Clay is a product of decomposition of rocks, particularly granite that is abundant all over the earth surface. Granite, which is also known as feldspathic rock, decomposed through time in many different physical actions such as wind, rains, erosion, earth movement: earth-quake, volcanic eruption etc., and eventually become clay as we know it. Clay is highly malleable substance and its plasticity: the important quality that enables it to retain a given shape when molded and shaped by hand or by machine. The bulk of most clay bodies is composed of the mineral Alumina and†† Silica, plus some small amount of other minerals. Chemically, clay composition would resemble this following formula:††† Al203.2SiO2.2H2O.†(also known as Alumina Silica Hydrate). The presence of other minerals in addition to alumina and silica, determines the firing temperature it requires to mature. Thatís why most porcelain clay-bodies is fired in much higher temperature than red earthenware clay. Therefore it is important to know how to differentiate stoneware from earthenware, or porcelain from white earthenware because in their unfired condition they may look exactly alike.
Let us take look what makes porcelain different from stoneware, and what makes stoneware different from earthenware. In order to do that, we need to visualize how clay formed in nature. Feldspathic rock, which is abundant on the earth surface, is the clay origin. When this rock decomposed through time- million of years in the process, by various actions: rains, erosions, earthquake, tectonic plate movement, etc., the rock gets pulverized into tiny particles. When these particles, in mass quantities, remain in place through time and not getting contaminated by other substances, then they become the purest form of clay that may resemble that hypothetical formula of clay. This type of clay then is known as porcelain. When the massive amount of particles of pulverized feldspathic rock come in contact with some contaminants, for instance during the process of erosion created by rain, where the rain water carves parts of soils that happens to be heavy in iron content, or some other metallic particles and minerals, these contaminants will change the appearance of the clay, before it is fired: the contaminants may change the color of the clay from white (the color of most feldspathic rocks) to grayish, or reddish.††
†† If the presence of these contaminants does not change the balance of the refractory elements of the clay (refractory makes clay mature at higher temperature) and the fluxing element of the clay (flux lowers the maturing point of a clay body at lower temperature) in any significant way, then this type of clay is known as stoneware. Most stoneware are fired at temperature that reach maturing point for porcelain as well, mostly in high temperature category.
†† When the presence of these contaminants alter the maturing point of the clay-body, where their presence is very noticeable, for instance they change the color of the clay to chocolate color, or reddish, then most likely the presence of iron (in the case of red clay) is very heavy. Iron, a metallic substance melts at a much lower temperature than pure clay. Therefore, heavy presence of iron in a clay-body will lower the maturing point of that clay-body significantly. Red clay such as terracotta is a good example of that. However, we can not assume that all terracotta, or low firing clay bodies are always red, because there are other contaminants that function like flux that are of different colors. They may be yellow, grey, or even white.
Glaze composition is not too much different from clay composition. Clay and glazes share similar components. Both have substances that increase or lower the melting temperature (flux and refractory), and some chemicals that exist naturally clay. As a matter of fact, glazes are combination of different clay plus the addition of metallic oxides (for color), and minerals and Silica. Silica or Flint, is the substance that forms glass that gives the glossy surface of the glaze, has its origin from sand (as in beach-sand). Some clay, such as EPK (Edgar Plastic Kaolin) has high silica content by nature. Therefore some glaze recipe may not require any addition of Silica/Flint, because some of the clay and minerals in the recipe already carry enough amount of silica in it to form the glassy surface desired. Silica or Flint requires high temperature to melt, therefore addition of flux is necessary to have the glaze melt at a desired temperature.