People who haven’t worked with clay often don’t realize that there are actually many different types of clay, and that each has specific properties that makes it more suitable for some uses than others. Clay suppliers formulate recipes for combining clays and other ingredients to create clay bodies that perform reliably under the conditions in which they will be used and give a satisfactory result. I work with three types of clay:
Stoneware and porcelain
Stoneware and porcelain are both high-fired clays which means they are fired to such high temperatures that the clay begins to coalesce into glass. It shrinks and becomes more dense which makes it more durable and chip-resistant than low-fired clays. Fired stoneware and porcelain absorb almost no water and therefore are not affected be freezing temperatures in the same way as terracotta. As long as high-fired items are protected from becoming encased in or filled with ice, they are pretty much alright to leave outside during the winter.
Stoneware clay itself is generally buff to brown in color, although there are some white or brownish-orange variations. Firing stoneware in a gas kiln gives the clay speckles and interesting color variations. These effects are sometimes imitated with electric firing by mixing additives into the clay.
Porcelain is noted for its pure white color and its fine smooth texture.
A lot of the chemicals that cause glazes to have bright colors, particularly reds, yellows, and purples, burn away or become black at higher temperatures, so high-fired ware generally has more muted, subtle colors and textures, more blues, greens, and browns.
Terracotta is a low-fired clay, generally reddish-brown in color. Low-firing leaves clay still comparatively open and porous so it readily absorbs water. If it’s not glazed, water will leach right through it, a quality that can be used to advantage in a number of ways. However, wet terracotta will break down if allowed to freeze due to the expansion of ice crystals inside it.
Low-fired clays can be glazed with a much greater range of bright colors so colorful decoration is often a much more prominent aspect of low-fired ware.