It is matter of general knowledge that pig-iron is made from iron ore in a blast or smelting furnace; that to secure this product the furnace is charged, first, with a layer of coke or charcoal, then with a layer of iron ore, mixed with broken limestone, and so on in alternate layers until the proper quantity of these materials is placed in the furnace. The fuel is then ignited, and, for the purpose of increasing the heat, streams of air are forced into the furnace by means of blast-pipes, the nozzles of which, called tuyeres, are inserted in openings in the walls of the furnace, usually from four to six feet above its bettom.
The limestone is used merely as a flux. The ore under this process undergoes a chemical change, and iron is formed and sinks in a molten state to the bottom of the hearth, by which is meant not only the bottom of the furnace, but its sides as high up as the foot of the boshes. The refuse left after the melted iron has dropped into the hearth is also in a molten state, and, being lighter than the iron, floats on its top. This is indifferently called 'cinder' and 'slag.' About three or four times in every twenty-four hours the melted iron is drawn from the furnace. This is accomplished in the following manner: The furnace is constructed with two holes, one called the iron and the other the cinder notch. The iron notch is made at the bottom of the hearth. The cinder notch is higher up the side of the furnace, just below the level of the tuyeres; so high that the cinder can be drawn through it without letting off the molten iron. These holes are kept habitually closed with clay or other similar material. At frequent intervals, and always just before drawing off the molten iron, or making a cast, as the ironmongers call it, the cinder notch is opened, and the cinder or slag is allowed to escape, and is carried away from the furnace in a trough made of moistened sand. The cinder notch is then closed and the iron notch is opened, and the molten iron is drawn off through a sand-trough, and conducted into moulds made in sand-beds, called the sow and pigs, where it is allowed to cool. The result is the pig-iron of commerce.
In the mean time, the furnace is supplied with constant charges of fuel and ore, mixed with limestone, in alternate layers, dumped in from the top; and this process is kept up without cessation for months and sometimes for years.
The sand-trough which connects the pig-beds with the iron notch is usually larger and deeper, but more elevated than the sow or general gutter which conducts the iron into the moulds or grooves in the pig-beds. When the metal is first let into the trough it accumulates so as to fill it nearly to the brim. As the flow from the iron notch decreases, the iron, and a small quantity of cinder or slag, which has been chilled by coming in contact with the cold surface of the trough, adhere to its sides and bottom. When the molten iron on the hearth is about exhausted, the blast is increased, and the material left on the hearth is blown out through the iron notch into the sand-trough. This also cools in the trough, and thus is formed what are known as trog h runners, consisting of iron and slag, which have been forced through the iron notch by letting on the blast as just mentioned.
A cupola furnace is one used for melting pig-iron for the purpose of casting it into useful forms and articles. It constitutes part of the equipment of a foundry. In shape it is generally a hollow cylinder. The iron is melted by substantially the same process as the ore in a blast furnace. The cupola furnace has an iron notch but no cinder notch, because there is generally so little cinder or slag in pig-iron, as to render such an opening unnecessary.
In order to reach the merits of the controversy, it is necessary to obtain a definite idea of what, if anything, the appellants are entitled to under Vinton's patent.
The specifications are ambiguous in respect to the particular kind of slag which is to be used in the process therein described, that is to say, whether it is the slag drawn off through the cinder notch, or the runners which are left in the trough through which the molten iron is discharged from the iron notch of a blast furnace. It appears, however, from the evidence that the use of the latter only is contemplated, the former containing such a very inconsiderable quantity of iron as to be valueless.
We observe, in the first place, that the patent cannot be held to cover the discovery that the slag, which is to be used in the process described in the specifications, contains so large a percentage of good metallic iron that it can be profitably extracted by again smelting it.