Charcoal and the General Steel Making Process

When one thinks of charcoal, we normally think of the most important historical use of wood charcoal; as the main ingredient in gunpowder. However, it was of major importance in metallurgical operations as a reducing agent, primarily in the manufacture of steel. The most common use in the modern era for charcoal is as a fuel source, but still has industrial application. The Charcoal Hearth Process was vitally important in the early industrial steel age.

Charcoal Hearth Processes were sometimes, but rarely, called forge processes, and bustling and buzzing are very obsolete terms. These processes are designed for the production of wrought iron (charcoal iron), usually by refining cast iron, occasionally by melting scrap before a tuyere with charcoal for fuel, the product being obtained in a pasty condition, and containing a certain amount of slag, but less than puddled iron. Only the latter method survived the early 20th century in this country.

In general the furnaces or hearths are like the Catalan and bloomary hearths for reducing iron from the ore, low, rectangular chambers, sometimes roofed, and with one or more tuyeres. The chief difference is that in refining cast iron much more strongly oxidizing conditions are brought about, chiefly by melting the metal down in drops before the tuyere, repeatedly, if need be, so that it passes in a state of minute subdivision and with great surface exposure through a part of the hearth where the atmospheric oxygen is in excess; and by the action of the basic ferruginous slag with which the metal is mixed during the earlier stages, and with which it is covered during the later stages, to ward off the strongly carburizing tendency of the charcoal.

Only a good quality of pig iron (nearly always charcoal pig) is used, as the process is expensive, and is employed only for a high grade of wrought iron. The pig iron is often given a preliminary refining to eliminate most of the silicon. The hearths are usually built of unlined cast-iron plates, at least in part water-cooled. Brick-work is avoided as the silica would enter the slag. The processes may be classified according to the number of times the metal is melted down before the tuyere, into single melting, double melting, and triple melting (or German or breaking up); into Walloon and non-Walloon.

Hydraulic shear