Prevent molten glass from cracking when cooling down 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
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.