. Calcination at temperatures below 1200° gives anhydrite and lime crystals of more comparable sizes, and avoids complete ‘dead‐burning’ of the lime.