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This body vitrifies to incredible strength on its own. But applying a glaze complicates that. The inside glaze, G2926B, is under COE glaze compression. The outside glaze (which I neglected to sieve) only covers about half of the surface. Another mug just like this, glazed inside and out with the same glaze, required a dozen hammer whacks to break (it sounded like a piece of steel on impact)! But the lower section of this broke off with the first impact and the other on the second. But notice that the entirely glazed handle has not broken. Notice wall inside the handle has separated latterly - the the handle is holding fast! This is a testament to how much the glaze strengthens the handle (by its full coverage) and how much it weakens the body (by partial coverage).
This mug is made from 325 mesh MNP, the strongest porcelain I have. Since the walls are of even thickness with no abrupt corners or contour changes and the glaze is thinly and evenly applied I thought I could follow a social-media-driven trend and glaze only on the inside. But I got glaze compression time-bombs waiting for hot coffee triggering! Three other mugs failed this same way! But four with this same glaze inside and out were fine. Why? The outside glaze counters the inside one pushing outward. And it closes crack initiation points.
I got lots of pushback on social media saying glaze compression problems are overblown. But I also got stories and pictures much worse than this (especially with thick and drippy glazes). But, some still feel that inside-only glazing can work by carefully tuning the thermal expansion fit between body and glaze. Or even by accident. Either way, there is still an elephant in the room: Glaze fit has to be just right - too much and pieces break, too little and the glaze crazes. That is a problem because it brings intolerance of even slight changes in body, glaze or firing.
Of course, by making thick-walled ware having a glaze thickness is not excessive, you may still get away with doing everything just advised against! But think about this: This coffee could have ended up in my lap rather than on my paper?
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