There are many advantages to be gained by cone 06 reduction firing of art glazes as opposed to high temperature firing. High fire glazes, between cone 9 and 11, require kiln temperatures of about 2350 degrees Fahrenheit (1288 degrees Centigrade), whereas cone 06 glazes require temperatures of 2200 degrees Fahrenheit (1204 degrees Centigrade). This 150 degree Fahrenheit (66 degree Centigrade) difference requires considerably more energy and stresses the kiln, wearing it out more quickly. Since it can take from 2 to 4 extra firing hours with the gas turned on high to go from cone 06 to cone 10, mid-range reduction firing can save a considerable amount of fuel, as well as time in the firing process. Firing only to cone 06 is faster, cheaper, and produces excellent results indistinguishable in most cases from high temperature firing.
Mid range firing was pioneered in response to the energy crisis in the 1970’s; and with today’s uncertain fuel costs it is an idea whose time has come. The limitations of mid range firing lie in the problem of which oxides needed to melt the art glazes at lower temperatures do not produce true glaze colors. Iron oxide based glazes, for example, work well at lower temperatures. Using a given base glaze and adding 1 percent red iron oxide gives a blue celadon; adding from 2 to 4 percent iron gives a celadon ranging from green to amber; adding from 5 to 10 percent iron gives temmokus; and adding from 10 to 20 percent iron produces iron saturates. By adding magnesium carbonate to temmokus in conjunction with cooling soaks, a teadust temmokus is produced. Adding magnesium carbonate and bone ash to temmokus produces kakis; and adding magnesium oxide to temmokus produces oil spots. Thus variations on one basic art glaze can produce many different colors. Similarly, copper red dry glazes contain high alkaline and low alumina with small quantities of tin oxide and cobber carbonate. Glazes of the oribe type utilize copper to obtain greens; glazes of the magnesium matt type with cobalt oxide produce translucent white and purple.
The only types of art glazes which don’t work so well in mid range firing are the feldspar-clay shino glazes. This is because feldspar melts at about cone 9; and with clay additions it only melts above cone 10. To obtain shino effects it is possible to use nepheline syenite, which is a feldspathoid rather than a feldspar. Since it contains less silica than true feldspar it melts at about cone 6; but the result is not identical with true shino glaze effects. Thus, cone 06 glazes work quite well for iron oxide and copper red, as well as a wide range of blues, greens, blacks, and carbon trap shinos (which contains soda ash which melts early). Only the traditional shinos don’t work as well at mid range. But all in all, cone 06 firing saves a considerable amount of fuel, thereby reducing the carbon footprint and being environmentally friendly. It also saves considerable time, reducing firing time from ten or twelve hours to only eight or ten. Finally, lower temperature firing reduces stress on the kiln, which means that it will last longer and perform better over the long haul.