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Glaze on
Coloured synthetic ash glazes, suitable for ‘glaze on glaze’ in oxidation
Glaze

by Mathew Blakely

I had been working with ‘glaze on glaze decoration’ in oxidation at cone 10 but the results were too static for me. The sponge and brush marks made when applying the glazes were still visible after firing and I didn’t want the effect of ‘wallpaper patterns’ masking the forms. I was also struck by a blue-green response I occasionally got from copper. So, my search was for a fluid glaze (that didn’t run off the pot) that gave a blue-green colour with 1% copper carbonate.
I had a wood ash glaze that was shiny and fluid, with no mattness where thin, so I chose to adapt this, replacing the wood ash with synthetic ash to remove its impurities. The synthetic ash recipe I tried was a hit and miss mix up, from trunk wood and branch wood recipes, in Phil Roger’s book , ‘Ash Glazes’. I removed the soluble salts to make the glaze easier to use.
Amazingly the results were pretty much what I was after and gave a lovely blue-green with 1% copper carbonate. It was fluid and crazed slightly on Clayworks JB1, so I did a line blend adding kaolin and silica in the same ratio of 1:6.3. As the glaze became stiffer the crazing disappeared but the copper gave a dull green. The final recipe is

The glaze needs to be fired to Cone 10, or an increasingly crystalline glaze is obtained. An addition of 5% Frit 4108 can be added to broaden the maturation range.

It is a good base for colour additions - as it is quite fluid. Cobalt carbonate dissolves well, not needing grinding or milling first. This can be softened with copper and small amounts of chrome oxide to give blues and greens. Increasing increments of iron oxide also give interesting results, ranging from pale honey to deeper warm browns with spangly crystals. I used a stiffer glaze from the lineblend for the honey, to cope with the fluxing power of the iron.

The only problems I encountered were when I used the glazes on a dark clay body covered with a white slip, resulting in blistering on the larger pieces. I think this was caused by a too rapid firing (probably between 800 and 1000(C) as I solved a similar problem in my wood ash glaze by soaking at this temp. Possibly the bone ash could be omitted as it can cause bubbling.
More Glaze on Glaze articles in Pottery in Australia 40#1