In these furnaces which consisted mainly of a shallow hearth filled with glowing charcoal, with tuyères for blast supply, the pigs of iron were heated until molten. At high temperatures the metalloids of pig iron will readily burn in a blast of air. After the air blown over and upon the molten metal has burned out most of the carbon, the main metalloid, the melting point of the purified metal becomes so much higher that the heat of the furnace is not sufficient to keep it molten. It becomes more and more pasty and stiff therefore. This is what the iron maker calls “coming to nature.” It is the signal that the carbon is about gone and that he must be careful or the iron will suffer in quality. At this stage he sees to it that the mass of pasty iron is well protected by glowing charcoal until it is removed to be hammered or worked into a bar.
This in general is the process which was used from about the fourteenth century up to 1783 when Cort invented the reverberatory furnace which has since been the type generally used.
In the processes just referred to, all very similar, it should be noted that the iron was in contact with the fuel, which, therefore, had to be charcoal, the fuel with little or no sulphur.
Cort’s Reverberatory Furnace
This is the type of furnace generally used to-day.
Charcoal was an expensive fuel, and, moreover, there was an insufficient supply because of the great destruction of forests necessary for its production. Naturally the thing to do was to substitute coal, which was plentiful. But the sulphur of the coal spoiled the iron.
This proved to be a great barrier until 1783 when Henry Cort of England succeeded in making wrought iron in a new type of furnace wherein the iron was refined in one compartment while the fuel was made to furnish its heat from another; i. e., the fuel and the metal were not in contact and the refined metal did not suffer from the sulphur content of the coal.
The figure illustrating Cort’s furnace plainly shows how this was accomplished and it represents almost as well the furnace used to-day. In the one-hundred and thirty-three years that have elapsed since it was designed, his furnace has been changed only in certain details, and but two important changes have been made in his process—the use of an iron bottom instead of the sand bed by S. B. Rogers in 1804 and the introduction of “pig boiling” by Joseph Hall in 1830.
Cort’s process is known as “dry puddling” and his trouble was the excessive loss of iron, due to his use of the sand bottom and the absence of a proper cinder. The loss was said to have been from 50% to 70% of the iron charged; i. e., it took about 2 tons of pig iron to make one ton of wrought iron. Because of the great demand for iron and the fact that he was using such a cheap fuel, coal, his process at that time was a success financially.
