Firing Log

I think the 11th firing worked out quite well.  I outlined my firing plan earlier this week, and now, some of the results are available in the photogallery.

It was a very hot firing — cone 11 laid flat front to back (I don’t have any higher cones).  However, I think getting it so hot is detrimental in that the glaze tends to run completely off vertical pieces.  There are two problems with this — first, the glaze is gone, and second, foot grinding is a huge chore.  Perhaps in future firings I will work on controlling the maximum temperature to no more than cone 11.

What was most excellent however, is that I avoided the gray-gunk I mentioned previously by not overstoking.  I figure that when overstoking, I probably have a lot of gritty unburned wood particles flying through the kiln.  With optimal stoking, I still got blackening on the front sides, but the carbon creates an inky black glaze rather than 30 grit sandpaper.  I’m completely OK with inky black glaze — it looks neat.

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I also finally managed to mimic the surface of the floor bricks on a piece, specifically, the blue marbled glaze that forms where the floor sand melts onto the bricks.  The sand itself is inexpensive white sand that I buy at Home Depot — last time it was around $5 for 100 pounds.  The result is beautiful and the piece below is one piece I feel completely happy about.  It makes me want to get a bright light and a magnifying glass and mentally tramp through the mountainous landscape.

First, inspirational bricks:

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A snow capped mountain range:

   Photogallery on this piece.

I also experimented with some local clay I was given a while back by a friend doing some construction work around his house.  The clay he gave me is everywhere around here, except it’s usually 4-6 feet down so getting it is difficult.  It is a blue-gray color in its natural state.  When I made some test pieces with it (full report here), the 100% “dug-up” clay melted into a beautiful glaze.  For this firing, I decided to try lining some of my pots with this clay.  The results were interesting.

First, the failed cup:

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Same clay used as a liner glaze:

   View more images here (pics 7-9) and here (pics 10-18).

Finally, I’m also very happy with my new kiln monitor setup.  I had been using the software that came with my Radio Shack Digital Mulitmeters.  It was lousy software.  Recently, QtDMM has been released for Linux and OS X.  I put together a system with an old Mini-ITX board powered by an aging VIA Nehemia 1ghz processor (think 5-6 years old).  I used Ubuntu 8.0.4 for the OS.

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The added processing power over my old system (it died between the this firing and last year’s) allowed me to set up a chimney cam — basically just an old DV camcorder connected to a firewire port.  I used Kino to display the feed.  This helped save my knees.  What I used to do is go up the steps to the mid-level, and peek at the chimney tip through a gap in the roofs covering the kiln — when I saw the flames die out, I’d run down and stoke.  Well, my knees are getting beat up by running up and down the stairs during these firings.  Now I can just stand by the kiln and watch the chimney cam.

This is what I used to see:

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This is what I see now:

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When all is said and done, I’m totally satisfied with this firing.

PS:  if you’ve made it all the way down to the end, enjoy a sunrise:

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Gray junk. Now I don’t mind koge and it looks great on certain pieces. I’m not talking about that. I hate how some firings tend to leave a layer of embedded soot on the front of my pieces. The results are decidedly not pleasant and should not be confused with koge. Over time, I have been letting more and more air into the kiln and I’ve had a reduction in the amount of ugly soot-glaze — but it hasn’t gone away completely.

This firing, I decided I would do no heavy stokes of the type that result in pillars of yellow flame emanating from the chimney. That yellow flame is merely an indication that large amounts of carbon failed to combust inside the kiln, and are instead, flaring off in the atmosphere. At least that is my thinking. The flaring is accompanied by temperature drops, and followed by temperature rise when it ends. Obviously, the kiln is in insane reduction at that point.

I am wondering if that level of reduction is necessary and so I decided that on this firing, I would avoid the massive yellow flames. I had plenty of red witch’s hat style flames, but that is more a glow of hot gasses than fingers of fire. It was very easy to raise the kiln temperature to very high levels by stoking small amounts (single stick) as soon as the witch’s hat faded away. It would return for a short time, fade, stoke, return, fade, stoke … etc. till exhaustion.

Cone 10 dropped after an hour around 34 mV, and cone 11 dropped at around 35 mV. I hit close to 37 in the front of the kiln and 38.2 in the back. So way overfired no doubt. Even with the avoidance of yellow chimney flames, I saw plenty of black smoke seeping out of the cracks in the kiln — I doubt this is a complete solution to my grey-gunk-but-not-koge issue.

Right now, the kiln is cooling and I won’t be able to open it for many days. As a result, I’m now second guessing my experiment and worrying that I made a terrible error. Sigh. I really dislike the space between closing the kiln and opening it.  Instead, I’ll think back to the start — to the time period when I’m full of hope and anticipation. Preheating. This is how I do it: