Abstract: This invention provides a method and apparatus for mixing calcined gypsum. The invention comprises a mixer which employs high pressure water to eliminate or substantially reduce the formation of lumps of gypsum inside the mixer which cause paper breaks when the calcined gypsum slurry containing the lumps is used to form the gypsum core in a gypsum wallboard product. High pressure water jets direct a spray of high pressure water at the surfaces within the mixer where the lumps of gypsum are formed. The method of this invention utilizes the high pressure water as part of the metered water that is continuously fed to the mixer, with the balance of the water being fed through low pressure nozzles.