Saturday, April 07, 2007

Falling Cones

Well, we tried to fire Monstro last weekend.

We started at nine in the morning, and Ronnie threw in the towel at ten at night. She got the kiln up to two thousand degrees, and there it stalled, bellowing flame and soot.

She called the manufacturer of the kiln, Olympic, and talked to them for a bit, and also brought in the gas company.

The first problem was that the kiln still had the natural gas orifices installed, instead of the propane set that I had. Now, I thought that I had swapped them out, before Steph was born, when we had the kiln sitting on the pad next to our house in Phoenix (a.k.a., The Ranch). We had had a studio added as a bonus room to the side of the garage, and were planning to get a propane tank to hook up.

Propane is more energetic than natural gas, so the orifices have smaller holes.

The other problem was that the kiln had been loaded incorrectly. Julie and Ronnie unloaded the kiln, and then reloaded with gaps between the sets of shelves, and rotated the shelves ninety degrees between levels. This is to force the hot gases to circulate more, and to help temperatures even out in the kiln.

We started at nine this morning. Every half hour, we would use a pyrometer to measure the temperature at the top and bottom of the kiln. We would then tweek the gas up a bit. Early on in the firing, we had a three hundred degree difference in top and bottom. We had the kiln climbing at about two hundred degrees every half hour, when, on one tweek, the kiln stalled, and we wondered whether we had given it too much gas. Too much gas is as bad as not enough, and the kiln stalls.

We backed the gas off a hair (there is a single valve that rotates about ninety degrees to control the inflow of gas to the kiln. There are four burners, with an autoignition ring, and a small thermocouple that shuts off the gas if all flames go out for some reason), gained another hundred degrees, then stalled again. Each time the kiln would stall, the temperature at the bottom of the kiln would creep up a little more, so the thermal gap between top and bottom was slowly closing. We then cracked the valve a little wider, and got it to climb again.

Ronnie had loaded three pyrometric cones into a cone pack (a bit of clay to hold the cones up. Ronnie's look like little Viking long boats with three masts). She uses a seven, a nine, and a ten. As temperatures climbed in the kiln, the cone seven would start to bend. A little later the nine would go, and finally the ten. When the tip of the ten touches the bent over cone nine, then the firing would be complete.

The inside of the kiln is really hot, over twenty-two hundred and fifty degrees at the end of the firing cycle. Everything in the kiln is glowing with a yellow-orange heat, and it is difficult to see the cones through the peeps in the kiln, because there is nearly no contrast between pots, kiln interior, cones, and flames. We squinted and stared a lot. Julie loaned us her darkest sunglasses out of the car to cut down on the glare.

After cone seven fell (bent over until its tip touched), we started monitoring the cones and temperature every fifteen minutes. As the tip of nine started to bend, we covered the opening in the lid of the kiln with a piece of broken kiln shelf. This is called 'damping the kiln'. Damping cuts down the gas flow through the kiln. A shaft of flames comes out, which is caused by insufficient oxygen in the kiln for full combustion, so the remaining fuel is burning outside of the kiln. The atmosphere in the kiln is in a state of reduction, and is pulling oxygen from anywhere it can, including the melted glazes on the pottery. This is what shifts a copper glaze from green to red.

A side effect of damping is that the bottom of the kiln starts to close the temperature gap with the top.

The whole kiln got within fifty degrees top to bottom, as the final cone (ten) fell. We shut off the gas, closed the damper, did a quick victory dance, then all went home. It took ten hours for the firing, but we think that we can shave a couple of hours off of that next time by turning the gas up quicker. We now need to wait at least twenty-four hours before cracking the kiln open.


I threw a rocket today.


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