Why does my kiln fire unevenly?

Question: “Why do I get different results on my cone pack based on which shelf– top, middle, or bottom– the pack is located?”

Our kilns tend to fire unevenly and I’ve also had the same experience. The new Cone Art kiln that I bought recently has much thicker bricks (better heat retention) than the kiln I had previously been firing for ten years. I used to have an old Skutt, a manual kiln sitter kiln. The bricks were all crumbling and the elements were spilling out of the grooves.

This was a free kiln that was given to me right when I graduated from clay school, so I made it work for me. But it did fire very, very unevenly. My bottom shelf was always cooler than the rest of the kiln.

Sometimes, kilns fire unevenly and that’s just how it is.

How to tell if your kiln is firing unevenly

Putting witness cones on all of your shelves is the best way to tell how evenly your kiln is firing. If you do that over two or three firings, you’ll see if it’s always the bottom shelf that’s the coolest, or sometimes it’ll be the top shelf that’s cooler than the rest. And sometimes the middle shelf is cooler than the top and bottom.

People often think the bottom will always be cooler because heat rises. But that’s not really how it works in a kiln. The way you load your kiln can have an effect on how much energy it takes to heat each shelf.

The way you load your kiln matters

If you have a shelf that’s very densely packed with lots of really small objects and a short kiln post (not much space between your pieces and the shelf above) then that shelf will take more energy to heat up than a shelf that has more open space. Air heats up easily. It’s the clay that takes longer to heat up.

So a shelf that’s densely packed is going to take more energy to heat up than a shelf that is more sparsely packed with a lot more open space. If you fire a large bowl, for example, there will be lots of empty space in the middle that is going to take less energy to heat up than a shelf with 100 cylinders on it.

Multiple thermocouples help regulate temp throughout the kiln

When we have a programmable kiln with multiple thermocouples, the thermocouples read the temperature at each level and the elements turn on and off in order to regulate the temperature. The thermocouples help to even out the temperature in the kiln.

If the top is heating up faster, the kiln will turn those elements off and let the heat in the bottom catch up until everything is at the same temperature. That’s the clicking sound you hear when you turn your programmable kiln on. The relays are turning the elements on and off, to regulate the temperature.

When we have a manual kiln, where we’re just turning switches onto low, medium, high, then that’s usually where we can start to get uneven heat work from shelf to shelf. It depends on how each shelf is packed and it depends on how many elements are exposed to that level of the kiln.

If your kiln is firing really unevenly, I always recommend leaving a good inch of space between the top of your pots and the shelf above it. Just so it’s not so dense and so there’s a bit of open space in there. And then try to load your kiln evenly.

I know it’s hard if you make large bowls and small cups and need to fit them all in. We have to use the kiln space as efficiently as we can. But we also have to keep in mind that the way we load our kiln can affect the heat work from shelf to shelf.

Get to know your kiln and load it strategically

Sometimes just knowing which shelf is going to be hotter or cooler can help you pack your kiln accordingly. With my old manual kiln that was really uneven, I used to always use a four inch post on the bottom level. This allowed me to put more cone packs where I could see them during the firing because I had four peeps which were 4” apart. 

But I eventually learned that when I put a taller 6” post on the bottom, even if my pots were shorter, that helped to expose one more element to that lower shelf, and helped to allow a little more heat down there. So that’s how I dealt with that.

I wasn’t really able to control it to my liking, which is why I eventually spent several thousand dollars on a new kiln with thermocouples. But I fired that manual kiln for many, many years before I was like, okay, I’m ready for an upgrade. Sometimes we just have to make do with what we have.

But the more you can get to know how your kiln fires, then you can load your kiln strategically. If it’s always cooler on the bottom, put the glazes that are okay being cooler on the bottom. Sometimes I had runny glazes that would run if they got too hot. I could put those on the bottom, or my clear glaze that didn’t care what shelf it was on, it always came out clear. And then the glazes that really needed the heat in order to look the way I wanted them to, I would put those in the middle or at the top, where I knew it was going to reach a hot cone six, which is what I was aiming for.

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