Recently, I got to go on an unofficial friend tour of an art conservation lab in a museum. We met one of the conservationists who was repairing a shattered ancient Greek pottery vase. She pointed out an imperfection in the restoration work and said that it could never be like it was again because pottery is under tension, which is lost when it breaks, and that’s why broken pieces will never fit back together exactly.
My question is: why is this? Where does that tension come from? I assume it comes from the firing process, but I’d like to know how. And why does glass not do this? Or does it?
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Things shrink when they cool. If a material goes from plastic to hard while it has a significant temperature gradient, then it will have internal stresses where the material froze at one size, and is forced to hold that size even as it cools off.
This does happen in glass. The best example of this is in Prince Rupert Drops. You can also see a form of this if you try to bake with non-baking glass ware.
The slower the cooling process, the less internal stress there will be at the end.
Glass does it too.
And yes it’s the firing process, or more specifically the cooling down process afterwards.
When the material is hot it expands slightly. Then when it cools down it tries to shrink again, but during the cooldown the mobility of the atoms slowly reduces so they kinda get stuck in the expanded shape.
How much tension there is depends a lot on the materials being used and how fast the cooldown happens. If you cool them too fast they can simply shatter
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