The concept of age of iron was used to the XIX ^E century by the archeologists to indicate a stage, succeeding the age of the stone or that of bronze according to the continents, in the general chronology of the history of humanity.

This concept applies to a level of technical development, and the archeologists endeavor to measure the consequences of the control of the metallurgy of iron on the history of the human groups.

The historians of the techniques, for their part, seek to give an account of the evolution of the processes by binding them to the changes of the companies.

The beginning of the age of iron is gone back to 1100 before J. - C. for the Mediterranean regions and of 700 before J. - C. for Jylland. If the archeologists continue to speak in certain cases about a first and a second age of iron, it is less to seize an evolution that to determine the disparities between certain areas, in reference, for example, with the archeological sites of Hallstatt (Austria) and of Tène (Swiss), from which one can define features of culture. Indeed, there exists, according to the places, of the ages of iron at different times.

The acquisition of the metallurgy of iron is a big step in the history of the companies, in particular by its repercussion on the agricultural techniques and its role in the installation of the powers. It is certain that the increase in the produced quantities of iron, the specialization which could result from it in the organization from the production had to notably influence the evolution of the powers and their distribution. But the methods of the intervention of iron in the political history are difficult to highlight.

The existence of a technical process appears not only by the direct traces which it leaves (objects, ingots, etc), but still by the vestiges of all that accompanies it. The metallurgy of iron supposes the installation of a circuit: ore and its extraction, the wood and its transformation into coal, materials of the furnace and its construction, the forging mill and its tools, and the know-how of the men, who leave “dumps”, witnesses of their habitat.

The mountain Holy-Cross (Poland)

In Poland, a team of archeologists studied, in the mountain Holy-Cross, nearly 2000 groups of furnaces, dating from thousand-year-old Ier of our era (of Ier front century J. - C. in VIIIe century apr. J. - C.). The archeologists showed that most these groups of furnaces formed an organized complex and corresponded to a rationalized production. Starting from the excavations, studies of the material and a work of experimentation, they tried to reconstitute the production and led to the following results: for furnaces whose crucible is of 45 cm approximately of diameter, for a 40 cm depth, and whose chimney measures 70 cm roughly, the block of slags found in excavation 100 kg border. To obtain such a quantity of slags, it is necessary to treat approximately 170 kg of ore, whose reduction requires nearly 300 kg of charcoal, corresponding to five or six steres. The reduction produces then from 15 to 18 kg of iron in the form of magnifying glass nut.

The division of the labor

The construction of the furnace occupied three men during three days; the crushing of the charcoal (cut and coal mining not understood), the netting of the ore and its crushing, the preparation of the tools, the reduction (with a continuous blowing by the service of two bellows) and shingling represented three workdays for five or six men. If one adds to this number the team of coal mining, it is necessary to estimate manpower for a group of furnaces at ten or eleven people. The conclusions lean, in this case, towards the assumption of a technical division of resting work on a thorough specialization of the men.

It is still difficult to today compare these observations, however already old, with those done on sites existing in other areas of the world and going back to different times, because this kind of work notably did not multiply. However, certain studies on African sites (the such area of Bassar in Togo, or Grassfields of Cameroun) lead to relatively close conclusions.

Iron is one of the most abundant elements among those which constitute the Earth: it represents 5 % of the weight of the Earth's crust. The first ore deposits started to be constituted there is billion years, when the terrestrial atmosphere became oxidizing, in the form of lists with ribboned iron.

Types of ores

The native ore is very rare, and exploited iron is a mixture in variable quantities of chemical compounds - among which one distinguishes oxides hydrated or not, carbonates, sulfides and silicates - and other elements.

If the content of iron of an ore were regarded a long time as essential in the success and the profitability of the operations of reduction, it is known today that the associated elements play a very important part. For example, sulfur associated with iron is regarded as the “enemy of the metallurgists”, because the product obtained is breakable; manganese, on the other hand, makes it possible to fix carbon and thus to obtain natural steels; phosphorus in small quantity supports weldability; silica facilitates the operations of reduction. The importance of the content of iron of an exploited ore is thus relative, which archeology highlights when it is about old metallurgy: the selection of the ores is the mark of a great technical skill.

Certain ores require a preparation - by washing, crushing, netting - according to their specific characteristics. Crushing, for example, equalizes the size of the fragments and the operation of netting (chemical reaction to high temperature) is necessary for a sulfurous ore.

Types of layers

The iron ore is in two great types of geological formations: magmatic formations and sedimentary formations. The mineral-bearing seams are exploited in general underground, whereas the sedimentary clusters are exploited in career. Beside the lists slickenside, resulting from the magmatism, the sedimentary lists were formed at the not very major marine resources. It is, in France, the case of the Lorraine ores, which were exploited up to one recent period. Lastly, there exist lists of surface origin, where the ore is concentrated in layers of surface of type the karstic, lateritic (crust) or lake. One finds some for example in Mali, in a lake zone where the ore concentrates at the bottom of ponds.

Concept of profitability

One understands today by layer a site on which the ore concentration is important and whose exploitation is a profitable economic enterprise, taking into account the infrastructure necessary to its implementation.

The content of iron of the ores, the elements associated as well as the economic context with manufacture of metal are as many data taken into account to determine the profitability of a layer. The size of the layer has little incidence on the choice of its exploitation in a context of production to small scales. Small layers were thus exploited, before the industrial revolution, in Europe and Africa. The quantity of ores with low content which was exploited also pleads in favor of a cost-benefit analysis where the content is put in keeping with the facility of reduction and with the quality of irons, variable according to the ores used. There does not seem to be in the history of real adequacy between size and quality of a layer, on the one hand, and development of a more important production of iron, on the other hand. Thus, in pre-Columbian America, there exist important iron ore layers but not old metallurgy. In the same way, certain zones of concentration on lateritic grounds, considered today as too poor for the exploitation, were at certain periods of the places where the production was intense.

In Antiquity and with the Middle Ages, it is hardly of areas of current France, except the plains and the plates covered with silts, which did not produce ore, in more or less great quantity. Sometimes of all small layers of a few hundred cubic meters were emptied by the men.

Iron, in general, is extracted from ground in the form of oxide ferric (Fe2O3). To obtain forgeable metal, it is necessary to separate iron from oxygen: this separation, called reduction, is done by carbon contribution thanks to a treatment higher temperature with 1000 °C. The two following processes of reduction are known.

The direct process

One obtains iron starting from the ore at the conclusion of one only technical operation. The reduction is done at a temperature lower than 1536°C, and the carbon contribution is weak. The iron magnifying glass thus obtained is ready to be forged with the workshop. The fuel used is the charcoal. This technique is the first known everywhere in the world. It makes it possible to as well produce soft iron, ductile but not very hard, that steel. Natural steel contains a small quantity of carbon. It is more rigid than iron.

The indirect process

It consists of a reduction of the ore to an higher temperature with 1536°C. There is cast iron of the ore, with important carbon contribution. The reduction is done in a blast furnace supplied with pit coal, or coke. One obtains what one calls of the cast iron, produces that one cannot forge because it contains much carbon and is breakable. To obtain iron starting from the cast iron, a refining still should be operated (to decarbonize it), this is why one speaks about indirect process.

The indirect process appears in China as of the VI ^E century, in Europe to the XV ^E century; it does not seem to be known in Africa before the XIX ^E century.

The forging mill

The magnifying glass of reduction is nut: the workmen hammer it to eliminate slags and the impurities, and to expurgate the bubbles formed by the gas pockets. This first forging, generally makes near the places of reduction, gives a metal mass which will be then transformed into end products. This second phase of the direct process still consumes calorific energy, but also much of mechanical energy.

After the shingling of the magnifying glass working comes from the objects: this one is done with the forging mill. Workshop of forging mill understands necessarily hearth, anvil, which can be out of stone or metal and which evolves in time to a better specialization, and a tool to hammer the iron, which can be him also out of stone or metal and which one also sees diversifying in time (specialized masses, hammers, grips, files, cutters).

Furnaces

One knows several types of furnaces, or low hearths, allowing the reduction by direct process.

The open hearth furnace is a simple hole in which one piles up by superimposed layers the ore and fuel. It does not have a chimney and one can reduce only one minor amount of ore, because fire is likely to be choked.

In the shaft furnace, such those used on the mountain Holy-Cross in Poland, pulling is artificial (using bellows). A tank is arranged which receives cast slag while the iron magnifying glass remains on the surface.

Furnaces with evacuation of slags, sometimes still upright, could be observed in Mali or Burkina Faso. Pulling is natural, a large number of conduits being distributed around the base of the chimney. A pit is arranged for the evacuation of the slag run in the course of reduction, which makes it possible to increase production capacity.

The Catalan furnace, which was still used in the Pyrenees at the beginning of the XX ^E century, has as a characteristic to be charged so that ore and fuel are vertically juxtaposed from one edge to another of the tank starting from the side conduit. Some were equipped with hydraulic bellows.

One can find at the same time of the different furnaces in service for precise uses: ones for a large reduction (annual production), others rather reserved for occasional productions.

Evolution of the direct process

The direct process evolves differently according to whether one seeks to increase the productivity by limiting painfulness and labor or to specialize the production, according to whether one tries to limit the fuel consumption or to exploit richer layers.

Evolution of the bellows

The bellows with angular displacement and automatic valve, such as one uses it today for our domestic homes, are known as of Roman Europe. The development of the hydraulic power starting from the XII ^E century led to the first mechanization with the hydraulic hammer, or trip hammer. With a latter date, still badly determined, the bellows, them, were also driven by the force of water, which led to a strong rise in temperature and leads to the cast iron of iron to 1536 °C, stage of the indirect process.

In Africa

The African continent, knows to him an evolution of the direct process to the XX ^E century, time to which colonization massively introduces the use of iron of recovery, which replaces little by little metal produced on the spot. The technique of direct reduction diversifies so much so that, in Burkina Faso, one could show the cohabitation of a massive production in large furnaces and of a quasi domestic production, more limited much and perhaps intended for repairs.

China and Europe

The history of the direct reduction of iron yields the place to that of the cast iron and the production of steel to very different periods according to the areas; thus, China knows the cast iron since 512 before J. - C, and the process of innovation intervened then is still badly explained: it is perhaps due to the coal-burning of ground, with the invention of bellows with cylinders and pistons. Europe, as for it, gives up the direct process to the XV ^E century, time to which the first blast furnaces are built.

The privileged field of the development of the production of objects out of iron - and place of the latter in the economy signs it - is agriculture, where, after a use of iron to arm the sharp part of the tool, appears little by little, then spreads, the manufacture of tools entirely out of iron. The marker of an increase in the production of metal often retained by the archeologists is the diversification of the tools produced. But one of course produces also objects for the armament or the ornament, and this sometimes earlier than for agriculture. Naval construction, the iron work, the clock industry call upon the forging.

Uses of iron

In Europe, in the neighborhoods of the XII ^E century, the sources attest the more frequent use of metal. The rise of the campaigns is related to the use of iron: manufacture of plow whose plowshare and coulter are out of iron, shovels, axes and saws to clear, horseshoes. To this request of the campaigns is added that of the aristocrats for the armament (swords and coats of mail). But the cities under development consume also an important quantity of metal for constructions (nails, chainings, etc), the tools of the craftsmen. The Gothic architecture was large iron consumer. To ensure the solidity of the buildings, with the increasingly hollowed out walls, the architects had recourse to iron chainings. The Ste Chapelle, for example, is surrounded by three metal belts.

The study of many old objects of Europe or Africa reveals the forging of composite objects with steel welding on soft iron, in general to ensure a better resistance of the sharp part of the tools. It is thus long Merovingian swords, entries in the legend with Durandal, of the knives, the points of minors of the XIV ^E European century, and even of many hoes of farmers.

Reduction or cementing

Starting from pure iron, produces more running of direct reduction, the blacksmiths could manufacture tools and sharp weapons by primarily using two methods to harden metal: the specific ore reduction, in particular those containing of manganese, to produce natural steel, or the cementing of iron by carbon with the forging mill. In this last case, the iron part, placed in charcoal at high temperature, is left there during hours. The operation is slow and delicate, this is why steel remained a long time, in most areas of the world, rarer than iron. It is by cementing that the scramasaxes (daggers) Merovingian were manufactured. The steel obtained by cementing can in addition undergo the hardening, which still confers a greater hardness to him.

Hydraulic power

Before the appearance in Europe of the indirect process, an important innovation allowed, starting from XIIIe century, a very strong increase in the production: the trip hammer. This hydraulic hammer is at the origin of the mechanization of the operations of forging mill and upset the conditions of production: mechanics replaces the man and the productivity increases. The iron and steel plant (low hearth and forging mill), which could up to that point be itinerant, is fixed close to a water arrival. These new installations require the mobilization of capital. Iron becomes then in an obvious way the stake of economic conflicts. The development, in Europe, of the hydraulic energy utilization in the iron and steel industry led to the adoption of the indirect process and the industrialization of the production process which we know today.

Scrap

The techniques of production by the direct process made iron a expensive product, in particular before the mechanization of the forging mill. Also this metal it often was recovered and reforged. The parts were welded, i.e. deformed and welded with the forging mill. Practically all the excavations of forging mill showed that the craftsmen had a stock of scrap intended for the re-employment. The metallographic study reveals on many objects of the traces of this re-employment: the object is not homogeneous and marks of the old parts are found.

The development of the metallurgy in China is incomparable with that which India, the Islamic world, Europe or the African continent knew.

Copper with steel

The historical transition in the use of metals seems to be directly made copper alloys with steel. For example, in the vocabulary, one does not indicate iron under his pasty state (result of the direct process) but under a liquid state (stage of the indirect process). One found objects whose core is out of iron, covered of a copper alloy: copper alloy knives whose heart of the handle is out of iron, arrowheads at end out of copper alloys and iron stem.

The first mentions of iron found in the Chinese texts date from the VIII ^E century before J. - C. approximately. To the III ^E century before J. - C., a text of Zuozhuan clearly announces the moulding of iron cauldrons.

Cast iron tools exhumed of the tombs of Huixian, in Henan, date from the end of the IVe-beginning of IIIe century before J. - C. These objects are preserved at the museum of the Palate in Beijing. The Chinese thus managed to exceed in their furnaces the temperature of 1560 °C, probably by burning pit coal. They also developed the piston blowing engine.

Advanced techniques

In the last half of IIIe century before J. - C., of the groups of contractors manage to pile up considerable fortunes by organizing a production on a quasi industrial scale. Some of them acquire a dominating place in the life of the country, thanks to the control which they exert on the industry of iron. One quotes the example of Zhao thus, deportee in Sichuan, where it founds a manufacturing plant of iron employing approximately 1000 men. From all this period, there remains also an interesting iconography.

One found traces of furnaces from 120 to 150 cm in diameter manufactured in large refractory bricks from 45 to 90 cm broad. The excavations of the foundry of Zhengzhou, in Henan, setting at the day in 1953, brought invaluable information on the techniques implemented. The site, wide on more than 2000 m2, revealed the existence of a complete workshop of production of Shang time. Eighteen furnaces among which some were used to reduce the ore and others to run the cast iron -, a forging mill and surfaces of ore processing were recognized there. Chinese technology as regards iron and steel industry was thus in advance of several centuries on the Western techniques.