Casting bodies can be toned to a point of success by a proper manipulation of carefully chosen clays, with particular emphasis upon the china clays. A simple casting test will shed light on their individual casting properties. Regulation of casting salts and slip density, and the employment of grog in casting bodies offer means for a certain amount of control, but the preeminent qualifications of a good casting body are a balanced clay content and a correct ratio of clay content to non plastics. The purpose of this discussion is t9 describe the practical application of those elements which make a good casting of 'a' 'lays body, there is not one which exceeds in importance the proper proportioning of the clay. To have the ratio of clay to non plastics correct is of course not enough. There must be painstaking care in the selection of .clays employed, and the basis of selection should primarily be casting properties. Aside from impracticability in firing, it is well understood that any body containing only ball clay, irrespective of what the ratio of clay to non plastics might be, would not adapt itself to successful casting. Ball clays to be sure contribute the very essential property of fluidity to a casting body, but the moment the mold has been filled, this most valuable property of ball clays, as far as casting is concerned, has largely played its part. The finely dispersed clay particles are rapidly floated toward the mold surface, upon which they build up with sufficient density to impede the passage of water from the center of the piece to the mold. It consequently becomes necessary to introduce into a casting body a tempering influence which will modify this behavior of ball clays. This is essentially the r61e of china clays. The limits of these two generalizations are, however, extremely far apart, encompassing clays of widely different characters. Viscosimeter tests of ball clay slips will credit some ball clays with extraordinary fluidity, whereas others develop it only to a moderate degree. As a class, English ball clays are highly fluid , the dark clays generally more so than the light.
*Courtesy of IHS engineering 360