|Monthly Tech-Tip |
Left: What GA6-C Alberta Slip rutile blue used to look like. Middle: When it started firing wrong, the color was almost completely lost. Right: The rutile effect is back with a vengeance! What was the problem? We were adjusting firing schedules over time to find ways to reduce pinholing in other glazes and bodies. Our focus was slowing the final stages of firing and soaking there. In those efforts the key firing phase that creates the effect was lost: it happens on the way down from cone 6. This glaze needs a drop-and-soak firing (e.g. cooling 270F from cone 6, soaking, then 150F/hr drop to 1400F).
Rutile variegates glaze surfaces. But it also opacifies at higher percentages. The blue effect is a product of crystallization that occurs during cooling, it is thus dependent on a slower cooling cycle, especially above 1400F. This is GA6-C Alberta Slip glaze with 4, 5 and 6% rutile. At 6% the rutile crystallization has advanced to the point of completely opacifying the glaze. At 5% the blue is still strong, even on a buff burning body. The loss of color occurs suddenly, somewhere between 5 and 6 percent. Rutile chemistry varies from batch to batch. The blue develops differently on different bodies. So do you want to play "at the edge", with 5% in the glaze, in view of these other factors and the finicky firing curve needed. Change in any of which could push it into the blueless zone?
The process of holding a kiln at the final temperature (or at other temperatures) to enable the heat to penetrate the ware or to effect or complete a glaze or body reaction
A popular cone 6 glaze that employed Gerstley Borate, it was very troublesome to use and to fire. Much work has been done to create alternative recipes.
Plainsman Cone 6 Slow Cool
350F/hr to 2100F, 108/hr to 2200, hold 10 minutes, fastdrop to 2100, hold 30 minutes, 150/hr to 1400