Firing

Firing makes pottery permanent. Before a piece is fired it is fragile and can easily become a lump of mud again. Firing applies intense heat, first to drive out all the moisture, then crucially to force the clay particles to bond permanently with each other, creating virtual stone. Firing is a science. Heating too quickly, going too hot or cooling down improperly can destroy work, as my melted earthenware mug pictured shows.

Pottery is usually baked in the kiln twice. The first is called a "bisque," or biscuit firing. This bonds the unglazed clay body so that it can no longer be dissolved by water. After this, glaze is applied and the piece returned to the kiln for a much hotter round two. Here, glaze melts and bonds with the clay surface, and the clay body matures, often vitrifying – forming glass inside the clay. Depending on the potter’s desired outcome or further surface decoration, it may be fired again and again, but the first two steps are the most important and as far as most pieces will go.

There are many types of firing and many types of kilns. The most common is the electric kiln, which uses coiled wire elements nested between heat resistant fire bricks. This type of firing has only existed for roughly 100 years. With electric heat, oxygen is present in the kiln so this is called an “oxidation” firing. The chemicals within the glaze react differently in the presence of oxygen than without it and colours can turn out quite differently with or without the presence of oxygen.

The other type of firing is called “reduction” and takes place without oxygen, often in the presence of an open flame that consumes the oxygen in the kiln atmosphere. Reduction potters use gas-fuelled flames or wood fuel in kilns that look very similar to electric kilns. However, for millennia ceramics have been and still are reduction fired in slow burning pit fires, cave kilns, even garbage can kilns.

Whichever method is used, ceramics are heated slowly to at least 1,000 degrees Celsius, often over 10 hours and sometimes over the course of several days. It is simply not an environmentally neutral process, so take care of your pottery. It took a lot to get here.