Abstract: The invention relates to a method for the production of hard plaster made from calcined beta hemihydrate plaster, comprising adding of plaster material mainly consisting of beta hemihydrate plaster to a post reactor (3), after-treating in the post reactor (3) for forming hard plaster, wherein the beta hemihydrate plaster is directly converted into alpha hemihydrate plaster at a temperature above the calcining temperature of 95 degrees Celsius, removing the hard plaster from the post reactor (3), and cooling down. During the adding of the plaster material to the post reactor (3) the temperature is adjusted to at least the calcining temperature, the conversion is carried out in the post reactor (3) at a dwell time of less than 30 minutes while externally adding water vapor, and the temperature and the pressure in the post reactor (3) are regulated such that the temperature is at least 95 degrees Celsius and an overpressure is present.