why does glass become less fragile when cooled down slowly?
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Glass it is a poor conductor of heat. When you pour a very hot liquid into a thick glass container what happens is the surface of the glass that comes in contact with the hot liquid heats up and expands, while the outer layer still remains cold and does not expand (because glass is a poor conductor and does not carry heat quickly). This leads to the hot expanding inner layer pushing against the cold outer layer which eventually causes cracking (like trying to fit a larger bowl inside a smaller one).
You can use this property to your advantage and have a glass that has an outer layer with an inner layer pushing from the inside in such a way that when it breaks it does it explosively without leaving large jagged shards. Such glass is called tempered glass.
The process to produce tempered glass, called “quenching,” cools down the surface of the glass much faster than it does the center. Because of this, the inside of the glass pushes towards the outer layer while the surface pushes towards the inside. This combination is what makes tempered glass so strong.
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