Japanese Shino Sake Set, stoneware, by Hironobu Nishitateno.
Every piece in the “Tabletop” exhibit starts with clay. If you look at the tags under “medium,” though, you’ll see a lot of more detailed terms.
Potters can use just a few words to say a lot about their process. But what’s the difference between earthenware and stoneware, or cone 6 and cone 10? Here’s a brief introduction to a few of the terms in this year’s “Tabletop” exhibit, open through this Monday, July 7.
When a potter uses cone numbers — for example, Compote With Four Dishes (photo) by Charlotte Martin, which is cone 6 porcelain — they are describing how hot the kiln was (and for how long) during firing. The word refers to an actual numbered cone that looks like these which gradually slumps over as it’s fired. As opposed to a thermometer, which measures the temperature at any given moment, the cones indicate the total amount of heat energy the kiln has put out.
Glazes are just a thin surface coating that fuses with the clay body during firing. As seen in Eric Botbyl’s Scraggleware Teapot (photo), an ash glaze uses some amount of ash mixed into the liquid glaze. Ash glazes were probably the first glazes to be used by potters. Raku technique, which is unrelated, can also involve ashes: in a raku firing, pots are heated in kiln and then cooled in the open air or placed in a container with combustible materials — which quickly catch fire and color the pots in unpredictable ways.
soda- or salt-fired
Each of these processes involves tossing a material — either salt or another sodium compound like baking soda — into the kiln during firing, which causes a glaze to form directly on the pottery inside. Both soda and salt firing produce a distinctive orange peel-like pattern on the finished piece. Green to Black Pitcher (photo) by Kyle Hendrix is an example of soda-fired porcelain.
earthenware, stoneware, porcelain
As seen in the graphic below, these terms refer to different mixtures of clay, which reach maturity at different temperatures. Earthenware is fired at the lowest temperature (with an orange-red flame in the kiln) and produces a porous body. Stoneware is less porous, and the highest-fired stoneware — when the flames are white-hot — is called porcelain. In porcelain, the clay has fused with the glaze and become vitrified (turned to glass) and strengthened.
Sources for this blog post: The Potter’s Dictionary by Frank and Janet Hamer and Ceramics: A Potter’s Handbook by Glenn C. Nelson.