Ceramic Industry

Cone 6 vs. Cone 9

June 30, 2010
Potters operating at stoneware temperatures traditionally fire pottery to cone 9 (2300°F), but many are now discovering a lower stoneware firing temperature at cone 6 (2232°F).

Soda fired covered jar (cone 6)

Traditionally, potters operating at stoneware temperatures fired pottery to cone 9 (2300°F), resulting in a dense, hard vitrified clay body and glaze. One characteristic of such high-temperature firings is the well-developed interface where the clay body ends and the glaze layer begins. The clay and glaze surfaces fuse together in a seamless transition, looking and feeling like one entity. Such integration of ceramic materials is often not the case with earthenware or in other cases where the low-temperature firing of clay and glazes takes place. Instead, the glaze layer often looks “painted” on the underlying clay body surface.

Many potters are now discovering a lower stoneware firing temperature at cone 6 (2232°F). While this lower temperature is often attempted in order to lower the cost of fuel or electricity (which by itself is a false economy, as labor and time are the overriding costs to produce pottery), it does result in faster firing and cooling times. On average, a 20 cu/ft kiln can take 18 hours to reach cone 9. Firing the same kiln to cone 6 will take 3 hours off the heating and cooling times. Staying in the studio and watching the kiln climb the last 68°F in temperature can be tiresome. An additional benefit is less wear on kiln shelves and posts due to the lower-temperature firing.

Soda fired bowl (cone 6)

What is needed when dropping down to cone 6 is compatible glazes and a dense vitreous clay body (if functional ware is required). Some cone 9 clay body formulas will, in fact, function at cone 6, but they may be porous and less dense. This can cause leaking of containers and/or delayed crazing in glazes. However, many ceramic suppliers do offer cone 6 clay bodies that will work fine with cone 9 glazes. A clear gloss cone 9 glaze at cone 6 will often be functionally correct but become satin matt opaque. With the ease of glaze calculation software programs, many glazes can be adjusted to fire at cone 6.

The largest problem is often making the initial change to the lower temperature range. However, with careful planning and testing, the pottery can be as dense and durable as in cone 9. In fact, it is possible to duplicate cone 9 clay body and glaze colors at cone 6, with the added benefit of some metallic coloring oxides offering a wider range of colors.