Why Your Kiln Is Firing Slower

Kiln elements almost never fail without warning. The warning is just quiet, gradual, and easy to explain away — right up until a firing stalls two hundred degrees short with a full load of glazed work inside.

What is actually happening

Kanthal element wire depends on a protective oxide layer that forms on its surface at temperature. That layer is what lets the wire survive firing after firing rather than burning up immediately.

But the process is not perfectly stable. With each firing the wire oxidises a little further and the effective cross-section carrying current gets slightly thinner. Thinner wire means higher resistance, and higher resistance means less current, which means less heat.

So the kiln does not fail. It gradually gets weaker. The elements are still intact, still conducting, still doing their job — just less of it than they used to.

The symptom is time, not temperature

Here is why this goes unnoticed for so long: the kiln still reaches temperature. Your controller still hits the target, your cones still bend, your work still comes out fine. Nothing looks wrong.

What changed is how long it took. A firing that ran eight hours when the elements were new might take nine, then nine and a half, then ten. Spread over months, each firing feels normal compared to the last one. The drift only becomes obvious against a number you wrote down a year ago.

Write down your firing times

This is the whole recommendation, and it takes ten seconds per firing. Note the date, the schedule, and the total time. That is it.

After a few months you have a trend line, and that trend is the most reliable early warning any potter gets. A firing time creeping up over months means your elements are ageing and you can plan a replacement for a convenient week rather than discovering the problem mid-glaze.

You can also measure resistance directly across each element with a multimeter on an isolated kiln — the reading climbs as elements age, and our resistance test tool will tell you what yours means. But firing times cost nothing and require no equipment.

Uneven ageing across rings

Elements in a sectional kiln rarely age at the same rate. Top and bottom rings typically work hardest — they lose heat to the lid and floor — so their elements degrade faster than the middle.

That shows up as uneven firing before it shows up as failure. If work on the bottom shelf is consistently underfired compared to the middle, or the top ring is lagging, ring-to-ring element ageing is a likely explanation.

Why the whole set, every time

When one element finally breaks, replacing just that one is tempting and almost always wrong.

A new element has considerably lower resistance than its aged neighbours, so it draws more current and produces more heat. Put one new element in a kiln full of old ones and you have created a hot spot — that ring now fires hotter than the rest, which is precisely the uneven firing you were trying to fix.

On top of that, the remaining elements have the same hours and the same oxidation. The second failure is usually not far behind the first, which means a second teardown within a few firings.

Every kiln manufacturer recommends replacing the complete set. The reasoning holds.

Making the next set last

• Fire no hotter than the work needs. Heat work is cumulative and non-linear — high-cone firing shortens element life sharply.
• Keep holds short. A long soak at peak does more damage than the ramp getting there.
• Vent properly. Kanthal needs an oxidising atmosphere to maintain its protective layer. A reducing atmosphere strips it.
• Keep grooves clean. A glaze drip on element wire eats through it locally and creates a hot spot that fails early. This is the most preventable cause of premature element death.
• Load with clearance. Work sitting hard against an element wall causes local overheating.