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? Glaze compression (the inside glaze is pushing outward) is important to avoid crazing. But, when suddenly heated it pushes harder. Outside surface imperfections, even microscopic in size, provide crack-initiation-points to relieve the stress (the solution is an inside glaze with carefully tuned thermal expansion or an outside glaze to counter the forces and fill in all the imperfections).
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