RELATION OF VOLUME SHRINKAGE TO FINENESS OF GRAIN.
The volume shrinkage of a clay is a reliable index of its drying be havior only within certain limits. Take for instance K-14 and H-17, which lie close to the extremes of minimum and maximum volume shrinkage; both require considerable care in drying. Roughly we can say that clays which exhibit an average shrinkage will dry safely, and that if the ware exhibits either a high or low volume shrinkage it can be assumed to be likely to occasion trouble in drying. But knowing this general fact, how can the drying behavior of a particular clay be esti mated ? It has been suggested that clays which have a fair range in size of grain, i. e., not too large a proportion of either the largest or smallest grains, can be dried with greatest safety. Jhis we proved to be true for the clays plotted near the middle of a diagonally drawn dotted line in Fig. 12 were the easiest to dry and those at the extreme ends the most difficult.
It was demonstrated, however, that while there may possibly be a reciprocal relation between porosity and fineness of grain in the naturally soft and loose-grained clays; there is no trace of such a reciprocal rela tion in the harder clays, like shales, because the cement which holds the grains is not broken by the methods of preparation usually employed. It has also been shown that there is no reciprocal or proportional rela tion between the porosity of the dried ware and the volume of shrinkage. This same lack of proportional relations was found between water of plasticity and volume shrinkage, as well as water of plasticity and por osity. The only factors that seem to exhibit any proportional relation with volume shrinkage are "excess water" and fineness of grain. These factors alone are not, however, sufficient evidence on which to base an answer to our query.