substance

Clay substance

The analytical data of clays is of the greatest service to the potter, but generally clay is consisted of:

Silica

Alumina

Ferric oxid

Ivime

Lime

Magnesia

Alkalis

Combined water

Quartz

The character of clay

The character of clay depends largely upon the mutual proportions of alumina and silica, so that, although these are generally the largest constituents of clay, it is important that they be determined with particular accuracy.

An accurate separation of mineral and quartz sand, by digesting the clay in strong sulfuric acid, and after washing out the excess of acid and the sulfates formed by the decomposition of the clay substance proper and the micaceous minerals, and removal of the separated soluble silica with a solution of alkali carbonate, described in text-books on analytical chemistry, gives data of great practical value.

When the sand thus found in clay is not pure quartz, but, as is generally the case, consists of the detrital matter of a great variety of minerals, it becomes very difficult to estimate its probable effect in its influence on the refractory qualities of clay, and the resulting coefficient of expansion of the burned body. The difference between the weights of the constituents found in the complete analysis of the clay and those of the sand, left by the sulfuric acid and alkali carbonate treatment, are calculated as constituting the "clay substance." Practically, the analytical data of clays is of the greatest service to the potter, who is familiar with "clay, flint, and spar" and knows how to vary their proportions for his various ends. It has resolved the composition of natural clays into components with which he knows how to deal, and made once unintelligible analyses serviceable to him.