Produced until the beginning of this century by varying methods, in earlier times it was made by direct reduction without further intermediate working.
The oldest process is extraction in a bloomery fire [Rennfeuer]. This was a simple firepit with a bellows bag and a sloping hearth so arranged that the slag could settle downward. This technique has been known for 4000 years and continued in use long after the closed furnace of mortar came into use to provide better heat containment and fuel economy.
Through all of antiquity and into the Middle Ages the process changed hardly at all. The result of the extraction process was the loop, a spongy, inhomogeneous mass of two or three pounds, but in a condition that could be forged straightaway.
The loop was forged at a welding heat, first with wooden hammers and then with iron ones. Only in the Middle Ages, after the development of the water wheel, was it possible to build larger furnaces and thereby produce larger loops. Over time, as they became larger and exceeded a couple of hundred pounds, the loop came to be called a bloom [Maß] and could be worked under a hammer powered by water. It was hardly posible to test the small loops for quality. They had to be worked up directly. The larger blooms, on the other hand, were rather like an onion and were segregated into a "husk" of low grade, a layer of better soft iron, an inner layer of high grade tough steel and a "heart" of very hard steel.
Until the 18th century iron production continued to be primarily by this direct reduction process and it had to deal with two problems:
The first was quality. Hardenable steel was only to be had by means of carburization of soft iron or by separating the heart steel from the other layers in the bloom. This was only done to obtain material for the edges of tools or for cutting tools.
The second was quantity: To extract a bloom from the ore a complete blast furnace batch operation was required. The furnace had to be fired up, the smelt run and then the furnace cooled again before the bloom could be pried out and the furnace readied for the next run.
This process demanded an unbelievable quantity of charcoal and manhours. An alternative was urgently required!
Because of the size of the furnace, the temperature often went too high and, to the dismay of the operators, resulted in fully melted iron [cast iron, in our terms] that no one, at least at for a long time, knew how to forge. They had already learned how to improve the quality of the outer husk of the bloom by heating the bloom to a welding heat in the finery, a forge with a powerful air blast, and compressing it by hammering and this was eventually found to work for the melted iron, too. The air blast oxidized the excess carbon in the iron. The result was good, forgable iron. The era of the direct reduction process was thus past, the quantities could be increased, the charcoal consumption was reduced to a fraction and the forests stopped dissapearing at such a rate. In Austria, for example, all the direct process furnaces had been converted by 1763.
The quality problem, however, remained. The new technology was again producing Schweißeisen -- practically identical with the earlier product. The only question is why this iron shows longitudinal fibers, as if it had grown like a tree. In the early bloomeries and the later bloomery furnaces, as well as in the forge refining and puddling processes, the work was carried out at a relatively low heat so that the iron oozed and dribbled rather than melting. These dropets collected on the hearth of the forge or furnace into a more or less massive lump that had a rather doughy consistency when brought to the hammer. At a welding heat, the droplets fused together inside the mass but also elongated as it was forged. Thus they remained visible as longitudinal fibers. The whole process depended on the tendency of relatively pure iron to weld together so long as this property wasn't forfeitted by the introduction of foreign material. Chief among such materials are sulfur, phosphorus, silicon and excess amounts of carbon. We've seen in this review that the ancient smelting technologies were essentially processes purely of welding rather than of melting. The product was worked in several different ways that were, however, all effectively the same. If a piece was too small or of the wrong consistency, it was welded to another. We admire the results today in the splendid ironwork of the Romanesque, the grillwork of the Renaissance and the Baroque as well as in their tools and fittings and especially their huge anvils that were welded up from many little pieces.
Once again it was an outsider that made the breakthrough. In 1740, the clockmaker Benjamin Huubman produced the first crucible steel in industrial quantities and reliable quality. The chief factor in producing crucible steel is the recognition that the iron and the fuel must be kept separated, an insight that led to the development of the puddling process by Henry Cort in 1780.
Puddling replaced the finery forge and remained for a hundred years the method of choice for converting raw iron into forgable iron.
The discovery by Henry Bessemer that it was possible to decarburize liquid iron by blasting air through it was major sensation, not least because the temperarure of the melt could be maintained and increased without the use of fuel by the oxidation of the carbon in the iron. After several experiments, Bessemer presented his idea in a proposal in 1856 and was able to put into immdiate practice with some success. Some problems remained, however, due to the phosphorus content of the ore.
Sidney G. Thomas was the first to solve the phosphorus problem by means of a basic converter lining [2] and the addition of quantities of quicklime to the melt. At this point the metallurgy of melted iron was mature and from 1879 spread around the world.
The problems of quality and quantity were finally resolved. The new steel technology accelerated industry and civilization and was simultaneously one of the greatest experiments of mankind.