The Nature of Clay
Return to Home Page
Return to More About Clay
The raw materials of pottery are found all over the world because the granite-type igneous rocks from which they come from account for most of the earth's crust. The hard igneous rocks are decomposed by hot gases from the center of the earth into feldspar minerals. Millions of years of exposure to weathering by rain, sun and ice break down some of these feldspars into the 'primary clays' which are always found where they are formed. When these primary clays are transported by the natural forces of wind, glaciers, rivers or seas they pick up impurities like iron and other minerals, as well as much organic matter. Their journey crushes and further weathers them, until by the time they are finally laid down, often thousands of miles from their place of origin, in river or lake beds, they are changed in color texture and particle size. They have become what we call 'plastic'.
Plasticity is the ability to be shaped or formed. The flat hexagonal shape of the clay particles and their size and, like sheets of wet glass they allow movement, and so shape, by slipping and sliding. The smaller the particle the more plastic the clay. As the water dries out they stop sliding and go rigid. So clay retains its shape, even when it is fired, it undergoes a chemical and physical change to become a hard and durable material. When the water dries out it will cause the clay to shrink, the usual rate is 12%, even to 17% or more, which can cause warping or even cracking. The potter must accept the fact that his pot will get smaller from the moment it is made to when finally it is fired in the kiln.
A raku clay is simply any clay which can survive the rapid heating and cooling process. Slip-cast, press-molded, coiled, or thrown, the tensions build up in the clay. A piece of work which has very thick and thin sections can shatter during either heating or cooling.
Anyone wishing to work in raku could best start by firing a range of different clays, some coarse, some fine, colored or white, to see what will survive. Most raku clay bodies for hand-building and throwing use high-temperature clays, such as fire clays and stonewares. At 1000 ºC (1830 ºF), and even up to 1200 ºC (2190 ºF), these clays are usually not vitrified. Spaces within the clays structure are necessary during heating and cooling. To increase this openness, grog sand or even pumice can be added in amounts up to 25 per cent of the total dry weight.
Return to Home Page
Return to More About Clay