The standard wire used in heating elements is Kanthal A1, an alloy of iron, chromium and aluminium. It is the standard because it isn't too expensive, it has an operating temperature of 1400°C (i.e. the wire temperature, not the kiln temperature), and it has good corrosion resistance. Some potters have played with Kanthal APM, which raises the operating temperature to 1425°C, but is significantly more expensive. There are other grades of Kanthal (A, AE, AF and D) that have lower temperatures of 1300-1350°C that may be worth investigating if they are significantly cheaper or longer life, and you fire at the lower end of stoneware or earthenware temperatures.
|Max. continuous operating temperature, °C||1400||1425||1350||1300||1300|
|Composition, Cr/Al/Fe %||22/5.8/72.2||22/5.8/72.2||22/5.3/72.7||22/5.3/72.7||22/5.3/72.7|
|Resistivity @ 20°C, 𝛺mm²/m||1.45||1.45||1.39||1.39||1.39|
|Temperature factor of resistivity, Ct @250°C||1.00||1.00||1.01||1.01||1.01|
|Coeff. thermal expansion, C-1, 20 - 250°C||11.1x10-6||11.1x10-6||11.1x10-6||11.1x10-6||11.1x10-6|
|20 - 500°C||12x10-6||12x10-6||12x10-6||12x10-6||12x10-6|
|20 - 750°C||14x10-6||14x10-6||14x10-6||14x10-6||14x10-6|
|20 - 1000°C||15x10-6||15x10-6||15x10-6||15x10-6||15x10-6|
|Thermal conductivity @50°C Wm-1C-1||11||11||11||11||11|
|Specific heat capacity @20°C kJ kg-1C-1||0.46||0.46||0.46||0.46||0.46|
|Melting point °C||1500||1500||1500||1500||1500|
|Tensile strength N mm-2||680||680||725||700||720|
|Tensile strength @900°C, N mm-2||34||40||34||37||34|
|Yield point N mm-2||545||470||550||500||520|
|Elongation at rupture, %||20||20||22||23||20|
|Creep strength N mm-2 @800°C||1.2||8.2||1.2||1.2|
|Emissivity, fully oxidised||0.7||0.7||0.7||0.7||0.7|
The kiln elements are protected by the aluminium in the alloy migrating to the surface, and forming an impermeable, stable protective layer of alumium oxide (alumina). This forms at temperatures above 1000°C, and is self-healing until the aluminium in the wire is depleted. Note that these are the temperatures of the wire, not the kiln.
Firing temperature is one of the biggest determinants of element life. Taking 1200°C as a base point, at lower temperature firings so the element only reaches 1100°C the element life will be 3.4 times longer, whereas at 1300°C it will be 3.3 times shorter. The table below shows relative durability of various grades of Kanthal at different temperatures.
Steam shortens element life, which is one reason to ventilate the kiln at lower temperatures.
The activity series says which metals will react with the aluminium oxide, reducing it to form their own oxide. These are restricted to magnesium, sodium, calcium, barium, potassium and lithium. Other metals such as zinc, aluminium and copper will also react with the elements. Lead is often quoted as being harmful to the elements, but aluminium oxide will not react with it, regardless of its low boiling point. Also, the salts of the alkali metals and boron will react with the elements, but only at higher concentrations than will be found in a potter's kiln.
If silica or other glass formers contact the elements, the aluminium oxide will melt into the glass former, just as it would in a ceramic glaze. This will significantly shorten element life. This means that bricks supporting elements should be low in free silica, with an alumina content of at least 45%. Sodium silicate should be avoided, and iron kept below 1%. The use of sillimanite or high alumina firebricks is recommended for high temperatures.
If fluorine is present, e.g. as an impurity found in some (mostly porcelain) clays, or intentionally e.g. as cryolite in glazes, the fluorine will bond with the aluminium oxide, and personal experience with cryolite glazes shows that this will reduce element life. This also applies to chlorine, bromine and iodine, though these are not used any more in pottery.