KILN ELEMENTS #1
A bit about kiln elements and their replacement.
Although there are exceptions, the vast majority of electric kilns sold today have heating elements made from a metal alloy called Kanthal. Kanthal is actually a brand of heating materials manufactured by a Swedish company called Sandvik. When the ceramics world says "Kanthal" elements, it is normally referring to Kanthal A-1 wire. It's good to 2460°F, which accommodates most traditional ceramic firing needs.
One of the reasons Kanthal is a good element material is that during the first firings, an aluminum oxide layer is built-up on it surface, which helps protect it in subsequent firings. This is the reason why purchasers of new kilns are told to do an initial empty-kiln firing to "season" the elements. One of the weaknesses of Kanthal is that the elements are quite soft at ceramic-firing temperatures, and if not well supported, can sag and distort during firing. If taken to too high of a temperature, they can begin to collapse. After the initial firing, kiln elements are also quite stiff and brittle at room temperature, and care must be taken when working around them or they can break.
Kanthal elements meet most ceramic firing needs, however they do not last well at sustained high temperature (say, cone 10 with a hold), commonly the type of firing sequence encountered with crystalline glazes. For crystalline glaze artists, it is normally a requirement that their kiln has APM elements installed.
APM elements are made from the same Kanthal material but are a specially constructed wire that is created in a process called sintering. In sintering, metal is turned to powder and is heated (but not melted) until it sticks to neighboring powder particles. This creates a wire that resists grain growth and has greater strength at high temperatures. Interestingly, the grain growth that APM elements resist is exactly the process you want to encourage in the crystalline glazes (thus why they are a good combination). APM elements are several times more expensive than regular Kanthal A-1 elements, so there is no reason to consider them for your kiln unless you have very specific high temperature needs.
HOW DO KILN ELEMENTS WORK?
Essentially, a kiln element is a piece of wire that is designed to resist the passage of the electricity. As electricity is forced down the wire, the "resistance" impedes this movement and subsequently causes the wire to heat up. Think of it as heat caused by electrical friction. (A scientist somewhere is having a heart attack over this description, but its accurate for our purposes.)
It's the very same concept as an electric cook top/oven, a toaster or numerous other heating appliances you have in your home. You may recall Ohms law from science class; V=IR. (Voltage = Current x Resistance). The utility company gives you the voltage, and the element has the resistance, which results in a current (amperage) draw. To get the heating power of the element (in watts), take that current draw and multiply it by the voltage. P = IV (Power = Current x Voltage).
HOW LONG WILL MY ELEMENTS LAST?
There are no absolute answers for this. Generally speaking, the lower the temperature you fire to, the longer your elements will last. And consequently, the higher the temperature you fire to, the shorter time the elements will last.
For example, if you fire your ceramics to cone 06, and never go above it, your elements may last 200-300 firings or more. If you fire to cone 5/6, maybe you'll get 50-100 firings. If you fire to cone 10, maybe 50 or less. This assumes there is nothing exotic going on inside your kiln (no reduction, for example) to accelerate the wear. Note that the number of firings mentioned here suggests that the elements simply wear out, and are not damaged/broken by impact, glaze splatter, etc.
WHAT ARE SOME OF THE COMMON MODES OF FAILURE?
One is "natural failure". The element simply fails due to the cyclic, high temperature environment that it works in. There are a number of factors that contribute to this type of failure, but ultimately this failure mode really can not be prevented.
You may notice that as your elements age, your kiln fires a little more slowly, or gets a little less hot. This is known characteristic of Kanthal, and is an indication that your elements are getting closer to wear-out failure.
Another common failure is caused by glaze splatter. For example, if a pot breaks in your kiln, it may fling glaze onto an element. If you do not get the glaze off of the element before the next firing, it will cause the element to burn out. Kiln wash will have a similar effect, as the silica in the glaze/kiln wash will attack the element wire.
Should a chunk of kiln brick nestle-up close to the element, that can cause the element to overheat in that area and it can accelerate it's failure. Or if a chunk of ceramic/clay gets up against an element, it can melt and cause the element to fail.
(These are among the reasons why it's important to inspect the element grooves/holders on your kiln for debris before every firing, and regularly vacuum those areas out. )
HOW DO YOU KNOW WHEN ONE OF YOUR ELEMENTS HAVE FAILED?
Typically, you will have trouble reaching the desired temperature with your kiln. If you have an electronic controller, it may error-out because the kiln is unable to increase temperature at an acceptable rate. You may also notice some part of your kiln will start under-firing, or you will see some other indication of temperature non-uniformity in the kiln.
IF I SUSPECT ONE OF MY ELEMENTS HAS FAILED, HOW TO I CHECK IT?
There are a few ways. One simple method is to turn your kiln on full-power for few minutes, then darken the room and open the lid. You should see all of your elements glowing, just like inside your kitchen toaster. If not, that may indicate that an element has failed. (Some elements, because of their design, do not glow much, so non-glowing does not necessarily mean it's failed, but glowing means it's working.) DO NOT reach inside the kiln when performing this test, as the elements are energized and you could be burned or electrically shocked.
Another method is to check the electrical resistance of the elements. In most kilns, the resistance of an element will be somewhere between 10 to 30 ohms. (It depends upon how the kiln is wired and how much heating the element is designed to provide.) If you measure anything too much outside of that range, specifically "infinite resistance", then you probably have a failed element. Measuring the resistance can be tricky, as you may have to electrically isolate the element to get an accurate reading. Also, making good contact between the element and your ohm meter is critical, otherwise you may get a false indication of infinite resistance (even though the element is fine). Note that these resistance measurements would be made WITH THE KILN UNPLUGGED.
Another thing to watch out for is a bad contact or crimp in between the element and the wire leading back to the relay. This is not an element failure per se, but it may certainly look or behave like one when firing your kiln, or performing the tests mentioned above. In these cases, you don't need a new element, you just need to re-establish a good electrical contact between your element and your power source.
Yet another detection method is close visual inspection of your elements. It's often easy to spot a broken/burned-out section on the element. For elements deep in your kiln, it can be helpful to use a mirror. In a odd way, it's always satisfying to find a physically broken element. At least then it's obvious what the problem is, and you know your time and money will be well spent replacing it.
We'll have more on more on kiln elements in the next tip (Tip 86), including how to replace them...
Browse our selection of replacement kiln elements.