Techno File: How Elements Die Craig Caudill and Ryan Coppage, PhD

Appears in the May 2023 issue of Ceramics Monthly.

Defining the Terms 

APM Elements: Heating elements for a kiln made by Kanthal Manufacturing. APM elements are extruded, ground up, reprocessed, and extruded again to reduce sagging and deformation to last longer than standard ones. 

Elemental Analysis: The measurement of scattered X-rays from the surface of a sample, which give percentage composition by element of a material. 

Oxidation: The loss of electrons from a metal and often the addition of oxygen atoms to its composition, i.e. aluminum (Al(s)) becomes alum (Al2O3(s)) in the process of losing 3 electrons, each.

Scanning Electron Microscopy (SEM): A microscope with an electron beam that scans across a small section of a grid, yielding extremely high detail of any object surface. It can also possess an X-ray detector, which allow determination of the elemental composition of the surface material and that of a few atom layers underneath the surface. 

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.

Analysis of Kanthal APM Heating Elements

For this study, heavily used heating elements were taken from a Skutt KM-614 kiln. Upon obtaining new heating elements, samples were imaged via SEM from both new and old coils and elemental composition data collected for both the heating-element cores and surfaces across both sets (2).

Observable in figure 2, the cores appear similar; however, there are significant differences between the surfaces of the new (2a) and old (2b) images. This is due to an oxide layer growth on the surface of the heating element during firing, which continues to grow and consume inner-core material until failure. Under magnification, the surfaces of new versus heavily used (E-1 failure) heating elements look very different. The new elements have some scraping/minimal texture from extrusion during their manufacturing (2a and 3a); whereas the old/failed elements possess significant wrinkling at the surface (2b and 3b). This is due to the formation of (mostly) an aluminum oxide layer at the surface during sequential heating and cooling cycles over multiple firings. Elemental analysis is an extremely useful tool to determine what elements are present in a sample.⁴ With Kanthal APM elements, the initial composition before firing is known through the manufacturer’s provided specification report; however, this elemental composition rapidly changes during a kiln’s extremely hot firing conditions.⁵ 

To determine how the elements change, relative mass percentages on the element’s surface and core are recorded between brand new and old/failed Kanthal APM elements. Mass percentages detail exactly what prevalence a specific element occupies in a larger sample. As shown later, the majority metals that compose the Kanthal APM heating elements (Fe, Cr) decrease in mass percentage on the element’s surface while the oxygen and aluminum mass percentages increase under the hot environment of the kiln. In figure 4, the new coil is mostly Fe, Cr, and Al.

These numbers greatly change for elemental analysis of the surface of a heavily fired heating element, as shown in
figure 5. 

The heavily fired (oxidized) samples have a lower mass percentage of iron (Fe) and chromium (Cr) on the element’s surface, but a higher composition of oxygen (O) and aluminum (Al), suggesting that the primary composition of the oxide layer is aluminum-based, though some iron and chromium are still present and likely part of the layer. 

Heating of Elements

As previously mentioned, when metals are 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 (6). 

Upon the next firing, that same oxide layer expands more than the inner heating element core, occasionally breaking open at the surface (6, middle). This exposes more of the core elements, results in more oxidation and a thicker oxide layer (6, 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.⁶ 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.⁷ 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.⁸