Designing a test kiln
Last year I came to the realisation that I needed a test kiln. I had developed a couple of glazes while I was in my final year at Clay College in Stoke-on-Trent, but I hadn’t managed to get to grips with the best way to fire them. As such I was having a huge loss rate from my main kiln (>50% in one firing!) which wasn’t at all sustainable. I really wanted and needed to nail down the best firing schedule for each glaze and make sure that the combo of glaze to clay body was correct.
A trawl through my books and online wasn’t as forthcoming with a pre-made design as I’d hoped. There are lots of designs for brick kilns but nothing specific for a small ceramic fibre gas version firing to stoneware. I did manage to find a couple of pictures of a likely candidate in use at the Thomas Town school in Ireland but there were no measurements to help with the build.
Fortunately, I have met a few of the ex-students from there over the last couple of years who were able to give me some rough dimensions to work from. I dug out some rudimentary ‘rule of thumb’ size ratios, a piece of graph paper and a pencil and doodled until I had something that looked viable. I needed it to be small, portable, able to have a controlled reduction atmosphere and almost most importantly of all - cheap!
In the end I based the sizes upon the smallest internal shelves I could find and worked out the rest from there.
Some items/elements could be compromised on, others not so. For example, I used basic builders mesh for the outer frame and some garden wire for the construction. A few pounds from my local Wickes all in. But, I had to use a decent gauge nichrome wire for the internal fixation. Thankfully, a good potter friend of mine was able to help me fire some porcelain buttons to help secure the ceramic fibre with the nichrome wire.
Speaking of fibre, its NASTY stuff! I wore a high spec face mask/filter throughout and once I’d finished all my clothes went straight into the washing machine and me into the shower. I used thick kitchen foil between the wire mesh and the fibre itself in an attempt to minimise any fibres coming loose during use or moving and handling of the kiln. There are no pictures during this phase of construction as frankly I just wanted to get it done as soon as possible.
Next came the burner. Ideally I would have a nice propane burner from Stedmark or similar. Maybe one if those nice raku ones… But, this would instantly double the budget, and for the size of this kiln it would be rather overkill. For anyone who has watched more pottery videos on Youtube than is good for them, you’ll know of a chap called Simon Leach. I watched a great many of his videos late at night trying to get my new born babies to sleep when they were very little. I remember him using a simple propane weed burner to fire his kilns. Worth a punt I thought so I got a commercial roofing variant. The hope being there that this would at least be built with expectations of prolonged use.
There are downsides: there is no flame failure built into the burner. A major safety consideration. It is also very noisy! Since I found that I really need to baby the firing of this kiln it is never left unattended for the duration of the firing. If anyone else follows this design please know that I recommend proper failure safe burners. I’ll get some too.
The last major consideration was the chimney. I made the mistake here of not trusting my maths ability. I originally worked out the diameter of the chimney, but it looked far too narrow. I figured that if I was a little more generous with the dimensions then I could always fix this with a fibre cover over the flue. In reality it didn’t work. It was FAR too big and allowed too much heat to escape. I ‘fixed’ it by putting more fibre around the inside of the chimney, ironically back to the originally worked out dimension. It looks a bit daft but it was the easiest solution.