Why do elements in electric kilns fail?
How can you save on kiln maintenance costs?
Most electric kilns today use coiled wire made of a special high-temperature alloy of iron-aluminum-chrome (sometimes known as Kanthal A-1 wire). The chief advantage of this alloy over others used in the past is that it resists very high temperatures (up to about 2400°F or 1315°C). It does this partly by forming a very tough alumina oxide coating on the outside of the wire. The alloy's disadvantages (or weaknesses) are that it becomes very brittle after firing, the wire grows with use, the resistance increases with age, and anything that harms the oxide coating can ruin the element.
Keeping these factors in mind, the kiln user must be careful not to allow any contamination of the elements. Reducing atmospheres, carbon compounds (such as might be found in some clays), glazes, oxides of lead, halogens, fluorides, and silica compounds (such as found in kiln wash) will all attack the alumina oxide coating. Reducing atmospheres will be caused by burning carbonaceous materials in the kiln - something never recommended by kiln manufacturers. Reduction firing should generally be done in gas kilns.
Even stretching of the coil is also critical. Once the elements are fired, they cannot be easily restretched. If the coils become bunched up in certain places because of improper initial stretch or rough firebrick, making the elements stick while they expand and contract, they will overheat and burn out where the coils touch each other.
The condition of the firebrick or element channels supporting the element is important. The brick or element channels must be clean and free of kiln wash, glaze, and crumbling firebrick. Grooves or holders must be in good condition. Many owners overlook this because it can be difficult to repair firebrick grooves. However, if an element droops from a broken groove, you may have lost that element. Although pinning the element in place with alloy pins can help, this is not an ideal solution because the pins tend to come loose over time (because their expansion factor is different than firebrick.) Ceramic channels are stronger and less prone to damage than firebricks in the first place and are easier to replace if they get damaged or contaminated.
Keeping the element as cool as possible is one thing the kiln owner can and should consider. The life of elements goes down logarithmically with element temperature. For instance, an element operated at 2100°F (1150°C) may last twice as long as an element operated at 2200°F (1200°C) but four times as long as an element operated at 2300°F (1260°C). There is a big difference between element coil temperature and kiln temperature. The coil may be 50°F to 150°F (30°C to 85°C) hotter than the kiln. The density of the load will influence this differential. Dense loads will absorb more heat, and the elements will work harder (and hotter) to get the heat into the kiln. You should never load your ware closer to 13 mm from the elements. It is essential to keep the load shelves away from the elements. You want to remove the heat from the elements and allow it to circulate within the kiln. Work or shelves that are too close to the elements prevent air circulation and reradiate heat back to the elements, causing them to overheat. If the element is buried behind insulating firebrick, it will get hotter than it needs to, and life will go down dramatically. Dense ceramic channels can help this condition because they transmit the heat from the element more efficiently.
Element design is often the paramount factor in element life. The three main aspects of this are watt density, which is the ratio of watts to the surface area of an element (this should be as low as possible), element stretch ratio, and the ratio of wire gauge to coil diameter. Unfortunately, there is not much the kiln consumer can do about element design except to trust the kiln manufacturer. However, remember that the relationships between the competing “ideals” in a design make element design difficult and time-consuming. The kiln consumer should know that cheap replacement elements from outside vendors may not have the proper design and may fail sooner. Also, several cheaper grades of iron-aluminum-chrome alloy are available, and there is no way for the average consumer to know that he or she has received the top grade (which is necessary for the ultra-high temperatures encountered in most ceramic hobby kilns.) Only buy elements from a reputable source.
You cannot do this cold if you ever have to reform an element (for instance, if an element gets out of its brick slot or element channel and droops down). The element must first be heated (not quite to red heat) and then carefully reformed with a tool such as needle nose pliers. The heating of the alloy will soften it (wear heat-resistant gloves and safety glasses if you attempt this).
Long soak times will accelerate the element aging process, so try not to soak at the final set point for any longer than you need to.
No matter what you do, elements will increase resistance over time. This is because the part of the wire that carries the current gets thinner as more aluminum is transformed into alumina oxide. Element resistance is measured in ohms. As resistance increases, the current draw (amperage) decreases, and the firing time increases. The only reliable way to know which elements need replacing when the kiln starts slowing down is to measure the ohms of each element with a digital ohmmeter. You must isolate each element for this measurement. Then, compare each value with the ohm values provided by the manufacturer. Typically, when the element has increased in resistance by 8% to 10%, it is time to change it. Remember that you may need to replace all elements as a set for kilns with graded elements (where elements vary in power output from top to bottom to avoid zone switches). In kilns with individual zone control, you can often adjust for variations in element output caused by resistance change.
Keeping your elements and element channels or grooves clean, using only good elements to start with, properly stretching and installing them, and making sure that they transfer their heat as efficiently as possible, you can do a lot to maximize element life and minimize your maintenance costs.
