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Firing Log

ancient kiln | 21st century logbook

May 2, 2007

A Single Piece of Firewood / I Stoked the Last of My Energy

Filed under: anagama, 9th,Firing,Pieces,sound & video — odin @ 9:25 pm

Fun first:

Here is a time lapse video showing roughly 90 minutes of stoking in two:
Nice quality, 65 mb, quicktime format.
Dreadful quality, 9.5 mb, quicktime format.

And of course, a photogallery of some of the work from the ninth anagama firing.

Now for the work:

My goodness, it’s been a long time since I posted. I finished up a firing at the end of March and then completely lost my motivation. I must have stoked the last of my energy into the kiln and honestly, I’m still waiting for it to come back — I’m burnt out. Even sitting down to write this entry is painful and I’ve been procrastinating for a month (!) despite the successful character of the last firing. I lament my lost motivation.

{… OK, let’s just do this …}

Overall, the ninth firing truly was good. I’m not 100% happy with the coloring or surface texture, but I’m satisfied for the most part. I received some nice pieces, had no shelf collapses, and internalized an important lesson. The firing can be summed up in three basic topics:

  • I finally know exactly how to make the temperature climb like a rocket.
  • However, I need to modify air inlet size, wood mixture, or both for brighter colors and
  • I need to work on my closing procedures.

Stoking One Stick:

Furutani wrote that sometimes during a firing, the temperature of the kiln can be perfectly adjusted with a single piece of firewood. Even though I’ve read that line a dozen or more times, it was only in the last firing that I actually put it into practice. The effect was astounding and the technique eminently simple: stoke one piece of firewood, watch the chimney till the glow at its tip died, repeat.

The stoking cycle was about one smallish piece every two minutes. I don’t know what temperature I hit, but the light from the kiln was blinding. It was blinding even with sunglasses behind welding goggles. It was like staring at the sun; a sort of fear inducing “holy cow — my bricks are gonna melt” kind of hot. Hot enough that I could imagine the devil requesting my consulting services when things get cool down south.

In retrospect, it makes perfect sense — firing the kiln is an exercise in finding the perfect fuel/air mixture. Anyone who has read anything about combustive firings (no matter what the fuel is) has heard that it is hard to raise the temperature in a reduction environment. I know I’ve read that all over the place — Furtani, Olsen, and from probably a half dozen other writers. Sometimes though, reading is not the same as knowing. After directly observing the effect of hitting the fuel/air sweetspot, I finally “know” this fact viscerally. I won’t forget.

Brightening colors:

As for the color of the glaze, it is less bright than the seventh firing’s results — there was less orange and more brown in this last firing. Early in the firing, I held the kiln in a reduction environment to build up a carbon layer on the pots. While it gave me some interesting glaze lines, and effects, I think I prefer a cleaner brighter look.

The carbon effect shows up In this example:

Wood fired stoneware vase with sooty glaze effect View Photogallery

This piece appears to have collected carbon soot. When the glaze layer formed and liquified, it caused the soot to streak. Once the glaze built to a certain level, it seems to have flowed separately from the carbon layer, particularly on the neck. This effect works well with my notions of fire as water. It’s as if a winter rain partially cleaned a soot coated object before freezing into a glaze of ice.

While I think I might like this effect, it doesn’t change the fact that many of the pieces were mud-toned. By the same token, it usually takes me about 6 months or so to really evaluate whether I like something … except for those truly awful pieces which are almost immediately subjected to hammer justice.

Aside from carbon, another possible factor in the muddy colors was my fuel composition. In firings seven and eight, I used mostly softwood for fuel. In the last firing, I used a 50/50 mix of hard and soft woods. I think I’ll try bumping up the ratio to 25/75, hard/soft because it may be that the type of fuel has a stong effect on the reds.

As for why this difference may exist, I have no answer — only some guesses. It may simply be that hardwood is denser and thus one armload of alder and birch is the same as throwing in 1.3 armloads of fir. In that case, it would be a simple matter of too much carbon causing the darkening. Or it could be that the there is a different chemical reaction from soft wood flame than that from hard wood. I suppose a better test would be to continue with a 50/50 blend but stoke less when stoking hard woods. My gut is telling me to change the wood though … yet my gut is often wrong.

Closing Procedures:

One easily corrected mistake I made was in cooling the kiln. This mistake caused me much anguish for the week I waited for the kiln to cool. The answer is to use an alarm clock. Here’s the issue:
I like to have a glaze that is semi-matte with glossy areas. I am not so attracted to extremely dry glaze. A surefire way to build gloss is to quick cool the kiln. In the seventh firing, I quickly cooled for about 90 minutes. In the eighth, about one hour.

In the ninth firing, I decided to go for a full two hour quick cooling period. Around 1:00 am, I capped the chimney, closed off the lower air inlets, but left the stoking door wide open. I then struggled to stay awake for two hours at which time I closed the stoke door but did not immediately seal the kiln with clay slip. What I wanted to do was cool for another hour at a moderate pace to let the coal bed reduce itself, yet not drop the temperature so much that it was impossible for a certain amount of matte glaze would to form (matte glaze forms when crystals grow in the glass — a process requiring time).

To repeat, my plan was to wait one more hour, and then completely seal the kiln.

Six hours later I woke up in a panic!

As a result of the excessively quick cooling, the pottery is excessively glossy and there is very little matte glaze at all. This drastic cooling may also have played a role in the lack of any of the brighter orange tones. There is absolutely no orange on any piece I pull out during the firing, rather, those pieces that cool from kiln to ambient temperature in a 30 minute period are very glossy, but completely lack red or brown tones.

Anyway, for not using an alarm clock, I swore at myself for days.

2 Comments »

  1. Hello Odin,
    Well I think you are being excessively modest with this firing. It must be because you got so tired stoking the kiln all by yourself. to my unrefined occidental taste, the pieces are absolutely gorgeous. I wish I could have Stone Vase number 2 or that vase that you show at your blog page. They are wonderful. And the shiny glaze is beautiful for a change. It gives such a smooth appearance. I saw clearly three colors at the pinched bowl you show. Again, beautiful.
    Claudia

    Comment by Claudia Borio — May 16, 2007 @ 6:05 am

  2. Hey, getting geared up for my next firing and noticed something in your work read your blog and it was not what i had expected. i have played with firing with diffrent wood and found the color of ash and clay is brown like you have gotten using mostly pine like you were stoking in the video. using hard wood cherry maple oak and bit of cedar got green jewel tone ash and bright flashing little or no browns which seems opposite to your results. which leads me to believe its more about atomsphere. browns came from a firing that took only 17 hours(120cuft kiln) and was mostly neutral with an end in reduction the longer firing with greater swings from reduction and oxidation gave resluts you were looking for. we plan on firing 48 hours minimum now. happy pyroing. Tim See pyronaut

    Comment by Tim See — May 16, 2007 @ 8:26 pm

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