WHAT MAKES POTTERY STONEWARE? In general, there are three major kinds of pottery. They are earthenware, stoneware and porcelain. Earthenware is fired at lower temperature than the other two which makes it more porous and not as strong and chip resistant . Earthenware is often made from natural surface clays like that used for common red bricks with very little modification. Other earthenware clay bodies are made from more pure clay deposits with additions of pulverized talc or other additives to help it fire to a strong product that is often almost white. Earthenware is usually fired to temperatures below 1900 degrees Fahrenheit. Earthenware is the product that in the past often used lead oxide as a flux to melt glazes at lower temperatures. Today most earthenware is lead free, but you can't tell by looking. Earthenware glazes can be very bright and beautiful. Stoneware pottery is divided into two groups, mid range and high temperature. For the last 20 years or so, there has been a gradual shift to more use of electric kilns and more mid range stoneware being made. Some of this is due to the difficulty and expense of acquiring and siting the gas fired kilns that are usually used for high fired stoneware. It is much easier to get started with mid range stoneware, and a lot of really nice glazes have been developed recently. Both kinds of stoneware can meet the technical requirement of being less than 1% porous, because it is easy to add more fluxing material to the clay body. Potters speak of cone 6 and cone 10 as mid range and high temperature stoneware. The cones are the pointed ceramic temperature sensors that are made to melt at a specific temperature. Cone 6 is reached at 2232 to 2269 Fahrenheit and cone 10 is 2345 to 2381 depending on the speed of temperature rise in the kiln There is only about 115 degrees Fahrenheit difference between the two cones, but the effect of more heat is important. As I understand it, the big difference is that the mineral mullite forms at the higher temperature which makes the clay body stronger and more chip resistant. It is also likely that a lot of cone 10 stoneware is fired a reduction atmosphere ( meaning a lack of oxygen ). This changes the behavior of Iron oxide which melts easier and becomes a strong flux in reduction. Reduction firing makes a whole range of beautiful colors and textures possible that don't happen in oxidation. Many of the famous Japanese and Chinese glazes are reduction fired. Porcelain is a similar story. There are both mid range and high temperature versions. A porcelain clay body is really a mixture of relatively pure kaolin clay, fairly pure ball clay, ground flint and ground feldspar. This kind of mixture tends to be much harder to work with than the more clay like stoneware. Porcelain is prized for its whiteness and it is often translucent which sometimes seems to make it glow. It is commonly used to show off the famous copper red and celadon glazes when fired in reduction. Another common variation of high temperature porcelain and stoneware pottery is wood firing. The smoke and wood ash blowing through the kiln often result in very beautiful glaze effects that are not achieved in any other way. Wood kilns can fire in either oxidation or reduction in the same way that gas kilns do.