This mug is made from the strongest porcelain I have, it is so vitreous that the bare fired surface does not even coffee-stain. So I glazed it only on the inside. That created a time-bomb waiting for hot coffee! Three others did exactly the same. Four other mugs glazed on the outside were fine. Why? Glazes need to have a lower thermal expansion than the body so they do not craze over time. When ware is glazed inside and the compressive forces the glaze finds itself under keep it crack free and also significantly strengthens the piece (like pre-stressed concrete). But here there is no outside glaze to be counteract the inside one pushing outward. When suddenly heated it pushes even harder. Structural weak points, outside surface imperfections or pronounced contour or thickness changes provide crack-initiation-points to relieve the stress. The only way to make this inside-only-glazed technique work is carefully tuning the thermal expansion of the inside glaze. That means a lot of testing and a lot of broken pieces.
In ceramics, glazes are under compression when they have a lower thermal expansion that the body they are on. A little compression is good, alot is bad.
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