Wednesday, May 25, 2011

The Downside to an Updraft and the Upside to a Downdraft

We fired the kiln from the last post, and it did pretty good. But the whole firing I was thinking that if it was a downdraft it would be way more efficient. As it was firing I kept imaging that the flame/heat was just going up the sides and out the top, never reaching the middle (which is where the pottery is). It did manage to reach a bisque temperature in only a few hours, but the firing was very uneven. A downdraft kiln will cause the firing to be much more even, and as I said before, more efficient. A downdraft kiln works by having the flue and the burners at the bottom of the kiln. The idea being that as the heat rises it will hit the top of the kiln and be forced back down to where the flue is. This causes the heat to work double, because it is touching the work twice instead of once like in an updraft kiln. So I decided to rebuild the kiln again and try to make a downdraft kiln. Simon Leach had done this to one of his electric kilns, and it seems to be working great (although his kiln is much larger than mine, but I’m told size doesn’t matter). The only thing holding me back was not having the materials to make the chimney that would be required. Lucky I have a good friend named Zack who just happened to have two spare kilns that he didn’t mind parting with. So now I had three kilns to play with. One of them was pretty beat up so I decided to cannibalize that one to make the other two kilns into downdraft kilns.

I started out by replacing the lid that I had cut a hole into for the updraft kiln, with the lid that had already fallen off the other good kiln. I then took the lid off of the bad kiln and put it on the other good kiln. I know that sounds kind of convoluted but it’s what had to be done. But then I had two kilns that where in good working condition.

I then started taking apart the kiln I was going to use for parts, this part went pretty quick.

After that I started work on where the chimney would go on the inside of the kilns. I did this by removing the part of the wall where the elements went; it was fairly easy with a hammer and a metal scraper. It did make a bit of a mess, and I would highly recommend wearing a mask or some kind of mask to help protect you from the dust.

I then had to decide on how I was going to build the rest of the chimney. I knew I was going to use the soft brick that I got from the bad kiln, but I didn’t know how I was going to put them in the kiln. My plan was to make these interlocking pieces that would go up the side of the kiln, and then put the larger bricks right up against them.

So I set out to make a bunch of these little interlocking pieces, and to cut the larger bricks down to a size that would work with those little pieces. Around this same time I was also thinking of how they would stay in place once they were all ready. I was going to use some of this old kiln wash that I was hopeful would act like glue. I was sadly mistaken; it didn’t work in the slightest. So I had to rethink the whole idea, and I was only going to be able to use the materials at hand. I don’t like the idea of buying materials just to see if something will work, if I could possibly use something I already have. That’s when I looked at the mess I had made while sawing through all those soft bricks.

It had made a lot of dust, and I thought if I could mix that with some slip (liquid clay) it would make a great mortar for the chimney. So I set out to make a lot more dust by grinding up the little pieces of soft brick that were sitting around. I used our blender to accomplish this task, because grinding them by hand would have taken forever (don’t tell Keturah, she doesn’t know and probably wouldn’t appreciate the time saving route I took). Just after I finished grinding all of the brick I thought I would need Keturah came home, and I showed her what my idea was for the chimney. She thought it was a lot more work than I had to do, and she suggested that I just cut the soft bricks edges at an angle so that the edges would be flush against the part of the kiln that I didn’t cut off. Of course she was right, and it would have saved me a lot of time if I would have thought of it myself. So I decided to make one kiln the way I thought of (mostly because I already had all the parts cut and ready for one of the kilns), and one the way Keturah thought of. Here are the two kiln chimneys. The first one is the one that I thought of and the next is done the way Keturah suggested. In the second one you can see the (barely) the hole at the bottom of the chimney to let the heat escape.

By this time I was feeling pretty good about myself, and decided I was done. As I was walking back to the house I thought to myself “Now how will the heat get out of the kiln…”, and then I realized that I forgot to cut the hole in the lid to complete the chimney. So I head back to the studio to cut some holes in the lids of the two kilns. The green stuff you see around the hole is chromium oxide, which I use in addition to a coil of clay to help me know where to cut the hole. I put the coil of clay around the top of the chimney, and then painted the chromium onto it and while it was still wet I closed the lid. This left a green mark showing me where the hole needed to be cut.

After this I was done, and I went inside to eat lunch and write this blog. Although as I’m writing I have realized that I forgot to cut the burner port out on the new kiln…

I know this has been a long post but if you could hold on for one more paragraph I would appreciate it. My friend Andy is participating in a fund raiser for Leukemia and Lymphoma Society. He and a number of other artists each made a limited edition print that are being auctioned off on EBay. So if you get a chance please go and check them out and if you can make a bid, it’s for a good cause. Here is a link to Andy's print, and a link to the prints by the other artists.

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