THE MEASUREMENT OF THE PLASTICITY OF CLAY SLIPS12

Abstract

1The paper emphasizes the great need of a quantitative measure of the physical properties commonly included in the term “plasticity” of clays and clay slips. Various indirect methods previously suggested are likely to lead to erroneous conclusions, while measurement of the flow or apparent viscosity of clay slips made up with a constant amount of water is shown to give misleading results. 2The work described in this paper is preliminary in nature and has been confined to a study of the properties of clay-water mixtures in the casting rather than in the molding range. In the light of these results it is recommended that, instead of the vague term plasticity, the properties of a given clay, at least for casting purposes, be expressed in terms of (a) the water content required to give proper working consistency, and (b) the resistance of the resulting slip to small deforming forces (such as gravity). Specifically, use is made of a modified Bingham plastometer, and the figures reported are (a) the amount of water required to give a slip of a specified mobility, and (b) the yield value of the slip thus obtained. 3The method is shown to be capable of giving quantitative figures for the yield values at constant mobility which correspond well with the customary qualitative ideas as to the plasticity of clays. The amount of water required to give constant mobility varies to a surprising extent, and does not appear to bear any definite relationship to the usual classifications of more or less plastic clays. It probably does parallel fairly well the shrinkage on drying. 4Small amounts of acid or alkali are shown to have a negligible effect on the mobility of a clay slip, but a very large effect on the yield value, the variations being greater than the difference between a non-plastic kaolin and a highly plastic ball clay. This brings out the necessity, and indeed the potential value, of carefully controlling the hydrogen-ion content of the water used in making the slips. 5Some preliminary data are given on the effect of additions of flint and similar non-plastic materials to clay slips. 6It is recommended that an attempt be made to agree upon standard dimensions for the capillary and the desirable mobility for various purposes, and that the plastometer then be used as a standard method of test in the ceramic industry.