Chinese N. Sung porcelain wine ewer and
warmer 960-1,127 AD - courtesy R&G
McPherson Antiques

The third type of ceramic is porcelain, which was originally made up of white China stone or petuntse and kaolin or white China clay. Porcelain also requires a high firing temperature between 1,280 and 1,480 degrees C, for a considerable time (days). It is a highly transformed translucent ceramic material, impermeable but not very attractive, so it is almost always glazed. When made into a relatively thin walled item, porcelain is highly translucent and remarkably strong.

Pottery is sometimes described as high or low-fired, and although not precise this is broadly split above or below 1,200 degrees C. The porosity (water absorption) of the three types of pottery varies from 0 – 0.05% for porcelain, up to 3% for stoneware and up to 10% for earthenware. This is why ordinary earthenware is not frost resistant. Conventionally, the word vitreous is used to describe a body with less than 0.5% porosity.

A significant characteristic of ceramics, like glass, is that they are much stronger in compression than in tension. For example the compressive strength of stoneware is some 50 times its tensile strength. So an ordinary earthenware flat tile on a flat surface can easily support a person, but if one edge is raised it will crack and break under little weight.

7.2 Clay

Clay is found naturally all round the world, but some regions are luckier than others. For example kaolin is found more frequently in tropical regions, and while kaolin proved elusive to find in Europe, it was more common in China, where it is also very pure, almost all “kaolinite”. Kaolin is derived from the decaying feldspar in granite as the result of eons of leaching, separating it from the other quartz and mica constituents. The availability of the right types of clay, particularly kaolin, in most parts of China, made the sustainable development of ceramics there that much more likely. The Chinese emphasis on pottery meant more resources being spent to develop the technology at a more rapid pace. From Central Asia westwards the use of metal vessels became more common, sometimes displacing pottery in importance.

There are several meanings given to the word clay. Universally it is a natural material, mainly mixtures of minerals such as silicates and oxides, dug out of the ground. Minerals are natural compounds with a definite chemical composition. As you would expect, this clay is very variable. Then there is the more scientific definition of clay, “chemical clay”, which relates to that part having particle size less than 2 microns. This is a very important distinction as the physical and chemical characteristics of this clay, particularly its plasticity, are key to the formation of ceramics, which will be described in more detail in Chapter 11. The third type of clay is “potters clay”, which is used by the potter to make his products. This is a mixture of “as dug” clay containing chemical clay along with a number of possible additives selected by the potter to meet his requirements.

The nature of the original rock that decomposed into clay, and any impurities picked up during natural transportation of secondary or sedimentary clays, affect greatly the characteristics of the clay. Feldspar itself disintegrates and the alkaline components dissolve away leaving alumina (Al2O3) and silica (SiO2). Chemists use a shorthand or special “language” to describe chemicals so that “Si” represents silicon and “O” oxygen. When they combine chemically in the ratio of one molecule of silicon to two molecules of oxygen they form silicon dioxide or silica (SiO2), which is a unique material with totally different properties to the two constituents. Similarly aluminium oxide has the ratio of two molecules of aluminium to three of oxygen. When alumina and silica combine with water they produce a pure white clay - kaolinite (Al2O3 2SiO2 2H2O). Kaolin is the name of a group of materials having this same basic chemical composition but different minor ingredients. “China Clay” is the somewhat confusing name given to the purest natural clays that are all or almost all kaolinite.