The kiln's oxygen environment fundamentally changes how iron, copper, and other colourants behave. Atmosphere is the second axis of firing — temperature is only the first.
The kiln burns clean — complete combustion, plentiful oxygen. Electric kilns are inherently oxidising. Iron oxide stays as Fe2O3: refractory, poorly fluxed, producing browns and yellows.[Digitalfire] Glaze palette is broader and more vivid; colours are predictable.
The damper closes, the kiln burns rich, and combustion becomes incomplete. Carbon-monoxide-rich atmosphere strips oxygen from glaze components: iron converts from Fe2O3 to FeO and acts as a flux, copper goes from green to red.[Digitalfire] The signature firing for celadons, tenmokus, shinos, copper reds.
A subset of reduction. Burning wood deposits fly ash on the ware; at high temperature the ash melts to form a natural glaze. Wood-fired surfaces show flame-paths and ash gradients impossible to reproduce in electric or gas kilns.[Trove]
