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Why are these vessels cracking when hot water is poured in?

They are time-bombs. Why? Yes, they have even wall thickness, the glaze is not crazing and the clay is vitrified. But, they are glazed only on the inside and that is a problem here. The glaze is under some compression, that is why it does not craze. That means that it is pushing outward from the inside. Ware can be very strong under glaze compression, but only to a point. When a hot liquid is poured into containers like these the inside glaze is the first to thermally expand, creating more even compression. The porcelain is under tension already, being stretched by pressure from within. And ceramics do poorly under tension. And third: The outer surfaces have incised lines that provide irregularities for internal forces to exploit and start cracks at. It is a perfect storm waiting for hot water to trigger it. Making the pieces thicker would help. Increasing the glaze thermal expansion would help. But unglazed outsides and incised lines will always be a weakening factor.

Why are these vessels cracking when hot water is poured in?

Related Pictures

Drip glazing and bare outsides: Deceptively difficult.

thick glaze drips on ceramic vessels

Why? Glaze fit. These are available on Aliexpress (as Drip Pottery) and they are made by a manufacturer that has close control of body maturity (and thus strength) and a dilatometer to precisely match the thermal expansion of the glaze. The glaze has to fit better than normal because of the absence of an outside glaze. Too low an expansion and it's compression (outward pressure) will fracture body (because these are thin-walled pieces). Too high and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same.

Links

Glossary Glaze Compression
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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