Firing Log

One of the ultimate joys of pottery is burning things. So when a friend of mine gave me a lump of clay that he dug up while in the process of building a foundation, I grew quite excited. Finally, I had an excuse to drag out the raku kiln, flare off some propane, and singe my eyebrows a bit.

Last Friday afternoon, I stopped by my friend’s office and picked up a chunk of his property — neatly contained in a ziplock bag. Then on Saturday, I pounded up the block by wrapping it in a heavy plastic bag and sieving out the powder.

Full Size Image

Full Size Image

Full Size Image

Once I had a bowl of dust, I took it into the studio and made four test clays (all measures volumetric, not weight based):

1. 100% test clay. Back right in photo below.
2. One part test clay, one part Helmar kaolin. Back left in photo below.
3. One part test clay, two parts Helmar kaolin . Front right in photo below.
4. One part test, one part Helmar, 2/3 part silica, 2/3 part Custar feldspar. Front left in photo below.

Full Size Image

I poured each test out like a pancake on the wedging table with Silver watching and perhaps wondering if they were tasty cakes for cats. After a few hours, I was able to wedge the clay into four little balls. From these, I quickly whipped up eight pinch pots and marked the bottom of each piece with the number corresponding to the test clay. Each of the clays was pretty short and edges tended to break and separate.

Full Size Image

Full Size Image

Full Size Image

Sunday came like Christmas morning — a burning day — and I returned to the kiln great anticipation. Unfortunately, Saturday night was comparatively cool and the pieces had not dried — they were at the perfect leather hard trimming stage. I decided to fire them anyway. Besides, what better to fire wet pottery than a wet kiln?

I haven’t used the raku kiln in a long time and so when I pulled the tarp off the base bricks, they had all turned green with mold. Additionally, an entire colony of snails was living quite cozily in the moist damp undersides of the tarp. As an aside, I wonder if this has been the source of the large snail population this year. I find it disturbing to walk around in the grassy area behind the studio — it seems that every fourth or fifth step results in the sickening pop-crunch of a snail being smushed.

Full Size Image

Full Size Image

After rescuing as many snails as I could find by putting them out in the tall grass areas in which I don’t walk, I set about loading the raku kiln. Because the pieces were substantially wet, I placed the shelf high above the burner port to reduce the risk of explosion. Then I candled the kiln at the lowest reliable flame, gradually increasing the temperature until the pieces appeared dry when I peered into the kiln. Once I got to that point, the fun began. I removed the regulator from the propane hose so I could a big jet of flame, and gradually began raising the temperature. Once I hit red heat, I let it roar. When that burner is tapping propane straight from the tank, it sounds like a jet engine.

Full Size Image

Full Size Image

I knew I couldn’t get to cone 10 with the raku kiln, but during the firing I began to wonder how high I could go. I grabbed a spare unused pyrometer I had laying around and inserted the probe between the brick base and the fiber kiln body. Toward the end of the firing, I stoked a medium sized log into the kiln — chopped up into kindling of course — stoking two small sticks at a time. Eventually, I was able to get to 2200 F and held it there for a short time (given enough time, this would equate to about cone 5 when firing quickly). It was a hot day and the wood chopping, such as it was, made it seem hotter. I figured that was good enough for a test and called the firing.

Full Size Image

I was intending to open the kiln raku style right then, but I suddenly realized I was wearing shorts and had brought no pants, so I just closed up the kiln and decided to wait till it reached a temperature that wouldn’t blister my legs. Eventually, the temperature dropped and I was met with an amusing sight — a lovely puddle of glaze where the “pure” dug-up clay cup had been:

Full Size Image

Full Size Image

Results:

#1: 100% dug-up clay. I love this glaze with the subtle variations of black and brown. A hot mud spring frozen while burbling — or a really flat cup (below):

Full Size Image

#2: 1:1 | dug-up:Helmar. This piece is slightly self-glazed (below):

Full Size Image

#3: 1:2 | dug-up:Helmar. This piece is quite dry (below):

Full Size Image

#4: 1:1:2/3:2/3 | dug-up:Helmar:silica:custar feldspar (below):

Full Size Image

#4 is rather interesting. It is self-glazing and incredibly porous — I know this because I broke it a little prying it off the shelf and the clay body is full of pinhead size holes (you can see the scar on the rim in the picture above). Perhaps it would make interesting insulating mugs. The porosity makes it feel extremely light though — so light that it feels “wrong” somehow.

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:

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.