Changing of volume of a melt during cooling,
(a) caused by crystallization,
(b) caused by formation of glass.

Crystalline materials go through an enormous variation in volume with the melting temperature T_S, they crystallize (red line). In contrast to crystalline materials glasses condense over a wide temperature range (blue curve) also beyond T_S. In the area of the transformation temperature T_g they change from the plastic state to the brittle state when cooled slowly (blue doted line). Rapid cooling leads to an undercooled liquid (blue line). 

T_R = ambient temperature,
T_g = transformation temperature,
T_S = melting point.