1. About metal chimneys. For anyone who is new to them, they need guys (wire or rope) to stop the wind from blowing them over ( the chimneys that is).. That's obvious. What might not be so obvious is that they grow during the firing. A six footer can grow 2-3 inches. So...the guy wires or ropes should be a little on the slack side or, what I do is introduce a few old bed springs to the wires, so they can accommodate both expansion and contraction. My learning curve has included:
A guy wire snapping (at midnight) and ladder to get on the roof to fix it "borrowed" by a neighbour, locked in his shed and him away on holidays.
Guy wires not snapping, so metal flue section crushing itself to accommodate the 3 inch expansion it wished to have and which I was denying it.
The stupid belief that I could squeeze one more firing out of a metal flue pipe that was obviously rusted to death. Wrong decision! When you buy one, actually buy two ( and don't lend it to your neighbour!!!!)
2. About the size of the wood for fast firing. In his Kiln Book, Olsen tells about a firing he did with Les Blakeborough somewhere a mile high in Colorado. Took 1 3/4 hrs to 1280C. Sounded a bit suss, so I checked with Les. "Almost true" said Les, "except the last 1/2 hour was a soak." Then he told me the secret. They fired with dry roofing shakes split down to chopsticks by willing helpers. Non-stop stoking but still one firebox at a time.
At right: guy wires “ a little on the slack side”
|More interactions between bricks, metal and heat
These paragraphs added in Nov, 2016.They are extracts from emails from Kevin
Exit flues blocked by rust
I've noticed from photos in books and ceramic journals that many potters use a metal flue for the last few metres of their chimney…as I did. I had a 2 metre, heavy guage galvanised iron flue extension on my bourry box kiln for several years until last firing, when I replaced it (because of rust collapse) with a stainless steel flue. I had noticed that the last few firings were not as smooth as they should be and blamed my new wood(quandong). The firebox needed constant raking and the mousehole behaved as if it wasn't even there. When I unloaded the last firing, I found one side underfired and the other side overfired…suggesting a blockage in the exit flues.
Sure enough…got out the flashlight and discovered 2 of the flues completely blocked and the other 2 were 50% blocked. I took an iron rod and hammer to it and found the culprit. Large flakes of rust had been falling down, accumulating, fusing together and gradually blocking the flues. Thanks to a hammer, sieve and ball mill, I now have clean exit flues and 3/4 kilogram of 80 mesh iron oxide, though buying the stuff might be cheaper than making your own and losing two kilnloads of pots.
Beware of relative expansion
(*) Sadly, Owen left Cherbourg before the kiln was completed, and none of the several potters who followed in the next decade knew how to fire the kiln with wood. I think it was Ray Harrison who converted it to oil, but he left to set up the Yarrabah Pottery before the kiln was fired. The next potter didn't know how to fire with oil and then the fire happened. I converted it back to woodfire and we fired it successfully just before my term there expired. After I left, it was used as storage space and was later bulldozed to make way for another activity. Sad story for a great kiln!
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