Copper and brass sheets are frequently shaped by hammering cold, as in shaping a hemispherical or other concave receptacle from a flat sheet, or in making bent copper pipes. The metal becomes more or less hard and brittle by hammering, and continued working would cause it to crack. At intervals during the shaping, when the metal gets hard enough to produce a metallic ring, it must be annealed. This is done by heating it evenly on the brazing furnace and allowing it to cool. Copper may be cooled quickly by holding it in an air draft or plunging it into water, but it is safest to anneal brass of properties not thoroughly known by letting it cool slowly where it is free from air drafts.