Currie glaze tests in pursuit of cobalt greens, page 1
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Glaze tests carried out using Ian Currie's grid method, as outlined in Revealing Glazes.
Tiles were fired in a Skutt 1027-3 kiln with controller. Firing schedule was 50C/hr up to 120C, 150C/hr to 525C, 75C/hr to 600C, 150C/hr to 1050C, 60C/hr to 1200C, hold for 20 min. then cool at 200C/hr to 1000C, 100C/hr to 800C, hold for 45 min, then cool naturally from there.
To see the same glazes on the same clays, fired to cone 7 with a different schedule, see John Post's glaze test page
Click on tile images to see a larger version
Corner C= 100% Frit 3134, colorants are 3% cobalt carb, 5% rutile, clay is Highwater P5 porcelain, fired to 1200C
Close-up of top row, showing cobalt greens
Corner C= 100% Frit 3134, colorants are 3% cobalt carb, 5% rutile, clay is Highwater Speckled Brownstone, fired to 1200C
Close-up of top row, showing cobalt greens
Corner C= 100% Frit 3134, colorants are 3% cobalt carb, 5% rutile, clay is Highwater Speckled Brownstone, fired to 1200C
I was curious why we didn't see more greens on the first sets. Because my cobalt green glaze tends to be blue when it is applied too thickly, I tried applying the same 35 glazes, but more thinly. Bingo, more greens.
To see what would happen on a pot, I mixed together all the glazes in the top three rows, giving some approximation of glaze #8 in a quantity large enough to glaze a small bowl.
Glaze #8 applied thinly to small bowl (3" diameter). Very sensitive to application -- thinner areas inside the bowl are very dark, nearly black.
Since the greens only appeared in the upper portion of the tile, I tried extending the tile upward by adding 30% EPK as if it were a flux, that is, adding it to Corner C. For reference, Glaze 18 on this tile is roughly equivalent to Glaze 3 on the original set. As you can see, there are indeed more greens "above" the original set, but they don't melt very well at cone 6.
I think it would be really interesting to see this set fired higher to see if the greens held up if the glaze were more melted. If I had to bet, I'd guess that at least some of the green would be lost since (I hypothesize) the alumina is allowing the green color to show up by stiffening the melt and inhibiting some kind of crystal formation that brings out blues. If you fired hotter, and the glaze became more fluid, perhaps the green would be lost.
On the other hand, if that is true, why should refiring bring out the green, rather than the reverse?
Corner C= 100 Frit 3134 plus 30 EPK, colorants are 2% cobalt carb, 5% rutile, clay is Highwater P5 porcelain, fired to 1200C
Corner C= 100 Frit 3134 plus 30 EPK, colorants are 2% cobalt carb, 5% rutile, clay is Highwater Speckled Brownstone, fired to 1200C
More Currie Grids in pursuit of cobalt greens, thumbnail version
Currie grids by Kathy Greaves, Talc Set and Roy/Hesselberth Glossy Base Glaze 2, thumbnail version
Currie grids by Debbie Engle, Talc Set on different clays, thumbnail version