Question

way after melting a cristal alum, after cool down it crumbles to the touch.
thanks for your attention.

Answer 1

Alum is the hydrated potassium-aluminium sulfate with the formula KAl(SO4)2•12H2O. When you are heating it, large crystals of alum are dissolving in the internal crystallization water, it looks like melting. When it cools down quick enough, it recrystallizes as many microscopic crystals, which are weakly coupled to each other. When you touch such recrystallized alum, the microcrystals disconnect from each other and it crumbles into a powder.