Although foundry iron is now selected by its composition as shown by chemical analysis, yet the designations of white, mottled, and grey irons, are still used to classify pig iron according to the carbon it contains. White iron, with no uncombined carbon, is too hard for castings which must be machined. Grey iron, with much uncombined carbon, is easiest to melt and easiest to cut. A mottled iron is strong, tough and not difficult to machine.

Cast iron containing much free carbon expands upon cooling, due to separating out of graphite from solution. This is an advantage in casting, as it balances the contraction due to cooling.

Scrap iron must not be used in high-grade castings unless it comes from similar castings.

Fig. 134. - Brass Melting Furnace.