Jeff Zamek Ceramics Consulting Services

Traditionally, potters operating at stoneware temperatures fired pottery to cone 9 (2300 0 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 low temperature firing of clay and glazes. Often the glaze layer looks “painted” on the underlying clay body surface.

Many potter’s are now discovering a lower stoneware firing temperature at cone 6 (2232 0 F.). While this lower temperature is often tried to lower the cost of fuel or electricity which by itself is a false economy, labor and time are the overriding cost 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. The same kiln firing 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 0 F. in temperature can be tiresome. Often what is not recognized is less wear on kiln shelves and posts due to a lower temperature firing.

What is need 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 may be porous and less dense which may cause leaking of containers and/or delayed crazing in glazes. However, many ceramics suppliers do offer cone 6 clay body’s that will work fine with cone 6 glazes. Often a clear gloss cone 9 glaze at cone 6 will 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.

Often the largest problem is making the initial change to the lower temperature range but 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.