GYPSUM PLASTER IN THE CERAMIC INDUSTRIES ¹

ABSTRACT This paper deals with manufacture, properties, and uses of gypsum plaster in the various branches of the pottery industry, i.e. , in white ware, stoneware, electric insulators, sanitary ware, etc., in the terra cotta industries, in plate glass, and for sundry minor ceramic uses. The methods of producing calcined gypsum are briefly described and then follows a discussion of the physical and chemical properties of the three principal forms of calcium sulphate as used commercially, (1) crude gypsum, (2) calcined gypsum (gypsum plaster), and (3) gypsum casts of set‐up gypsum plaster. The proper methods of mixing, blending, and casting plaster are described, with graphs showing effects of varying such factors as consistency, mixing time, etc. Methods of testing plasters are briefly discussed. Considerable attention is given to difficulties commonly encountered in the use of plaster, with causes and methods of coffeetion. This includes the use and abuse of plaster molds, such as “rotting” of molds, “burning” of molds, efflorescence, etc. The properties of gypsum plaster can be varied within wide limits to meet special requirements. It is shown how the plasters used for pottery and terra cotta molds are quite different from those used in bedding plate glass. The purpose of the paper is to give a more complete picture of how gypsum plaster may and should be used in theceramic industries.