Under which situations can an amorphous substance change to crystalline form?

Continuous slow heating and cooling of amorphous substances over long periods of time causes small portions of the substance to turn crystalline. Crystalline solids have a sharp melting point. On the other hand, amorphous solids soften over a range of temperatures and can be moulded and manipulated. On heating they become crystalline at some temperature. For example, some glass objects from ancient civilisations are found to become milky in appearance because of some crystallization in their structure.