Cold rolling is a metallurgical process in which metal is passed through a pair of rollers at a temperature below its recrystallization temperature. This process hardens the metal, by compressing and stretching the metal crystals. During the rolling process, the metal is annealed by heating it above the recrystallization temperature after every few rollings, to prevent it from becoming brittle and cracking. Cold rolled metal is given a "temper" rating based on the degree it was compressed. "Skin-rolled" metal undergoes the least rolling, being compressed only 0.5-1% to harden the surface of the metal and make it more easily workable for later processes. Higher ratings are "quarter hard," "half hard" and "full hard"; in the last of these, the metal is compressed by 50%. Metal hardened by cold rolling can bend without breaking, although ductility decreases as the hardness increases. Quarter hard metal can be bent entirely back on itself, while full hard can only be bent 45°.