The best results for strength and elasticity are obtained by mixing a number of carefully selected kinds of iron, as such a mixture gives higher tensile strength than the average strength of the different samples cast separately. When all the carbon in iron is in a combined state, the fracture of a freshly broken piece has a silvery white color. When only a little carbon is combined, most of the carbon particles crystallizing separately, the fracture is gray in color, and the iron is weaker and more fusible. Hence the founder must exercise good judgment in selecting suitable mixtures of different sorts and qualities to obtain the desired strength, softness, hardness, toughness, and clearness of grain for different castings.
Fig. 186. - Drawing Iron from Cupola. When the iron has been melted down in the furnace it is drawn from the cupola into large ladles which are picked up by the overhead traveling cranes and taken to the flasks where later it is poured.