Reduction is a new concept for a lot of people, that’s because the majority of people are firing in electric kilns, which basically only fire in Oxidation (there are some ways to do it, but they aren’t the safest or most stable).
Most people think about Reduction in reference to copper reds, which we have been talking about recently. But there are a few materials which also can be changed by reduction, and one of those is simple, unassuming Tin Oxide (SnO2).
Tin is considered an opacifier, which we use to make glazes white. It used to be used a lot more, but now it is very expensive and Zirconium Silicate opacifies for a cheaper cost.
But Tin still has a place in glazes like Copper Reds and Chrome Tin pinks, that we were discussing a few weeks ago.
But Tin is important to understand, because in reduction it changes from SnO2 to SnO, which changes the way it interacts with the glaze.
Here we have one glaze (4-3-2-1, which you can find on our Glazy page, linked in our profile), all four tests are from the same batch. We mixed it and dipped two tiles, then added 5% Tin and dipped two more. We then fired one of each in a oxidation firing and one of each in reduction. You can see that only one of the glaze (upper right) is white (opacified) the others are all clear.
That is because those glazes either didn’t have Tin at all (bottom row) or the Tin was reduced(upper left), changing its properties and removing its opacifying abilities.
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Matthew and Rose Katz - Founders of Ceramic Materials Workshop
This is a place online to understand and explore how and why our Clay and Glazes work (and don't work).