ACTION WITHIN THE BLAST-FURNACE—FUSION.
The contents in the boshes and near the hearth, being raised to an intense heat by the combustion of the fuel, are reduced to a pasty condition; the limestone parts with its carbonic acid some distance above its point of fusion, but the lime is more readily fused than the ore, and commences to flow earlier than the earthy ingredients of the ore; with these, however, it combines, aids their fusion, and forms with them a liquid slag or cinder. This flows from the lower part of the boshes to the hearth, while the oxidized ore, partially free from its silicious or earthy parts, on arriving before the blast is reduced to a liquid metal and falls through the cinder in drops or small streams. Any remaining earthy matter is taken up by the flux during the passage of the iron through it. The metal, being heavy, remains in the bottom of the hearth, while the flux or slag, being lighter, floats on the top of the iron.
The cinder is allowed to flow almost constantly from the top of the hearth, while the metal is tapped off occasionally, or about once every twelve hours.
The slag indicates by its appearance the manner in which the furnace is working. Thus, if the cinder is liquid, nearly transparent, or of a light-grayish color, and has a fracture like limestone, a favorable state of the furnace is indicated. Tints of yellow, blue, or green show that the furnace is working cold. A deep brown or black color indicates that the supply of fuel is not sufficient to deoxidize the ore, or that the ore and coal have not been in contact a sufficient length of time for the purposes of deoxidiza tion and carbonization.