Clay is a natural product dug from the earth, which has decomposed from rock within the earth’s crust for millions of years. Decomposition occurs when water erodes the rock, breaks it down, and deposits the eroded particles. The particle size of these minerals is extremely small. Because it is dependent on the type of rock that it decomposes from, clay is specific to a geographic area. Hence, different types of clays, each with its own particularities, are found in different areas of the world.
Two properties of clay
Clay differs from the inelastic earths and fine sand because of its ability, when wet with the proper amount of water, to form a cohesive mass and to retain its shape when moulded. This quality is known as clay’s plasticity.
A second property of clay is that of shrinkage and rigidity when air-dried or heated. The more water clay holds, the more it will shrink when it dries. To prevent excessive shrinkage which will deform a finished piece, a potter can add in small quantities of materials that do not absorb water to the clay, such as feldspar or flint, called additives.
Additives: what are they for?
Different types of additives lend different qualities to the clay. Different types of clay can also be mixed together. Ingredients may be added to aid plasticity, change its fired coloration, or to lower or raise the firing temperature. To make the clay materials fuse better, a flux may be added. A common flux agent is wood ash, which was also used extensively in Cambodian and Thai pottery as glazing.
Sometimes potters also use clay that has already been fired and then ground up to add to their mixture. This type of substance is called grog. Grog can be used to add colour to a piece as well, as can flakes of rust or adding chemicals such as manganese dioxide.
The clay and additives are then mixed thoroughly together with water to form a liquid mud, called slop. Once the ingredients are sufficiently blended, the slop must be air dried, so that excess moisture may evaporate, otherwise the clay will not have enough consistency to be shaped.
The clay body
The resulting compound is the clay body, although we generally use the term clay to describe this composite material that is used to make pottery.
A third property of clay
A third characteristic of clay is that when heated to high temperatures, it partially melts, resulting in the tight, hard substance known as ceramic material. The higher the temperatures, the harder the clay becomes, depending on the type of clay used. Firing also changes the colour of the clay and makes it less porous. Thus, firing and type of clay can influence the look and properties of the finished piece.
Pottery, therefore, is not made only from raw clay but a mixture of clay and other materials, and clay bodies come in a wide spectrum of colours, from white, to red, to black, and many colours in between.
De-airing before shaping
Before a lump of clay can be fashioned into a ceramic object, another step of preparation is needed.
Besides forming a malleable clay body with different additives, the mixed up clay has to be de-aired to remove any air trapped within it, as air bubbles in the clay can cause cracks in the final product when it is fired. This is because the trapped air will expand in the hot kiln atmosphere and sometimes even cause the object to explode in the kiln, destroying other objects in the process.
Nowadays, a machine called a vacuum pug mill can be used, but in ancient times, a potter had to wedge it. This just means kneading the clay in different directions to push out the air bubbles. Wedging can also help to ensure an even moisture content throughout the body. It is safe to say that this technique of kneading clay has probably remained the same for millennia.
Only after all this is done can the clay be shaped into a desired object.
Pottery: an art and a science
Pottery-making is a long process. In modern society, clay is readily bought and ceramics can be mass produced. But imagine when potters had to dig their own clay from the banks of the river, remove the minuscule impurities from it, mix it with various ingredients that they had to collect themselves, fashion an object from the resulting clay compound, monitor the drying process, fabricate their own glazes, construct their own firing chambers (kilns),gather their own fire wood and even sell their own products.
Little wonder then that the objects that have passed through the ages now command such respect from us; respect for their art and craft, for those anonymous, ancient potters, of whom we know so little.
Clay from Bat Trang, northern Vietnam