Before we could start the Barrel Fire Research Project, we needed to have some idea of what we were doing. Armed with various articles on the subject, I took a bunch of test tiles to Red Kiln and spent a couple of days lighting various fires.
First Attempt: A small metal garbage can was filled with brush (dried lavender bushes), sticks and sawdust. The work was placed in the can so that the work was half out of the sawdust. It was then lit. I immediately had problems getting flame because there was no air circulation around the sawdust. Eventually, I got enough flame to get some smoke and covered the can. I knew I had little heat and expected nothing from the copper, salt, etc. Although much of the sawdust was untouched by flame, the smoke quickly turned the work very black. The portion covered by sawdust was protected and stayed white.
This raised the question, "How little is needed." The amount of black made me curious as to what it takes to make clay change color. The answer is - very little. I placed a tile on a handful of corn chips and lit them on fire. They caused crusty grey/black variations. I flattened a wad of newspaper and placed a tile on top. It made nice black edges, with grey on the front fading to white in the center where the paper did not have oxygen to burn. A wad of newspaper in a tall saggar with pottery in it made lovely variations of grey/brown/black as the smoke swirled around the clay. None of these took more than 5 minutes to fire.
Armed with the knowledge that black and grey will happen no matter what we do, I tried to get color. I suspect that much of the color becomes covered in the black. So we need to not only get color, but reduce black. Final Test Fire: A large can was filled with newspaper, 20 sticks, lavender brush and loose wood shavings. Tests were wrapped in copper scrub pad, steel wool, salt, copper carb or copper sulphate. It fired with flame for about 15 minutes, but then I was out of wood. I covered the can for 30 minutes. The color was mainly black/white but the coppers clearly made ocher yellow and bright green as well as the common black lines copper scrub pads cause. A bright iron orange from the steel wool occurred one piece. So the higher temp is obviously needed for color. But we should do even better with a good supply of wood. I think more wood will make more heat and cause areas of the carbon black to be removed and return to white - hopefully showing more color.