(Cones are small triangles of clay formulated to melt at precise temperatures.
There are two types of stoneware clay: midrange stoneware and high-fire stoneware. Midrange stoneware fires to maturity between 2200º and 2283ºF, or cones 5-7. (Cones are small triangles of clay formulated to melt at precise temperatures. Potters place them in the kiln and use them to visually determine the temperature inside the kiln when firing). Midrange stoneware pottery is sometimes referred to as “ovenware” due to the practice of using it to cook in ovens. Midrange stoneware clays are easier to finish out in a home studio. They can be fired at a lower cost in electric kilns, with beautiful results, or, less commonly, they can also be fired in gas or wood kilns. High-fire stoneware is nonporous and very durable because of the high firing temperature. It fires to maturity at 2300°-2400°F (cone range 8-11) and is the most popular clay for functional pottery. High-fired clays are often used for salt-glazed and wood-fired pottery because a higher temperature is required for the salt or wood ash to melt and form a glaze on the surface of the pot. High-fired clays are commonly used for gas-firing as well. Some very striking glazes and effects are possible with these clays—effects that are not possible to achieve in lower temperature firings. Although the results of high-firing are well worth the effort, this type of clay is expensive and time consuming to fire, and kilns used to fire it wear out faster due to the stress of the high temperatures.