Wednesday, June 22, 2011

Summer Solstice a Hot Day for Firing

We blew a bit of extra ash into the kiln during this firing.

My daughter holds a board in front of the stoke hole
with a bit of ash deposited on the end, and I use an extractor
fan for an electric kiln to blow the ash into the kiln.

Thanks to Levi's midnight to 5 a.m. shift, we were able to finish up the firing today by 8 p.m., a 20-hour firing. We had three large pots fired green, so we took everything real slow.

WARNING: The following might be too much information if you're mostly interested in pottery and not pottery making.

We had thought we would be firing until after 10 p.m., but the kiln's temperature shot up quickly at the end, giving us a little scare at first.

We watched cone 10 drop in the front of the kiln, and an hour later cones 13 and 14 dropped. It usually takes about three or four hours to get cone 13 down after cone 9 begins to bend. I added cone 14 this time, thinking it wouldn't bend, so I was surprised when I noticed it was all the way down.

Cones (called pyrometric cones) are one of the most accurate ways of measuring temperature and heat work in a kiln. You place them in a kiln so you can see them during the firing and gauge your firing time. Cone 10 is about 2,340 degrees F. And cone 14 is about 2,530.

After we noticed all our cones in the front of the kiln had dropped, we fretted a while about the possibility of overfiring. Certain clays are designed to be fired to specific temperatures, and some of the clay we use undergoes a lot of stress at these high temperatures.

In the end, we decided to stoke until cone 9 was down in the back of the kiln (naturally a cooler area of the kiln because it's furthest away from the firebox). Once again, we essentially ignored the temperature in the front of the kiln to get the back up to a decent enough temperature. One of the reasons, we kept going was that we realized that the kiln didn't actually feel as hot as we normally get when at the end of the firing. I wonder about the accuracy of cones sometimes when firing in a wood kiln. All that ash melting on the cones...

We had some bloating (probably caused by overfiring) in one of the clays we used during the previous firing, so we were a bit worried about getting things too hot this time. We got cone 10 down in the back last time (our highest temperature yet in that section of the kiln). So, this time we shut the kiln down when we saw cone 9 down.

Everything looked good during my last peek into the kiln before sealing it up and going upstairs for some dinner.

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