Thursday, 11 April 2013

Post 87 Even kiln temperature

When I bought my secondhand kiln it came with a number of shelves and assorted props. The big shelf on the floor of the kiln was cracked - right across the middle. When I had the kiln delivered and gave everything a closer examination that crack in the shelf remained a mystery - it looked like a stress failure; if it had been dropped it would be in many more parts than two. This was the start of another discovery thread.

I'd read recommendations that you should have the first shelf on the floor of the kiln - why I'm not sure; perhaps to protect the more fragile insulating bricks. The shelf material is an amazing refractory material, really hard, really rigid and able to sustain high temperatures, often just supported only around the edge, without significant movement. But sitting on the floor of the kiln it would have seen quite a temperature differential between its upper and lower surfaces during firing. The upper surface would have tried to expand more than the lower and the shelf would have tried to bend, arching upwards, but being of a rigid material all it could do instead was to  crack.

Having come to that conclusion then, from the first time I fired the kiln, I set up a bottom shelf supported on stilts about 25mm off the floor. I had plenty so used about 12. Over a few firing cycles it seemed that the pots in the bottom of the kiln were not as completely fired as in the top. A gas or wood fired kiln sees the movement of the combustion gasses through the kiln and the forced convection of the draught exposes the work to a uniformity of heat work. In the unvented electric kiln which relies more on radiant heat at the higher temperatures, convection does however come into play and you can expect up to 25 degrees C differential in temperature from top to bottom. This can have quite an impact.

So I bought some pyrometric cones, tested and confirmed that this was indeed happening and it was a big difference, with the biggest step occurring right at the bottom of the kiln.
My first step to overcome this problem was to reduce the width of the shelves above the first one. You can see that in the picture below. This reduced the 'working shelf space' in the kiln but it made a big difference in even temperature. Perhaps with a tighter wall/ shelf clearance convection and radiation were impeded.

The last couple of times I've glaze fired I have raised the lower shelf even further, now to 100mm off the floor and more importantly above the height of the first heating element. This leaves some 'dead space' in the kiln but I'm getting a better result throughout the kiln. Perhaps the lower element can now play on that lower space to provide some underfloor heating to the bottom shelf that previously wasn't happening. If the shelf is hotter then the work on the shelf will be too.

Modern kilns which offer multiple heating zone controls would be a joy to operate, but if you haven't got that you have to see what happens and respond. Mine is just single zone control and the ability to preset the whole firing schedule and walk away is just so good. There is no substitute for vigilance however because particularly at the higher temperatures as the kiln elements age they may not deliver the ramp you are asking for. Because both time and temperature are so important, spending longer, at elevated temperatures, than you think, will have a big result on the cone firing result you get. I'm tracking the total elapsed time of my firing schedule, firing by firing, as a measure of element condition. The other valuable practice is to plot a graph of temperature vs time and compare this with the set schedule; very telling. At the moment once over 1100C the best rate of heating I can get out of mine is just a bit more than the 85C/hr I'm looking for, while at the lower temperatures 200C/hr is achieveable.
When I last fired the kiln as setup above I placed a single cone 6 cone on the second top shelf and another on the bottom shelf.

This is what they looked like after firing.

The one on the left, from the upper shelf ,has started to fall and the other from the bottom shelf was very close to completion. By my reconing I'd say I reached cone 5.95 on the upper one and 5.8 on the lower. After more than 3 hours at temperatures over 1000C the schedule was probably within 2 minutes of having the upper cone come to the horisontal. The very top shelf would no doubt have reached cone 6, so overall an excellent result, confirmed by the even fired appearance of the pots.
 As usual it's about a lot more than just making pots.

No comments:

Post a Comment

Happy to hear your advice, feedback or questions