Tips for Extending the Life of Your Kiln Elements Craig Caudill and Ryan Coppage, PhD

If you’ve been in ceramics for very long, you’ve most likely changed elements in a kiln. Each time you do this, you lose out on firings and studio time, and it costs several hundred dollars—a huge chunk of change for a potter. More often than not, this happens every year or two. If you’re lucky, you’re working in a facility or university art program where some other poor soul changes them for you and you don’t know any better. Kiln elements work like any normal heating elements in an oven: a current is passed through the wire, certain metals resist that current/voltage flow, and that resisted flow is converted to heat by the element. These are typically called “nichrome” as the more traditional heating elements contain nickel and chromium; however, Kanthal A-1 or APM makes iron-chromium-aluminum (FeCrAl) alloy elements. Calling them nichrome would be misleading, although it is still commonly done.¹

Elements, when new, are glossy and metallic. After being fired, the surface appears matte to the eye and lighter gray in comparison to the new elements (1). This is because a handful of processes are taking place at the surface of the element. As brand-new elements resist current flow and heat up, they expand. At the highest temperatures, the metals at the surface of the expanded heating element will absorb oxygen from the air, lose electrons, and form a metal oxide layer.² Of the three metals in Kanthal elements, iron has the lowest energy of oxidation at +0.04 V (Fe ‡ Fe⁺³), chromium the second at +0.91 V and +0.41 V (Cr ‡ Cr⁺², and Cr⁺² ‡ Cr⁺³), and aluminum has the highest free energy of oxidation at +1.66 V (Al ‡ Al⁺³).³ What this means is that aluminum will oxidize first—and do so aggressively, often outrunning the others, forming a layer of aluminum oxide on the surface of the heating element and slowing down the oxidation of the other metals in the alloy. This creates the gray matte appearance of used heating elements.

Heating of Elements

As previously mentioned, when metalsare heated, they more easily oxidize and form oxide layers at the surface. This absorption of oxygen also adds mass to the surface layer, expanding it. When those same heating elements cool after firing and begin to shrink, the oxide layer also shrinks, but does so at a greater expansion/contraction coefficient than the inner metal core. This results in wrinkling at the surface of the oxide layer, as it bunches up around the core (2).

Upon the next firing, that same oxide layer expands more than the inner heating element core, occasionally breaking open at the surface (2, middle). This exposes more of the core elements, results in more oxidation and a thicker oxide layer (2, right), repeated over and over until the inner core cannot meet the heating schedule and the kiln shuts off (for Skutt, an E-1 error).

What It All Means

Effectively, this process is happening to your heating elements every time you fire. It is inevitable and will result in the formation of a metal oxide coating.⁴ The core of the alloy elements will slowly be eaten away and form an additional oxide layer through the course of the heating elements’ working life. To minimize these effects, there are a couple key strategies.

The first is to use a kiln that easily functions above your working temperature—it can provide more net voltage to your elements, heat up faster, the elements are made for larger heating loads, and you keep them hot for less net time. A kiln that is working right at its top temperature load is going to keep elements hot for a much longer time and result in more oxide layer formation and shorter heating element life.

It is also very important to vacuum inside the heating element channels when replacing elements, as debris can be picked up by new elements and affect the first oxide layer formed on the surface, which would then change element core consumption during all subsequent firings in that area.

Finally, the first firing should often be done without any pottery in the kiln, to cone 04, and the kiln should be vented, such that off-gassed components are not absorbed by the first oxide layer and cause the layer to grow more rapidly/unevenly over the lifespan of the elements.⁵ It helps to have a vented kiln (most manufacturers make kiln-venting systems that draw air down through a kiln, and out through small holes in the bottom), to pull off-gassed vapors out, as these also contribute to shortened lifespans of kiln elements.⁶