The mug on the left is made from a whiteware body (Plainsman M370), the one on the right is a highly vitreous translucent one (Plainsman Polar Ice). Both have been over-fired slightly. The Polar Ice mug has stuck to the shelf somewhat, taking chips out of the base on the outer perimeter of the bare porcelain (a fault called Plucking). If the shelf had been better dusted with alumina powder (or a kiln wash based on it) rather than a cheap kiln wash made from a kaolin:silica mix there would have been no problem.
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