A method for the calcination of powdery or fine-particled plaster, comprising two steps: the plaster is subjected to a flash-calcination in a calcinator and the hot plaster is post-calcinated in a reaction vessel. Post-calcination is carried out in the reaction vessel by adding humid gas, said reaction vessel not being heated. The postcalcination takes place over a long period of time, that is at least 10 times, preferably 50-100 times longer than the amount of time taken for flash calcination. Calcination takes place without expending additional energy, and the remaining dihydrate produced during the flash calcination is also transformed into semi-hydrate and undesired anhydrite fractions are reduced. The method can ensure consistency in the product quality and also increase product quality. The temperature in the upstream calcinator can be lowered thus saving more energy. The method can also be used to accelerate the ageing of calcinated plaster.