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Glazing black clay bodies stained with manganese is just about impossible with typical transparent glazes. They over-flux the clay surface and decompose it generating LOI gases. In the process they ruin its color, and if it doesn't blister, the glaze does. How about transparent glazes over a black engobe instead? At least the body color is not lost. But the wrong transparent glaze can do what you see here.
These mugs are a buff stoneware, Plainsman M340. A black engobe was applied by pouring the inside and dipping the outside two-thirds of the way down. The outsides are glazed with GA6-B.
Left: A L3954F black engobe was applied inside and upper-outside at leather hard. The piece was fired at cone 6 using the PLC6DS schedule. The inside, totally clouded glaze, is G2926B. This is crystal-clear on M340 itself.
Right: The whole mug was dipped in GA6-B.
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