Grades of Pig-iron
Pig-iron produced from the same ores differs in its nature and quality, and must be rated or graded in such a manner as will indicate the special purpose for which each is applicable. Broadly stated, the classifications are commonly understood as gray, mottled, and white--a condition discovered by the fracture; but the gray iron is subjected to still further divisions, termed No. 1, No. 2, No. 3, etc., according as the fracture indicates the various degrees of hardness, commencing at the softest, No. 1, the numbers advancing as the hardness increases up to the point where they cease to be suitable for general foundry purposes, and are classed as forge-irons, being fit only for conversion into malleable iron in the puddling-furnace. The lack of fluidity common to the latter grades is a quality which makes them highly desirable for the puddling process, as in melting they pass, just before fusion is complete, into a pasty mass favorable for decarburization much easier and with less loss than could be possible if the pig-iron were gray and more fluid. See MALLEABLE IRON.