how long should glaze ware be fired in a typical school kiln?
I have a Jen-Ken Kiln and typically for the unglazed pottery I set it on low for an hour, medium for an hour and then High until the cone drops.
I am not familiar with a Jen-Jen Kiln. And yes, it does depend on the type of glaze being used. When pottery is being fired you have to go through the dehydration process, and then the organic matter burns out, then the clay matures (becomes hard).
A standard practice for glazes is generally a rapid fire (meaning approximately 170 degrees Centigrade per hour / 340 degrees Fahrenheit per hour). Glaze fires take longer than bisque fires.
Overglaze colors / enamels. . .Cone 018 – 720 C -1330 F
Chrome Red glazes . . . . . . . . Cone 015 – 805 C -1480 F
Lustre glazes . . . . . . . . . . . . . .Cone 012 – 875 C -1607 F
Low fire lead glazes. . . . . . . . .Cone 09 – 930 C – 1706 F
Low fire earthenware. . . . . . . .Cone 07 – 990 C – 1814 F
Earthenware glazes. . . . . . . . .Cone 02 -1125 C – 2057 F
Semi-Vitreous glazes. . . . . . . Cone 2 – 1165 C – 2129 F
China glazes. . . . . . . . . . . . . . .Cone 4 – 1190 C – 2174 F
Stoneware & Salt glazes. . . . .Cone 10 -1305 C – 2381 F
Porcelain glazes. . . . . . . . . . . .Cone 13 -1350 C – 2462 F
Now divide the glaze temperature by the rate of heat rise.
For Cone 018 it would be 720/170, or Four and a quarter hours. For Cone 015 it would be 875/170, or a little over five hours. For Cone 02 it would be 1125/170, or about six hours and forty minutes.
To be on the safe side, if you have a peep hole use the proper pyrometric cones for each fire. In a rapid fire, a cone requires a slightly higher temperature to bend than it does when the fire is slower and the heat has more time to “soak” the ware.
