FINENESS OF GRAIN.
By fineness of grain or texture of a clay is meant the size of its min eral particles. Experimental evidence indicates that variation in grain controls many of the physical and pyro-chemical properties exhibited by clays. Plasticity, shrinkage in drying and burning, tensile strength, drying properties, rate of oxidation, rate of vitrification, toughness of burned ware, and finally, to some extent pyrometric value of the clay, are all influenced by fineness of grain.
The grains of many clays are so cemented that they resist separation in the ordinary pug-mill or blunger. When two or more particles are thus cemented they operate as a unit in their influence upon plasticity, _ tensile strength, drying behavior, etc. This accounts, in part, for many of the apparent exceptions to the general rules deduced from experiment al evidence, for, in the usual methods applied for determining fineness of grain, special effort is made to separate the particles completely.
This raises the question whether separation of the particles should be carried to such extremes when attempting to trace direct relations be tween fineness of grain and the physical properties developed in the process of manufacture of clay into wares. On this point, however, we have no direct evidence, except perhaps as shown in Figs. 4 and 5, so the question will have to remain unanswered for the time being.
It is known, however, that it would be well-nigh impossible to determ ine how far a mechanical separation of the particles should be carried in the laboratory to make the test comparable to the separation effected in the pug-mill, wet pan, or blunger. For this reason it would seem as though the most useful data concerning texture or fineness of grain can not be obtained by the present method of analysis.