GRAIN

unit of weight equal to 0.065 gram, or 1/7,000 pound avoirdupois. One of the earliest units of common measure and the smallest, it is a uniform unit in the avoirdupois, apothecaries', and troy systems. The ancient grain, varying from one culture to the next, was defined as the weight of a designated number of dry wheat (or other edible grain) kernels taken from the middle of the ear. The Sumerian shekel equaled the weight of 180 wheat grains; the British silver penny sterling was set at the weight of 32 wheat grains. The metric grain of 50 milligrams is used to weigh precious stones. in metallurgy, any of the crystallites (small crystals or grains) of varying, randomly distributed, small sizes that compose a solid metal. Randomly oriented, the grains contact each other at surfaces called grain boundaries. The structure and size of the grains determine important physical properties of the solid metal. Grains of a metal ingot can be elongated and locked together by rolling to improve the mechanical properties in the direction of grain length. Internal stresses at grain boundaries may be relieved by annealing to restore exhausted ductility in certain alloys or to harden other alloys.