25 May 2005

Pit Firing

A quick update while Mica's asleep in her swing and her daddy is getting ready for work.

Today, Kim is planning a pit firing and I need to drive the pieces that I unloaded from the kiln this morning to her studio. The twistie beads look alright now but, with any luck at all, they'll look stunning with metalic reduction.

The pit firing process that we use is a little different from that most commonly used. Kim and I pre-fire our work in an electric kiln. That, in itself, is not unusual but we actually glaze the pieces with modified raku glazes and pre-fire the glazes to maturation in the electric kiln. The result is a finish which is very similar to and easily confused with raku.

"So why not just raku fire your beads?" I'm asked... Raku firing involves firing pieces until the glaze matures and removing the red-hot pieces from the kiln to transfer them to a post-firing reduction chamber. That chamber is commonly a trash can with straw, leaves, grass clippings, shredded paper, or other combustible materials inside. The heat from the red-hot pottery ignites the fuel and then a lid is placed over the chamber so that the flames are smothered. Since fire needs oxygen to burn, once the lid is placed on the chamber the fire consumes all of the oxygen in the chamber and then steals oxygen molecules from the glaze. The result is that the mineral oxides which color the glazes loose the extra oxygen molecule and return to a mineral state.

The most common example of this is copper oxide. When fired in an electric kiln with lots of oxygen, copper oxide makes a green glaze. It is the same principal which causes copper roofs like on a state-house dome to tarnish and turn green. In the absence of oxygen, metals will not tarnish. I'm sure that a chemist would probably have some corrections to make to my description but, it will suffice for now.

When you raku fire beads, or anything without much thermal mass, it has been my experience that the pieces cool too much between the kiln and the post-firing reduction chamber to ignite the combustible materials. As a result, very little reduction is achieved. I know people whose efforts have brought them much different results but I've not had the liberty of exhaustive experimentation and, as a result, am happier with the pit firing method that we've developed.

Pit firing may seem, at first, more labor intensive. First there is the digging of the pit. Kim has a pit next to her studio which has become a permanent fixture, she even had a metal shed built over/around it so that it would be somewhat protected from the elements. Next there is the gathering of wood. We use a lot of sticks, twigs, dead-fall branches, etc. Since Kim supplements the heat in her studio with a wood-burning stove, there is also cord wood available. With the pit dug and the wood gathered, the first step is to lay a bed of sticks, branches and kindling in the pit. Onto this bed we begin arranging the pieces which we'll be firing. I never actually count how many pieces go in, but I know that it would take several raku firings to equal the number of pieces in the pit. Once the beads, vessels, and other pieces are loaded, we start laying more wood on top of the pit. Starting with branches and twigs, sometimes adding heavier boards, etc.

The twigs and branches burn fast and hot leaving a bed of coals. Once the fire has burnt down to embers, we bury the whole pit in a combination of sawdust and soil. At this point we usually go to lunch or something to kill time for an hour or two before the un-earthing. Once the pit has cooled some, we dig out the contents using a shovel and some barbeque tongs. It is a slow process, each shovel-full of dirt and ash must be carefully examined to ensure that there are no beads mixed in. A bucket of water comes in handy for quencing those pieces that are still hot, and a garden hose allows us to put out smoldering hot spots if we find them while we dig.

Once the pieces have been recovered from the earth, they must be cleaned. Many of them come out of the pit looking less appealing than the charcoal remnants of our firewood. With careful scrubbing we are often amazed at what we discover. The crackling of the glazes caused by thermal shock and uneven expansion and contraction because of the play of fire... The smoking which highlights those craze lines and gives the pieces an ancient and timeless quality... and most magically of all, the reduction. It is unpredictable at best. Two pieces placed right next to each other and uncovered in the same shovel-full may be as different as night and day. A single piece may seem barely effected by the process except for one tiny area which was kissed by a hot ember when the pit was buried and reduced to a bright copper flash.

Kim and I loose a lot of pieces in the process. Sometimes the glaze boils and bubbles. Sometimes a piece is broken by the shovel. (or perhaps by thermal shock caused by the extreme variations in heat) And some pieces get lost in the ash only to be found much later after one or 5 firings, if ever. But the end result is worth it, at least to us.

I may go back to my experiments with raku. I have an electric kiln which I'll use for PMC once I get it hooked up again, and I could probably do some raku in it. I doubt, however, that either Kim or I will ever give up pit firing completely in favor of raku. There's just something too magical about the process. It is like alchemy and the primitive experience of fire and smoke is now in our blood.
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