The clear glaze on this cone 03 mug survived a 300F-to-ice-water thermal shock without crazing (IWCT test). However, in the process, water was absorbed by the bare foot ring and dispersed into the porous matrix of the lower part of the mug. Moisture expansion occurred as a result and produced the crazing. Over continued use (and rewetting of the base) the entire piece would eventually craze. Calcium carbonate is often added to low fire bodies to prevent this expansion.
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