The first firing of the green ware, also called raw ware, requires several precautions. Since the pieces are not glazed, they may be stacked together up to a point where they will not break or fit together too tightly when shrinking occurs. They chip easily in this dry state. Green ware and glazed ware should be fired separately whenever possible. The green ware may break and small particles stick to a glazed piece. Excess glaze may drip upon an unglazed piece.
Tripods for raising the glazed ware from the shelves come in a number of sizes. Fit a tripod on the bottom of each glazed piece and place it in position upon a shelf in the kiln. There should be only a thin coat of glaze on the bottom of the piece. It is not actually necessary to have glaze on the bottom. Care must be taken so that the pieces do not touch. They should fit solidly upon their tripods. There is a certain amount of movement of the pottery as the maximum temperature is reached. Sometimes one piece will fall over and ruin several others during the kiln firing. Powdered flint sprinkled over the shelves and bottom of the kiln prevents the drops of glaze from sticking.
To gauge the temperature of the firing, pyrometric cones are placed within view of a peephole in the door of the kiln. The cones should be chosen to represent the lower, medium, and high range of temperatures desired for that particular firing. They are set in wet grog clay at a slight angle pointing to the right. See Figure 17. The cones are arranged so that the highest-temperature cone is at the left, the medium is in the middle, and the low is at the right. An average firing would require cone .05— 1900°; cone .06—1850°; and cone .07—1814°. These readings are Fahrenheit.
The length of time required for reaching these temperatures differs with the kiln, the fuel, the weather conditions, and the kiln load. After the kiln has reached a pink-hot color inside, it usually takes two or three hours to reach cone .07. When that temperature is reached, the clay in the cone vitrifies and it bends over. Usually the .06 cone is allowed to bend before turning off the kiln.
Figure 17. The cones are placed in grog clay at an angle.
The kiln should be allowed to cool from 24 to 48 hours before opening the door. It will still be warm inside and cotton work gloves should be worn for protection from the heat and from sharp pieces of glaze. The unloading of the kiln is a most exciting time in the ceramic artist's work. His art is truly proved by the fire.
Some glaze may have dripped at the bottom of a piece, or a tripod may have stuck to the base. The projecting parts should be carefully tapped off and the sharp edges ground smooth on an emery wheel. Some pieces will need reglazing, but it is difficult to reglaze a piece. Do not attempt to apply another color of glaze, since the chemical reactions may not be satisfactory.