Test kiln - the results.
I was emailed recently to ask about the outcome of this project, and I realised that I hadn’t uploaded the second part! An appalling oversite, so here it is:
This kiln has some rather major limitations but some benefits too. I'll start with the good. Turnaround time is fantastically quick. Because there is so little thermal mass it cools wonderfully fast. If I fire it in the day to finish late evening then I can open it early in the morning. With this kiln I can completely recreate my firing schedule for my larger 'Savit' kiln. This has let me nail down my firing schedules quite nicely without losing large amounts of work to my bigger kiln, along with the substantial fuel cost saving.
I made this with 1400 deg Celcius rated fibre which worked out well, especially as I over fired it quite badly the first time around. (The higher rated fibre is really really nasty stuff though so use lots of caution if you do the same). On which note; it fires to stoneware temperatures rather easily and frankly I fire it much slower than it is capable of for the temperature rise. It is small, lightweight and easy to set up and store away. It is also very frugal on fuel use. I can fire this to 1300 Celsius 3 times on a 19kg propane tank easily. That is with a prolonged firing pattern, if I just went on a race to the top and back I am certain it would be able to do more.
Now the downside. The main problem is that this is an updraught kiln and inherently inefficient. I can get the bottom shelf to cone 12 easily if I want to, but the top shelf will only be cone 9 at best. The temperature difference is quite startling and I've tried many different things to try to get the top hotter. This style of kiln means that the heat loss from the top will always mean that the top is cooler. One of the biggest mistakes I made was not allowing enough of a gap between the shelves and the walls of the kiln. It needed to be about an inch wider on the internal wall all the way around, especially on the bottom shelf. I got around it by putting perforations in the shelves and 'scalloping' the edges. The wonders of diamond cutting discs!
The other idiosyncrasy of the kiln is how delicate the fine tuning is to achieve a reduction atmosphere. Because the overall volume of the kiln is so small it is quite a fine control on balancing the air intake at the burner port with the exit flue size. I use a piece of broken kiln shelf over the chimney as a damper. The difference between a full oxy atmosphere and a heavy reduction can be as small as 1-2mm.
I didn't use any rigidiser on the fibre or any other layer with kaolin on the inside. This was mainly due to costs. I needed to make the whole thing as cheaply as possible! I'll admit that I have considered putting a rigidiser on it but so far not as yet.
The main benefit of using fibre has got to be speed and therefore also the cost effectiveness of the firing. But, that also means that there is no specific benefit in using it for long firing schedules. In any case, when firing for stoneware the fibre needs to be 2" thick. Less than that and it loses its thermal integrity too quickly and you'll really struggle to get anywhere near cone 10, especially with a prolonged firing schedule.
Things I'd change if I made this one again:
I'd make it slightly wider and shorter. I'd make the chimney ever so slightly narrower.
But, if I was to do the whole project again, I'd make a down draught kiln. So much of the heat energy put into the bottom of the kiln is lost out of the chimney. It's a real shame. It would still need to be as thick in the wall but I think I'd make it as a box shape. I'd never used an updraught kiln before and to be completely honest I probably won't ever make another again after this one.
I still use this kiln from time to time to check new mixes of glaze before committing a whole batch and for checking new recipes. But, when I'm in full swing I put my tests in with my main firing.
Should anyone be in need of a test kiln and be considering this design then I think you should consider what it is you want your kiln to do. Are you testing the firing schedule or are you testing the glaze fit? Mine was for the schedule and for which it works perfectly. If it's for the fit then it might not be the design for you. It'll work fine but it there are better and more efficient designs to use. With any design you won't get quite the same overall glaze results from a lightweight fibre kiln as you will out of a proper brick kiln. This is not necessarily a problem, just an observation.
