There are two types of kilns. In the direct-fire kiln, pronounced "kill," the fire comes in contact with the pottery. Marie Martinez and other primitive potters, as well as certain commercial potteries, use this type. The muffle kiln is like a kitchen-stove oven. There is a large firebox with a smaller box inside. The fire goes between the boxes and the pottery is placed inside the smaller box. The muffle type is the most commonly used.
There are two kinds of muffle kilns. The wall-muffle kiln is less efficient. The heat travels up between the outer and inner walls. The tube-muffle kiln has tubes of fire clay lining the kiln all around the inside. The front tubes are set in place and sealed at every firing. The heat passes up through the tubes. There is greater radiation from the surface of the tubes than from the flat wall-type muffle. It requires less fuel and time to attain the desired temperature. The fuels used in heating muffle kilns are wood, gas, electricity, coal, or oil.
The following instructions for building an economical low-firing electric kiln were made by J. Sheldon Carey, ceramics teacher in the design department of the University of Kansas. Plate L gives the plans for the electric kiln.
Any handy person can build this simple electric kiln, using only a few tools which can be found in the average home. It can be plugged into a 110-volt AC or DC line that is properly fused. Heavier than standard home fuses should be used, since the amount of current drawn is more than is used by the average home appliance.
An old stove with an insulated oven, picked up at a junkyard, will serve admirably as the outer framework. Lined with firebrick and equipped with heating coils, the kiln will produce pottery as beautiful as pieces fired in expensive factory-built equipment.
The back and front of the oven should be drilled or punched as indicated in the scale drawing. The peephole is in the front and is kept covered during the firing. Porcelain tubes are inserted in each of the drilled holes at the back to prevent the wires from coming in contact with the metal sides of the oven.
PLATE L. Plans for building an electric kiln.
The nichrome wire is wound around a 1/4-inch dowel stick or rod so as to form a tight coil of uniform diameter throughout. To save time, this can be done on a lathe, drill press, or with a hand drill. Each coil is made of 154 inches of wire, leaving about 7 inches at both ends for making connections. The coils are made separately rather than in one continuous length so as to make replacements quick and inexpensive, should one of the seven sections of wire burn out.
The firebrick used is of the type made by burning out cork that has been mixed with clay, thus leaving a very porous and light material which is easily rasped or drilled. The bricks must be arranged in the kiln so as to fit tightly in place without cement. Determine which bricks require trimming, and rasp them with a wood rasp or coarse file. When trimming is completed and the bricks fit correctly, remove them and lay out the sides and bottom in panels, separating only one brick at a time to avoid a mistake in connection with drilling the 3/4-inch holes into which the heating coils will be inserted. The drilling can be done easily by using a brace and electrican's bit or an electric drill. Try to keep the holes 1/4 inch below the surface, although no harm will be done should the bit break through the sides of the brick, exposing the coils, when they are in place. As the drilling of each panel is completed, insert the coils and put the whole panel in place in the oven. When all the bricks are in place, the ends of the coils are connected in series at the back of the kiln, using the 1/2-inch copper jumpers and bolts. Be sure to connect one coil with the next, not to both ends of a single coil. The wire at each end is connected to the 30 - ampere switch which is mounted either on the kiln or on a nearby wall, fused for 30 amperes.
At 117 volts, the kiln will draw 23.7-ampere current, the resistance being approximately 4.5 ohms. This is equal to 2772.9 watts, costing about 50 cents for 6 hours' firing.
List of Materials for Electric Kiln
I. Insulated Oven—from an old electric or gas stove
Inside dimensions—18" x 18" x 14" (any oven of similar dimensions will do.)
II. 30 Insulating Bricks
A. Size—standard 9"x41/2"x21/4"
B. Heat resistance—2500° F.
C. Place purchased—Armstrong Cork Co.
D. Price—14.7 cents each packed in lots of 2 5
Total cost of brick...........$ 4.41
III. 80 feet Nichrome V Wire
A. Size—No. 10
B. Electrical resistance—.0648 ohms
C. Temperature resistance—2000° F.
D. Place purchased—Driver Harris Co.
E. Price—16c a foot
Total cost of wire............ 12.80
IV. 14 Porcelain Wall Tubes
A. 3 inches or more long
B. Place purchased—any hardware store
C. Price—2c each
Total cost of tubes............28
A. 6 Copper strips—18 gauge 1/2" wide x 72 long
B. 14 Stove bolts—1/4" x 1/2 with nuts
C. 5 feet No. 8 wire—Single conductor rubber cover in
B.X. cable or conduct switch 30 amps
D. Total cost of accessories............. 1.51
Grand Total $19.00
A. 117 Volts—A.C. or D.C.
B. 4.5 Ohms Resistance
C. 23.7 Amps
D. 2772.9 Watts
E. 6 hours firing—Cone .06 or 1830° F.
F. 16.6374 Kilowatt Hours
G. at 3c: a Kilowatt Hour
H. 50c total cost of firing