Kilns are where the magic of pottery happens. The process of firing turns clay into ceramics and your raw work of art into a finished masterpiece. While early kilns consisted of a bonfire over a hole in the ground, technology has vastly improved to create sophisticated ceramic kilns. How a ceramic piece is fired has a huge impact on the look of the finished result. Firing is a craft in itself, and requires an open mind and a good deal of experimentation.
Kilns are freestanding, and their size can range from the size of a stove to a full room. Most kilns contain shelves, where the pottery is lined up. It's important that glazed pieces don't touch, or they can adhere together. There's a method for firing large un-glazed pieces that's called tumble stacking, where pieces are carefully stacked on top of one another. This is a job best suited to a skilled potter.
The three most common types of kilns are electric, gas and wood. Electric kilns are probably the most common type of kiln used in ceramics. They're comparatively inexpensive, and small ones can plug directly into a 120-Volt wall socket, making them accessible to small pottery operations. These kilns always fire in oxidation, meaning there's oxygen present in a completely controlled environment, which yields consistent results with glazes. This control is important for a potter who wants to replicate their work.
Gas kilns run on natural gas and fire in reduction, which doesn't allow oxygen in during firing. Reduction firing results can be unpredictable, but typically yields rich, earthy colors. It's difficult to maintain consistency, so gas kilns are most ideal for one-of-a-kind creations.
Wood kilns are fueled by wood and have been used for thousands of years in pottery. They're very labor-intensive because they need constant stoking and re-fueling of the fire to keep the wood at consistently high temperatures. A wood kiln firing can take three times as long as it would in an electric or gas kiln, and it needs to be monitored the whole time. The kiln creates its own glaze when the wood ash lands on the pieces as they're firing. Many potters feel that the unique results are worth the extra work.
There are also a couple types of specialized kilns that produce specific surface results. Salt kilns, also known as soda kilns, produce a bumpy glaze that's most commonly found on stoneware pieces. This happens when salt is introduced during the final stage of firing. The heat puts the salt through a chemical reaction that leaves a residue glaze resembling an orange peel. This glaze also ends up all over the kiln, which can decrease its lifespan.
Raku kilns are another type of specialty kiln that yields a specific finish. Raku pottery is heated until it's glowing, and then pulled with tongs out of the kiln. After it cools, the piece is immersed in cold water, creating a crackle effect. Unglazed areas of the clay become black from the carbon in the burning fuel, and when the carbon is scrubbed off, a bright metallic finish is revealed.