Slag
Cupola slag is sometimes called "scoria" or "cinder," and consists of the fused compounds of the silica and alumina in the lining, daubing, and dirt which are too frequently thrown in among the iron and fuel. Impure fuel also adds its quota to this readily fusible mass, in addition to that given off by the metal itself. If, on account of burnt or dirty iron being used, it be deemed necessary to employ limestone or anything else as a flux, the quantity of slag is augmented considerably, and means must be provided for conveying some portion of it away, otherwise it will prove a serious detriment to the effective action of the cupola, as at every rise of the metal in the bottom this superfluous slag is forced upwards among the fuel in the immediate vicinity of the tuyeres, where by the action of the blast it is converted into an impenetrable mass, through which ultimately no air at all can be forced. Another evil attendant upon this over-abundance of slag is that it must inevitably make its appearance at the tap-hole on the instant of the issuing stream failing to completely fill the orifice, defiling ladles, and everything else it comes in contact with; making it necessary sometimes to clear all away, and permit its being blown out at the tap-Lole into the pit below. An effective remedy for this is to copy the tymp and dam stone of the smelting-furnace, and, like the smelters, allow this slag to flow away, while the molten iron is allowed to collect comparatively clean on the sand-bed below.
By making a large tap-hole some distance below the tuyeres, at a convenient part back of the cupola, a spout may be attached for leading the liquid slag away. This hole is to be prepared somewhat after the manner employed for the tap-hole, and kept securely plugged when not required. When it is thought advisable to flow off superfluous slag, allow the molten iron to rise in the bottom until the slag makes its appearance at the hole, when the clay plug may be taken out, and it will at once issue forth. If this operation be conducted in a proper manner, and repeated from time to time, there need be no trouble from this source at the spout; and if due attention is paid to the tuyeres (see TUYERES), the duration of a heat in the cupola may be prolonged indefinitely.
When clean pig iron, of good ordinary quality, along with pure cast scraps, free from sand and rust, is melted with coal or coke comparatively free from impurities; and when the operation is conducted according to the best rules for practice, in a well-kept cupola supplied with blast at an adequate pressure for perfect combustion, and no more; and allowing that the melting is not protracted beyond a reasonable time--there is really no need for a flux; and as, under the conditions stated, little or no slag would be likely to gather, it is plain that the need of slagging cupolas will only occur in proportion as such conditions fail of being met. See CUPOLA; CHARGE; RATIO OF FUEL TO IRON; SCRAP; BUGS.
Slag on the Surface of Castings.--See FACING-SAND; SEA-WATER.