Reduction firing is a way of running a kiln with less oxygen than the fire wants. It sounds like a technicality. It is the reason gas-fired pottery looks the way it does.
Oxidation and reduction
Fire needs oxygen. In an oxidation firing, like most electric kilns, there is plenty of it, and the metals in clay and glaze stay in their oxidized state. In a reduction firing, I cut back the air at the right point in the firing. The flame, hungry for oxygen, pulls it out of whatever it can, including the metal oxides in the glaze and clay body.
What that does to color
When those oxides lose oxygen, they change. Iron that would turn amber or brown in oxidation can turn soft green or deep, broken black. Copper that would turn green can turn red. The clay body itself darkens and can show warm speckles where iron comes through the glaze.
Why no two pieces come out alike
Reduction is never perfectly even across a kiln. One shelf gets heavier reduction than another. A pot that sat in the flame path looks different from one tucked behind it. That variation is the whole point. You can't fake what fire does.
Is reduction-fired pottery food safe?
It can be, when it is made with food-safe glazes fired to maturity. My own work is fine art: reliefs for the wall, collector pieces and limited-edition earrings, made to be looked at and kept.
Read how the kiln itself works in How gas firing works, or see reduction-fired pieces in the shop.