By about 300 b.c., iron production was common throughout Europe. The abundance of iron ore, however, was offset by the limitations of the bloomery process through which iron was produced. Furnace temperatures could not reach iron's relatively high melting point. When iron ore was smelted, the iron was reduced to metal in the solid state, leaving a spongy mass (called the sponge or bloom) with slag still trapped in pores. A smith reheated the bloom in a forge to soften the metal and liquefy any trapped slag and then hammered it repeatedly to force out as much slag as possible while shaping the iron into ingots or finished forms. The wrought iron so produced was relatively pure and therefore not very hard. The smiths learned that they could harden the iron by placing it in the forge in contact with organic materials. It is now known that this technique, called case hardening, works by introducing carbon into the surface of the iron, converting it to steel. The process was labor-intensive and difficult to control. Furthermore, a great deal of fuel—charcoal, produced from wood—was needed for both smelting and forging. Although wood was readily available in barbarian Europe, procuring the wood represented another labor-intensive step in production.
Ironworking in this early era was carried out in many settlements of various sizes. The level of production was small-scale, the political economy had to support a full-time specialist, and the quality of the product could not always be assured. As a result, iron was used primarily for weapons, funerary goods, and other items with a strong political and social component and only to a very limited extent for agricultural tools.
The nature of iron production began to change with the rise of urbanism in Late Iron Age Europe. After about 200 b.c., large, complex settlements began to emerge in specific areas of Europe. These oppida were based in part on long-distance trade with the Roman world as well as control of local political, social, and economic networks. Evidence of large-scale iron production occurs on most of these sites, and some even appear to have specialized in iron production. Several well-excavated oppida in Bavaria, such as Manching and Kelheim, have provided evidence of every facet of ironworking, from mining through forging, and the analysis of the finds from these sites confirms the view of site specialization and of trade with Rome. The Roman need for iron may have led at least in part to this urban phenomenon. In any event, the formation of large centers with higher population densities and greater social differentiation and specialization certainly allowed and encouraged the support of large-scale iron production, which in turn made iron more important to the economy. Not only do a wider variety of tools and weapons of iron appear, but evidence also includes the appearance of iron bars that seem to have been used as a kind of currency. The use of the iron plowshare almost certainly had a major impact on the rest of the economy. Ironworking also continued to be carried out on the smaller settlements, although their economic relationship to the centers is not clear.
In addition to the changes in the quantity of iron, there were qualitative changes as well. First, the simple shaft furnaces were replaced by slightly more-advanced domed furnaces, which did not create much greater temperatures but were more consistent and had larger capacity. Archaeometallurgical analyses from many parts of Europe have shown that the smiths learned that steel could be reheated and quenched to produce an even harder substance and that the resulting quench-hardened steel could be reheated to achieve a balance between hardness and toughness. This technique was not known in the Early Iron Age and would not have been obvious to early metalworkers because it does not work on other metals such as bronze. The smiths also learned how to weld a steel edge onto a soft iron back without accidentally decarburizing—removing the carbon from—the steel, a difficult process that leads to a superior tool or weapon. Various finds of smiths' tools also attest to the range of techniques available to them. They did not, however, learn to "pile" steel by alternating thin layers of iron and steel, as was done in the Classical world.
There is some debate as to what extent the smiths of the barbarian world developed these techniques independently owing to their long experience with iron and to what extent the technology diffused from the classical world. On the one hand, at the time of the Celtic invasions of Italy in the third century b.c., classical sources make reference to the inferior nature of the barbarians' swords. On the other hand, by the second century b.c., the sources speak of the outstanding quality of the steel from Celtic Iberia. After the Roman conquest of central and western Europe, Noricum—now the province of Carinthia in the Austrian Alps—became the major steel supplier for the empire.
The situation of barbarian iron production outside the Roman limes after the Roman conquest until the fall of the empire was a mixed one. Some areas, such as the Holy Cross Mountains in Poland, continued to specialize in and produce large quantities of iron for local consumption and trade with Rome. Other areas underwent a decentralization and technical regression. Still others, such as Ireland and Scandinavia, which had originally been outside the zone of increased and improved iron production, gradually developed their own industries, probably under the influence of their trading and raiding relationships with Roman territories. It is safe to say that, after the fall of the Roman Empire, the barbarian world was everywhere an iron-based economy but one that depended on relatively basic techniques and somewhat decentralized production.
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Michael N. Geselowitz