Currie glaze tests in pursuit of cobalt greens, page 2
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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.
Click on tile images to see a larger version
Since the cobalt greens showed up only in the top rows of the tile in the original set, we decided to try increasing the alumina in the set. In this set, 2% alumina oxide was added to the frit, (effectively simulating an alumina-bearing frit).
Just to be inconsistent and nonscientific, I changed the colorants in this set, reducing the cobalt. In my studio work, my cobalt green has worked with either 2% or 3% cobalt. The primary difference seems to be that more cobalt produces a richer blue in the thick areas of glaze.
Corner C= 100% Frit 3134 plus 2% Alumina Oxide added as a flux, colorants are 2% cobalt carb, 5% rutile, clay is Highwater P5 porcelain, fired to 1200C
Close-up of top rows, showing cobalt greens. Click image for macro view.
Corner C= 100% Frit 3134 plus 2% Alumina Oxide added as a flux, colorants are 2% cobalt carb, 5% rutile, clay is Highwater Speckled Brownstone, fired to 1200C
Close view of top rows. Click image for macro view.
In this set, I experimented with a high-boron frit that also contains alumina, frit 3195. You can't see it on these tiles, but the areas that look black in the middle rows show some olive green under strong light. However, in general the greens are not in evidence. One difference between 3195 and 3134 is that 3195 has less calcium, so the next set may involve adding a calcium-bearing flux material such as whiting.
Corner C= 100% Frit 3195, colorants are 2% cobalt carb, 5% rutile, clay is Highwater Speckled Brownstone, fired to 1200C
Closeup of part of above tile, click image for macro view.
Corner C= 100% Frit 3195, colorants are 2% cobalt carb, 5% rutile, clay is Highwater Speckled Brownstone, fired to 1200C
Closeup of part of above tile, click image for macro view.
Back to first set of Currie grids