What Is Clay?
Clay is the result of the crumbling of a certain kind of rocks called feldspars. When feldspar is exposed to the action of the weather, it crumbles slowly at the surface and the little fragments combine with a certain amount of water, forming clay. Pure clay is white and is used in the manufacture of china and porcelain. The common clay that we usually think of when we think of clay, is generally yellowish, but there are many different colored clays. Most of these colors, particularly those of red clay, yellow clay and blue clay, come from the iron which is present in the clay. Clay which contains iron is useful for making bricks. Bricks are made from clay by first softening the clay and pressing it in molds, the size of a brick. When dried for a time in the sun they are put into an oven and baked in great heat and they become quite hard and generally red. Most of the clay from which bricks are made turns red when baked, whether blue, yellow or red, because the iron which is in the clay is generally turned red when subjected to heat.
For making porcelains it is desirable to use the kinds of clay which contain nothing that melts when heated to a high degree. Clays which contain substances which melt in strong heat are, therefore, not good for making porcelains. There is a pure white clay called Kaolin which is very excellent for this purpose. Clay out of which we make firebrick for lining stoves and fireplaces is free from substances which melt. Several kinds of clay are good for making paints.