FIRING (1) Clay for pottery.
Firing: Clay – process: The firing of clay transforms it into bisque and is known as “Bisque Firing”. This changes the chemical composition of clay so that it becomes a stone-like material, breakable but still porous to water. Heat should build up slowly during the first hours of firing until the temperature reaches 600º C, which is the moment when it ceases to be clay and becomes bisque, a transformation known as “Ceramic change”.
Clay pieces should be generously spaced to allow air and heat to go right round them when drying or being fired. The clay must be dry for bisque firing. The ware that is not decorated with any type of glaze can touch other pieces; one piece of ware can be put inside another or boxed edge to edge but do not overdo it. For example, placing cups upside down one on top of the other would allow heat to spread evenly round each piece, but to put one or more flat tiles on top of each other would create a barrier. Heat would rise quicker on the outside than the inside and lead to warping, cracking and breaking. I only work with clay for items in relief and tiles of a special size. If the pieces are big, scrape out as much clay as possible at the back and leave to dry for a long time. When starting to fire, bring up heat slowly, so that they can dry out completely and all the chemicals and water can escape.
During the whole process of heat rising, clay shrinks and changes color. The more slowly the heat rises during the first hours of firing, the less likely you are to have problems. Normal temperatures for bisque firing are between 850º C-1300º C. The higher the temperature, the lower the porosity will be in the fired clay. During the first hours of firing, many gases have to escape. For this reason the kiln peepholes must be left open. When firing work in relief at a final temperature of 1300º C, I make the kiln take about 5 hours to reach 600º C; the rest takes about 6 hours more. The following are temperatures for different clays. When you buy your clays they usually come with instructions that should be followed.
Red Earthenware: 900º C – 1100º C
White Earthenware: 1060º C – 1180º C
Stoneware: 1200º C – 1300º C.
Porcelain: 1280º C – 1350º C
Bone China: 1200º C – 1300º C.