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My mother works in a ceramics workshop. Besides giving lessons she creates various ceramic dishes, cups and plates. She often uses the effect of colored glass melting in the product and then leaving beautiful patterns at the bottom.

However it always cracks a lot as cooling down. Sometimes, it looks good when it's cracked, but other times she's trying to make it look like a gem - and cracked gem is not very gemmy.

So I was curious if there is something non toxic, insoluble in water and alcohol, that would melt at the similar temperature as glass and would prevent it from cracking in the cool down phase.

The glass doesn't have to be temperature resistant in the final products. We just want to avoid the cracks

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    $\begingroup$ This might be better asked in an art exchange. (Zoinks! There's no Art Exchange on here!?!) In any case, the way that glassmakers usually avoid this problem is through the use of an annealing oven. The forces involved in it cracking are very large; I wish you the best of luck in finding the sort of product you're after, but I'm doubtful about its existence. I'll be delighted if I'm proven wrong though. $\endgroup$ – Jason Patterson Oct 1 '14 at 3:38
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    $\begingroup$ At first I would suggest using glass with low coefficient of thermal expansion, e.g. en.wikipedia.org/wiki/Borosilicate_glass And then of course annealing, as sugested by @JasonPatterson, see en.wikipedia.org/wiki/Annealing_%28glass%29 $\endgroup$ – ssavec Oct 1 '14 at 12:33
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    $\begingroup$ You might not need a specialized annealing oven if you can control the kiln correctly. Just cool it down VERY slowly, maybe over a 24 hour period. This would require the kiln to allow some sort of temperature gradient and I don't know anything about kilns so I don't know if yours will do that. If you can adjust the temperature without turning off or opening the kiln you can probably do this manually. Every few hours reduce the temperature by a bit until it's off. This could require staying up night and playing with how quickly you cool it down until you find the right conditions. $\endgroup$ – user137 Oct 1 '14 at 18:06
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The surface cracks due to a mis-match in the temperatures inside and outside the ceramic. Things contract as they cool. So, the cooler outer surface is contracting against the hotter inner surface, which is still not contracting. The outer surface wants to be closer together than the inner material, but the inner material forces the outer material to stay apart. When the ceramic on the outside transitions to a solid (at what is called its "glass temperature"), it can no longer flow like a liquid. The only way it can continue to stay apart due to the forces from the inside is to "fall apart" ... i.e. to crack.

Your mother may be able to coat the pottery with a different glaze that forms a glass at a lower temperature than the one she has now. Also, the suggestions to cool VERY SLOWLY are directly on target. The cooling should happen in a place that has no air flow, ideally in a closed box. Absent an annealing oven, a different option may be to put the pots in an conventional NON-CONVECTION oven that is held at a temperature just below the glass temperature of the glaze being used. When the entire pot reaches that temperature, it can be removed to cool further in a box at room temperature, again with no air flow.

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