In today’s post, an excerpt from Electric Kiln Ceramics, Richard Zakin explains the features that one should look for in an electric kiln. And if you’d like to start off small and purchase a test kiln, Richard has some tips for getting the most out of those small, but handy devices. – Jennifer Harnetty, editor.
Electric kilns are used by contemporary ceramists more often than kilns of any other type. They are produced in large numbers and are sold at relatively low prices. Their economy, simplicity, reliability, and relatively benign impact on the environment guarantee their great popularity. They are somewhat limited in the eyes of many ceramists because they do not lend themselves to the rich effects of reduction and flashing that characterize fuel-burning kilns. On the other hand, they have many virtues – they lend themselves to a wide color range, are simple to load and fire, and are reliable and efficient.
Electric kilns are very simple structures. They are essentially closed boxes made from soft, porous, highly insulating bricks. Inside the kiln, running along channels grooved into its walls, are coils made from a special alloy. Heat is produced by forcing a great deal of electric current through these tightly wound coils. The result is friction and the result of the friction is heat. This heat is even, easily controlled, and quite reliable.
Electric kilns may be fired manually or with the aid of control mechanisms. Older models are limited to terminating the firing when the kiln reaches maturity. Newer, computer-controlled designs can be used to control the firing from the beginning till its end, raising and lowering the temperature as required.
Though electric kilns are useful at all parts of the firing spectrum, they are particularly suitable for low- and midfire work. They are not quite as well suited for work at the highest part of the firing spectrum: coils that are used for high firings tend to wear out more quickly than those used only for low-temperature firings. However, if careful glazing and cleaning procedures are used this need not be an insurmountable problem. At present the manufacturers of coils for electric kilns recommend that they not be fired above cone 8 (1263°C/2305°F) but if care is exercised they will last fairly well even if fired to cone 9 (1280°C/2336°F).
Purchasing a Commercially Made Electric Kiln
Most electric kilns are purchased completely assembled and ready to plug in. Their design and construction vary a great deal: it is no easy matter for the ceramist to make an intelligent purchasing decision.
At one time most electric kilns were front loading. Kilns of this design are highly durable for they must be heavily braced. This makes them very heavy and bulky. While frontloading kilns are expensive, this design results in a kiln that lasts a long time and can be loaded quickly and easily. Toploading electric kilns have been very popular for years because they are relatively inexpensive. These kilns must be carefully designed for they are subject to mechanical and heat stress, particularly in their roof and hinge areas. If you choose a top-loading kiln, make sure the roof is replaceable. The flat roof of a top-loading kiln will eventually crack under the stress of normal use. The roof hinges, also points of stress, should be designed with strong elongated arms to keep them away from the heat path. Many newer top-loading electric kilns are segmented. The electrical connections between each segment can be a source of real problems. Look for connections made with industrial grade cables which can withstand the stress that results when the heavy segments are assembled and disassembled.
The gutters that hold and support the coils should be deep and set at an angle to hold the coils securely. The coils should be pinned to the soft brick with refractory metal pins to insure that they will not come loose and sag during the stress of the high fire. The coils should be made from an alloy that resists high temperatures (such as Kanthal A1); they should be easily replaceable and fairly thick (thin coils burn out very readily), and should be consistently wound to avoid hot spots. The switches, wiring harness, and connecting wires should be heat resistant and of the highest quality. Connections to the power source should be secure: a poorly connected coil will soon burn out. The insulation should be effective and durable.
Look for kilns that fire evenly. Floor-mounted coils help keep an even heat throughout the kiln. They add to the expense of the kiln but are a mark of a professional design. Computer-controlled zone firing has proved very effective in assuring an even firing. Each zone is furnished with its own pyrometer and the computer is programmed to direct current to those coils that need it most. Originally computer control was envisaged as a way to automate the firing. An even firing was an unanticipated benefit.
Electric kilns are high current devices and they require special, high quality, high capacity fuses, cables, and outlets. For the installation of an electric kiln there is no substitute for the services of a qualified electrician.
Most small test kilns are very simple devices and they can be easily built or purchased. They are electric fired and work from normal house current (110 volts U.S., 100 G.B.). They have small firing chambers, usually under a square foot in area, are portable, quick firing, and inexpensive. Because of the quick pace of the firing, pieces fired in these kilns are likely to crack or explode. Furthermore, their quick cooling adversely influences the look of the glaze. In a normal firing cycle, glazes have a chance to develop a crystalline pattern during the cooling period. Most glazes derive a great deal of their character from the process of crystallization, a process which opacifies and modulates their surfaces. A glaze fired in a test kiln that has been allowed to cool quickly will not only lack character, it will also not look the same as a glaze fired in a standard kiln. To control the firing cycle and encourage rich glaze surfaces, the current flowing to the test kiln must be controlled so that the coil runs only part of the time. This can be done with a reliable and inexpensive device called a “current interruption” switch. If you purchase a test kiln, you may be able to find one with the controller already installed.
The following firing cycle is recommended for test kiln firings: 10 minutes on very low current with the kiln lid open, then close the lid. After 20 minutes, turn up the switch. Continue to turn it up every 20 minutes, switching the current from low to medium to high. Leave the kiln at the high setting until the cone bends and the kiln has reached the desired temperature. Now the kiln must be fired down. Turn the switch down every 30 minutes, switching the current from medium to low to very low. At the end of this procedure turn the switch off and allow the kiln to cool. An hour later, partially open the lid. Forty minutes later take off the lid and empty the kiln.
You can fire finished pieces as well as tests in these tiny, quick-firing kilns. While you must tailor the form and size of the piece to the limitations of the kiln, in many cases this presents an interesting challenge. Small-scale ceramic objects such as jewelry are perfect for this kind of kiln. Since the kiln is so portable and may be installed anywhere, it is conceivable that a ceramist who is on the move might find it a useful tool.
For more great information on firing kilns, be sure to download your free copy of Techniques and Tips for Electric Kilns: Inspiration, Instruction and Glaze Recipes for Electric Kiln Firing.