Claymonk's Journal

A couple weeks ago I was invited to lead a pit firing at Lin Holley‘s place on Vashon Island near Seattle. It was the first time I was put in charge of a pit fire with a group of experienced potters who nevertheless were pit fire newbies, and I worried beforehand that the firing would not turn out well enough, the pots would not pick up colors, and the participants would be so disappointed that they would never pit fire again. So I studied my notes and videos from previous firings carefully, and came up with a relatively conservative firing strategy that I hoped would maximize the chances for getting strong colors onto the pots. Luckily for me and for the others as well, the strategy worked, and everybody seemed very happy with the results (figure 1).

figure 1: most pots got some colors!

N.B. if you have done pit firing, or if you have read my post on a previous firing, you’d know that the colorful pots in figure 1 above had already been handled before the picture was taken. The telltale sign is the jet black area on each pot — these should be on the side that’s still buried in the embers and thus invisible to us at this point.

All in all, I was very happy with this firing, and I thought it had served its main purpose well, i.e. as an introductory firing for first timers. It was done quickly, less than 6 hours from lighting the fire to taking the last pot out; it generated very little smoke; it resulted in just one broken pot and one slightly damaged one out of a total of 20 or so; and it got strong, dramatic colors on most pots. Below I’ll document the timeline and the process of this firing, so I’d know how to repeat it the next time, and hopefully you can use this as a reference for your own firing as well.

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I have often had problems with flaking and peeling terra sig ever since I started pit firing. But since it doesn’t happen every time, I procrastinated in tracking down the root cause of the problem. Well, last Sunday I had a very successful firing — except for this flaking, peeling problem that happened to just about every pot I had in the pit, including some otherwise really gorgeous ones. Ouch!

Didn’t I say once that I learn more from my failures? So this time I fired up Google searching for “terra sig peel”, and found a bunch of great links on the Web:

• First and foremost is Vince Pitelka’s comprehensive treatise on the topic. This is a must-read for anyone working with terra sig. I wish I had read it when I first saw my terra sig peeling off back in 2002! :-(
• Clayart Forum thread "cracks in my terra sig"
• Clayart Forum thread "terra sig with no chipping/peeling"
• This article from CLAYART archive of Septermber 2001.

So what was my problem? Apparently I over-burnished my pots before I applied the terra sig, resulting in a poor bonding between the terra sig and the pot surface. And why did I sometimes get away with it, and sometimes not? My guess is that the degree of thermal stress and the presence of various chemicals played a role too.

I’ve noticed the peeling and flaking usually happens on the up side, i.e the side of the pot that was exposed to the flames, and not on the side buried in the sawdust or dung; it also tends to happen more frequently where there was a lot of fuming, often as a result of direct contact with salts or metal. The “quiet” side is generally quiet with terra sig peeling as well.

Anyway, time to start thinking about a new burnishing strategy…

Now here (figure 1) is an even more ancient looking pot than the last one. Again, the clay body and the bisque temperature (white porcelaneous stoneware, cone 06) are the same as the other pots in this firing. What’s different here is the colorant used in the terra sigillata. In this case, a white terra sig with manganese dioxide mixed in was used to give an earthy brown tone to the piece. (This is what I think anyway — see below for an alternate explanation of where the brown might have come from instead.)

figure 1: real ancient looking pot

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In my last post I wrote about the use of cobalt carbonate to make a blue terra sigillata; here in the picture below (figure 1) is a different pot — but same white clay body and same cone 06 bisque — burnished with a pink terra sig, which was made from a white Tennesee ball clay base using red iron oxide as the colorant. As is in the case of the blue pots, the underlying pinkish red tint influences the final hues of the color flashes; here it gives the pot a warm but muted, almost aged look overall.

figure 1: pot with pink terra sig

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I wrote about trying — and failing miserably — to copy Dick Lehman‘s method for transferring carbonized images of plants onto pots in a previous post. Well, that pot was just one of the two on which I had attempted that trick. Here is the other one, which seems to have fared somewhat better — at least here we get a hint of the leaves (if they are indeed what I think they are) I had placed under a clay mask on the pot (figure 1). The clay mask is simply a big wad of moist clay fixed to the pot with newspaper and twine before the firing.

figure 1: image of leaves (I think)

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Some of my best pit fired pieces came from my very first pit fire in 2002. I had just taken a saggar firing workshop with Maria Spies, where I learned a few things about terra sigillata and packing materials for fuming colors onto pots — all saggar techniques directly transferrable to pit firing. Also, by this time my potter friends Dan Ebert and Gary Georger had already done two or three pit fires with various degrees of success. So one sunny Saturday morning in early September I joined them and a few others at the beach and had my first pit fire experience.

I’ll write about the firing itself next time; here I just want to show a few pictures of the firing results, along with some description about the packing materials and the kind of terra sigillata, if any, used for each pot.

figure 1: subtle hues from smoke and fumes

This bottle with a small neck (figure 1) stands about 6 1/2 inches (17 cm) tall. Clay body is a white porcelaneous cone 6 stoneware clay. A white terra sigillata with titanium dioxide added (about 2 teaspoons to one cup of liquid terra sig) was applied, burnished to produce a high sheen, and then bisqued to cone 06 before the pit fire. The titanium dioxide in the terra sig gave the pot a creamy white surface that seemed particularly receptive to subtle hues from the smoking and fuming that occurred in the pit during the firing.

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I promised to write about this pot, so here it is. First some technical details: the piece stands 4.5 inches (11.4 cm) tall; the clay body was a commercial porcelaneous white stoneware clay (OH6) from Clay Art Center of Tacoma, WA. It was thrown on the wheel in one piece and burnished with a smooth pebble just after the leather hard stage. A white terra sigillata was applied, and the pot further burnished — now with a piece of lightweight clear plastic wrapped around my index finger — just right before it turned bone dry. It was then bisqued to cone 06 in an electric kiln. After the bisque it sat on my shelves for months until last Thanksgiving, when I pit fired it at my brother’s place out in the country. The process of the pit firing (in a BBQ smoker using wood and horse dung as fuel) was described in detail in a previous post so here I’ll just show some photos of the firing results.

figure 1: orange blushes of salted raffia

Many of my pit fired pieces have one side that’s more quiet, with less fire marks and fuming actions. The quiet side can have its own charm, though. Here in the “back” side of this piece (figure 1) we can see some beautiful, subtle grays (from the horse dung perhaps) on the neck, and then a range of orange and yellow-ish blushes near the bottom — probably caused by the salt from the salted raffia that I used to wrap the pot with (see my previous post for the “before” — as in “before vs. after” — pictures.)

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