These two mugs are the same dark burning stoneware (Plainsman M390). They have the same clear glaze, G2926B. They are fired to the same temperature in the same firing schedule. But the glaze on the left has 4% added iron oxide. On a light-burning body the iron changes the otherwise transparent glass to amber colored (with speckle). But on this dark burning clay it appears transparent. But amazingly, the bubble clouds are gone. We have not tested further to find the minimum amount of iron needed for this effect.
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