Currie glaze tests in pursuit of cobalt greens, page 2
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.
The following two tests were done by accident -- I grabbed the bag of Alumina Oxide when I was going for Strontium (that's what I get for mixing glazes too early in the morning, I guess.)
Nonetheless, the results are interesting. Glazes 24 and 25 have an especially nice, smooth but not too shiny, unpinholed surface and interesting look, worth testing further.
Corner C= 90% Frit 3134, 10% Alumina Oxide added as a flux, colorants are 2% cobalt carb, 5% rutile, clay is Highwater P5 porcelain, fired to 1200C
Corner C= 90% Frit 3134, 10% Alumina Oxide added as a flux, 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.
Another approach was made by Bonnie Hellman, adding a flat 10% EPK to all glazes, as if it were a colorant. Interestingly, the greens don't show up at all on this tile -- why?? The tile looks rather similar to the set above using Frit 3195, in the sense that the glazes go from blue to nearly black rather suddenly.
Corner C= 100 Frit 3134, colorants are 2.5% cobalt carb, 5% rutile, and 10% EPK, fired to 1200C, clay is white stoneware.
Same glazes, on a light buff stoneware clay.
More experiments with cobalt greens
Back to first set of Currie grids, thumbnail version
Currie grids by Kathy Greaves, thumbnail version
Currie grids by Debbie Engle, thumbnail version