MIXING AND MELTING SCRAP IRON. 297
a little ferro-manganese to the molten metal. This will greatly combine with the oxide and come to the surface as slag, which can be skimmed off. Oxide of iron combines readily with silica, and for this reason when there is any rust on scrap, or old iron, it is often desirable to have some sand (which is silica) on pig iron, that it may be charged with the scrap iron to assist in forming a slag to be carried off by fluxing. This will greatly absorb the oxide and give a cleaner iron for pouring castings.
The oxide of iron caused by the oxidation created by the blast, in the case of strictly clean iron, may at times be insufficient for the amount of sand on pig iron, etc., to form the right combination for making a good fusible, or thin slag, to carry off the ash of the fuel and other dirt out of the cupola. In such cases an addition of rusty scrap, etc., may sometimes work well. However, it would be better to add limestone or other flux to make a fusible slag than to increase the oxide of iron or rust, etc., in a cupola. In cases of excessive oxide of iron being present, it is absolutely necessary to use limestone or other flux in order to make a good slag. It is claimed that high cupolas may have a reducing action on oxide of iron, so as to obtain more metal from rusty scrap, etc., than low cupolas. High cupolas should at least cause a greater loosening than low cupolas of the scale from iron, and often permit more of it being blown out of the stack to remove some of its evils. However, in striving to obtain very soft or clean castings, rusty or burnt scrap of all kinds is best avoided where practical.stings as they are poured. Where there is any apprehension of such difficulty, it is often well to addo break, he can, by " siz-