Abstract: An apparatus for preparing multiphase gypsum and particularly wall plaster which has an exactly defined composition comprises a plurality of heating stages including at least one high temperature stage and at least two first and second separate low temperature stages. Heating gases are directed from a combustion chamber first to the high temperature stage and subsequently into each of the lower temperature stages. One of the low temperature stages is employed as a pre-burner and it is fed with materials from a common storage bin to pre-burn the material and then direct it to the high temperature stage. The other stage is provided with the heating gases from the high temperature stage and in addition it is controlled as to temperature, humidity and quantity of heating gases in its products together with the high temperature stage products after treatment are delivered to a common cooling device.