At about half past four on a firing day, with the kiln at roughly a thousand degrees, I close the damper a little and the kiln changes its voice. The steady roar of the burners goes softer and lazier. A short orange flame starts licking out of the spy hole at the back, and the studio fills with a smell that is somewhere between a bonfire and a struck match. That is reduction. For the next few hours I will be deliberately starving a kiln full of pots of oxygen, and almost every colour I am known for depends on it.
I get asked what reduction firing actually is about once a week, often by people who have fired electric kilns for years and never needed to know. So here is the plain version, from someone who learned it by ruining a lot of shelves.
Starving the fire
A gas kiln burns fuel with air. Give it more air than the fuel needs and you have an oxidising atmosphere: clean, efficient, roughly what an electric kiln gives you all the time. Give it less air than the fuel needs, by closing the damper or turning down the air on the burners, and the fire goes hungry. It starts producing carbon monoxide, and carbon monoxide is greedy. At these temperatures it will pull oxygen out of anything that has some to give, and the things in my kiln with oxygen to give are the metal oxides in the clay and the glazes.
That is the whole trick. Reduction is a way of changing the chemistry of the colourants while the glaze is molten, so they settle into a different state when it cools. The same bucket of glaze fired in an electric kiln and in my gas kiln can come out looking like two unrelated recipes.
Iron, mostly
Nearly everything I make is coloured by iron, because iron is cheap, safe to handle and astonishingly versatile. In oxidation, a glaze with a small amount of iron in it fires to a pale straw or amber. In reduction, the iron loses some of its oxygen, dissolves into the glaze in a different form, and turns that same glaze a soft grey-green or blue-green. That is a celadon, and my pale green glaze, the one I call Shale, is nothing more exotic than a clear glaze with about one and a half percent iron oxide, fired in reduction. Put it in an electric kiln and you get a slightly yellow clear. I have tried, and the pot was perfectly fine and completely uninteresting.
Push the iron much higher, up towards eight or ten percent, and reduction gives you the dark, glossy brown-black glazes that break to rust on the rims. My Turf glaze is one of those. Where it runs thin over an edge, it goes the colour of a wet bog road.
Reduction changes the clay too. The body I use has a little iron scattered through it, and in reduction those particles come to the surface as dark freckles that bleed up through a pale glaze. The toasted, speckled look that people sometimes ask me how I paint on is not painted on. It is the kiln pulling iron out of the clay.
Reduction is a way of changing the chemistry of the colourants while the glaze is molten. The same bucket can come out looking like two unrelated recipes.
Copper, occasionally
Copper is the showier one. In oxidation it gives greens. In reduction, a small amount of copper can give a deep red, the kind I chase on my tall bottles. It is also the most temperamental glaze I own. Too little reduction and it comes out a muddy pink. Too much, or reduction that eases off at the wrong moment, and it goes liver-coloured or clear. I fire copper reds a few at a time, in the parts of the kiln I know best, and I still lose about one in four.
Why no two firings match
Reduction is not even. My kiln has a hot front, a cooler back, and one corner at the top where the flame always seems to linger, and the glazes record all of it. The same mug can come out green on one side and grey on the other, because one face was turned towards the flame. I log every firing: the time I start body reduction, the damper positions, the cone packs at the top and bottom, what came out of which shelf. Those logs are the reason I can repeat anything at all, and the full ones go to the patrons on my Patreon, which you can find on the official links page.
None of this is new. Potters have been doing it for well over a thousand years, without thermocouples or gas meters, by reading flames and smoke. When I stand at the spy hole at half past four, watching that small orange flame, I am doing something very old, slightly badly, and I enjoy it more than almost anything else in the job.
One practical note, because people always ask: carbon monoxide is dangerous, which is why my kiln lives in a properly ventilated shed with a CO alarm on the wall. If you are setting up your first gas kiln, start with the ventilation and work backwards.