Brass, an alloy of copper and zinc, is of historical and enduring importance because of its hardness and workability. The malleability of brass depends on the zinc content; brasses that contain more than 45 percent zinc are not workable, either hot or cold. Unified Numbering System for metals and alloys is the best way to know the composition of the metal and predict its applications. The letter C indicates brass is a copper alloy. The letter is followed by five digits. Wrought brasses — which are suitable for mechanical forming — begin with 1 through 7. Cast brasses, which may be formed from molded molten metal, are indicated using an 8 or 9.