Silicon Carbide Heater

Ageing is the defining property, and the furnace has to allow for it

A silicon carbide element does not fail one morning. It gets steadily more resistive over months as it oxidises in service, and at a fixed supply voltage that means steadily less power. The symptom is a furnace that takes longer each season to reach temperature, and it is very often chased around the control panel for weeks before anyone measures an element.

Design for it from the beginning: a step-down transformer with several tappings, and a routine of measuring each circuit’s cold resistance once or twice a year. When the numbers say so, move up a tapping and the furnace is back to its original performance.

Never put a new element into an old circuit

Because elements age, the ones in service are not interchangeable with new stock. A new low-resistance rod in a parallel bank of aged ones takes far more than its share of the current and burns out quickly. In series, it takes too little and leaves a cold spot. Replace a whole circuit at a time, or buy replacements matched by resistance to what is already there.

Suppliers group new elements into resistance bands for exactly this reason. Note the band when elements are fitted, and keep spares from the same band together.

Most failures are at the contact ends

The hot zone in the middle usually outlasts the ends. The cold end is where a sprayed aluminium contact, a braided strap and a clamp all meet, and oxide grows between them. A slightly loose clamp heats a little, which grows more oxide, which heats more, until the end of the rod burns off — and the element is scrapped for a fault that had nothing to do with the ceramic.

Clean and re-tension every clamp at each service, keep the terminal area ventilated and free of dust, and never leave a strap hanging on a rod’s own weight.

Treat them as ceramics, because they are brittle

Silicon carbide rods break from levering, from being clamped too hard, and from their own weight if they are unsupported over a long span. Support long rods, or mount them vertically. Let them hang free so they can grow in length. And if a rod has bonded to its bush with kiln deposits, work it loose rather than forcing it — the rod will lose that argument.

Spacing keeps everything alive

At 1500 °C a rod radiates fiercely at whatever it can see. Too close to the lining and it overheats the brick; too close to the work and it marks it; too close to another element and both run above their design temperature. Keep at least twice the rod diameter clear all round, and remember that a load stacked against an element in a kiln is a hot spot for both of them.

Atmosphere shortens or lengthens the life

Silicon carbide is happy in air, which is its main advantage. What it does not like is a furnace full of alkali vapours, halogens or reducing gases — glaze volatiles, salt, some fluxes. If elements in one kiln consistently last a fraction of the time they do in another, the difference is usually what is being fired, not the elements.