Annealing. Annealing is a process by which the extreme brittleness common to glass and some metals, when cooled suddenly after melting, is avoided or removed. Such brittleness appears to be occasioned by some disturbance in the regular arrangement of the constituent particles in rapid cooling, and is remedied by cooling very slowly and regularly. Glass-houses are furnished with large annealing-ovens, in which this object may be attained by the gradual removal of the glass articles from a hotter to a cooler part, or by allowing the heat of the oven to subside slowly. Analogous operations are employed in the manufacture of cast-iron and other metals. Some malleable metals which crystallize on cooling are brittle in their crystalline state but are rendered tough by heating and rolling. Zinc, for example, though incapable of more than very slight extension under the hammer, without cracking, becomes almost as flexible and tough as copper after being rolled at a moderate heat.