Facing
Foundry facing, employed for intimately mixing with the facing-sand, or spreading upon the surface of moulds, to prevent the molten metal from penetrating the sand, as well as to impart a smooth, fine skin to the surface of the casting. When properly used it is an unmistakably good help in making smooth and clean castings, profitable alike to the moulder and the employer. Facings are composed of finely ground and bolted fire-proof substances, principally carbon, as charred wood, coal, graphite, etc. They are now so well understood by the manufacturers that, by making application and stating for what description of casting it is to be used, founders can be supplied with a facing exactly suitable. See BLACKING-BAG; SEA-COAL; GRAPHITE; FACING-SAND. Facing-sand is the sand used for facing or covering the surface of the mould, and necessarily that with which the metal is brought into immediate contact when the casting is poured. Sand for this purpose should possess properties that will enable it to resist pressure and heat from the molten metal as well as permit free and uninterrupted egress to the gases which are generated. Hitherto this subject has been left to mere chance, trying first one sand and then another, with the usual loss and disappointment which attend such methods, until the right substance has at last been found. It is now positively known just what sand will give a clean casting, free from adhering sand. To be a positively good moulding-sand it must contain no substance that will act chemically upon the molten metal, nor should the high temperature of the metal affect it adversely. The difficulty in meeting these conditions is apparent when we consider that the higher the temperature of molten metal the fewer become the substances that will successfully resist it. Three per cent of metallic oxides in sand seriously diminishes its refractory qualities, and one per cent of lime present measurably lessens its value as a good moulding-sand, as the carbonate is acted upon by the intense heat and caused to give off carbonic-acid gas, which disturbs the surface of the mould during its escape, causing honeycombed and rough surfaces on the castings. Should caustic lime be present, its fluxing properties will manifest themselves by melting into the form of a slag and adhering to the surface of the casting.
Sands which contain the largest proportion of silica, from one to three per cent of magnesia, with as much alumina as will impart cohesiveness and plasticity, are under almost all circumstances the best for facing-sand. Lime should not be present in even the smallest proportion. It is seldom that sand with the above proportions can be found in nature, but a chemical knowledge of these matters enables us to choose such grades as will, by suitable admixture and blending of two or more kinds, produce a mixture containing known proportions of the elements necessary for meeting all emergencies. Facing-sand is termed strong or weak according to the amount of alumina or clay which enters into its composition; and the same term is applied with reference to a high or low percentage of coal used in the mixture. See ANALYSIS; VENTING; DRY-SAND FACING; OLD-SAND; ROCK-SAND.