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The wrong transparent glaze on a black underglaze or engobe can do what you see here (inside left). 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.
L3954F black engobe was applied inside and upper exterior to both leather hard mugs. After firing to cone 6 using the PLC6DS schedule, G2926B, which is crystal clear on M340 itself, became completely clouded over the engobe because bubbles generated during firing remained trapped in the melt.
Right: The entire mug was dipped in GA6-B. The Alberta Slip particles and the melt characteristics of GA6-B promote bubble coalescence and escape, producing an exceptionally glossy jet-black surface over the same engobe.
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