400x photo of the clay/glaze interface. Notice the vitrified clay body (white) is pulled up into the cobalt glaze. Similarly, the cobalt glaze is pulled into the clay body. The interface is more than just a bond between the two: it has a mechanical structure that locks the two together. Notice the white flecks of clay floating in the cobalt blue: you are seeing the visual results of "leeching."

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Comment by Tom Anderson on October 18, 2016 at 9:23am

Hi Denice:

All oxides will leech: alumina is probably the most well known because it hinders glaze run. If the iron content is high enough in stoneware: it will change the color of the applied glaze. You see that often: a glaze is one color on porcelain, and different on stoneware. Stoneware clay can also have high levels of magnesium: which shows up as a grey cast in the glaze. In the picture above, I reduced the amount of flux in the glaze because the clay has a higher than normal level: creating the translucency. Pending the formulation: porcelain can have a high level of titanium dioxide: which causes over nucleation in a crystalline glaze. Potters get stumped by that one: it nucleates one way on a porcelain body, and over nucleates on another. Often adjusting their formulas thinking it had something to do with it. Clay can cause, and does cause more problems than most potters realize.

Tom

Comment by Denice E. Demuth on October 18, 2016 at 6:36am

Very cool, I had no idea that clay and glaze mechanically locked.  Do all oxides leech or just cobalt.   Denice

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