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Improving Your Kiln's Element Life

By Stephen J. Lewicki

From the November/December 1997 Clay Times.

Why do elements in electric kilns fail? What can you do to extend element life and save on kiln maintenance costs?

Most electric kilns in use today use coiled wire made of a very special high temperature alloy of iron-aluminum-chrome. The chief advantage of this alloy over others that were used in the past is that it resists very high temperatures (up to about 2400F). It does this partly by forming a very tough alumina oxide coating on the outside of the wire. The disadvantages (or weaknesses) of the alloy is 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 on the elements. Reducing atmospheres, carbon compounds (such as might be found in some clays), glazes, oxides of lead, halogens, fluorides, 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 only 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 because of rough firebrick making the elements stick while they expand and contract, the element will overheat and burn out where the coils touch each other.

The condition of the firebrick or element holders supporting the element is important. The brick or element holders must be clean and free of kiln wash, glaze and crumbling firebrick. Grooves or holders must be in good condition. Because it can be difficult to repair firebrick grooves, many owners overlook this. However, if an element droops down out of a broken groove, you may have lost the 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 from firebrick). Ceramic holders have the advantage of being stronger and less prone to damage than firebrick in the first place, and easier to replace if they do get damaged or contaminated.

Keeping the element as cool as possible is one thing the kiln owner can and should think about. The life of elements goes down logarithmically with element temperature. For instance, an element operated at 2100F may last twice as long as an element operated at 2200F but four times as long as an element operated at 2300F. There is a big difference between element coil temperature and kiln temperature. The coil may be 50F to 150F hotter than the kiln. This differential will be influenced by the density of the load. 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 than 1-1/2" from the elements. It is especially important to keep the load shelves away from the elements. You want to get the heat away from the elements and allow it to circulate within the kiln. Pots or shelves that are too close to the elements not only prevent air circulation but they radiate 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 the life will go down dramatically. Dense ceramic holders can help this condition because they transmit element heat more efficiently.

Element design is often the paramount factor in element life. The three main aspects to this are watt density, which is the ratio of watts to surface area of element (this should be as low as possible); element stretch ratio; and the ratio of wire gauge to coil diameter. There is unfortunately not much the kiln consumer can do about element design except to trust the manufacturer of the kiln. However, keep in mind that the relationships between the competing "ideals" in a design make element design difficult and time consuming. The kiln consumer should be aware that cheap replacement elements from outside vendors may not have the proper design and may fail sooner. Also, there are several cheaper grades of iron-aluminum-chrome alloy 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.

If you ever have to reform an element (for instance, if an element gets out of its brick slot or element holder and droops down) you cannot do this cold. The element must first be heated (not quite to red heat) and then carefully reformed with a tool such as needlenose pliers. The heating of the alloy will soften it.

Long soak times will accelerate the element aging process, so try not to soak at final set point for any longer than you absolutely need to.

No matter what you do, elements will increase in resistance over time. This is because the part of the wire that actually carries the current gets thinner as more of the aluminum is transformed into alumina oxide. Element resistance is measured in ohms. As resistance increases, the current draw (amperage) will go down and the firing time will increase. The only reliable way to know which elements need replacing when the kiln starts slowing down is to measure the ohms of each individual 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 9% it is time to change it. Keep in mind that for kilns with graded elements (where elements vary in power output from top to bottom to avoid zone switches) you may need to replace all elements as a set. In kilns with individual zone control switches you can adjust for variations in element output caused by resistance change.

Keeping your elements and element holders 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.

 
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