Making an object indicates the ability to know how to produce an artefact that does not exist in nature, always providing the same result in the best possible way with procedures already tested through the simplest operations of acquisition and transformation of natural materials.
The starting point for the production of a metal artefact originates in the identification of the deposit and the organization of the mineral extraction.
Once the material was identified, a series of procedures were then initiated: the first is that related with the extraction processes of metals from minerals which include a sequence of mechanical and chemical operations to be obtained through different mining and metallurgical technologies used depending on the knowledge in a given era and in a specific social context; furthermore, the more important the deposit is, the more difficult it is to date the evidence while taking into account the broad chronological period of its use.
Once the extraction had been carried out – the oldest technique of which involved following the metalliferous mineralizations that emerged on the surface by digging open-air quarries, which often wound in a series of tunnels, and to follow the metalliferous veins that extended into its depths – it was therefore from the phase that refers more specifically to mining technologies we move on to that of metallurgical technologies.
The minerals were therefore subjected to the actual metallurgical process through the enrichment of the “metalliferous” component before being sent to the reduction process. The concentration of other elements, mainly associated with the mineral, called gangue, affect the yield, the extraction time, the necessary temperature and the process to transform the mineral into metal, which is why the mineral was sent to the smelting furnaces, as far as possible, after it had been separated from the gangue.
The minerals were subjected to a crushing process and the pieces obtained were selected taking into account their colour and weight; generally the fragments were then finely ground and the proceeds were subjected to washing, in order to separate the metal from the rock residues by exploiting the difference in specific weight, using for this purpose a stream of water through which the heavier metallic mineral was deposited, while the sterile was removed. An interesting system of washers is the one brought to light in Greece, in the Laurion mines where a hydrodynamic current was used.
During the roasting of the mineral, which occurred only in the case of sulphides, temperatures between approximately 500°C and 800°C were reached in open installations with natural ventilation; it was necessary to arrange the mineral, for example, on stacks of wood in the presence of air, this procedure served to eliminate the crystallization water, carbon, chlorine and sulphur, to transform chlorides and sulphides into oxides, to make the mineral more crumbly, and therefore divisible into minimal portions in order to make it more reactive in reduction. Roasting allowed the metal to be oxidised, eliminating sulphide and volatile elements such as arsenic, antimony and bismuth. The oxide thus obtained was sent to the extraction process. Oxide and carbonates, on the other hand, could be sent directly to the extraction phases.
The use of the furnace allowed the extraction of the mineral and its transformation into metal. The metal’s melting point could only be obtained through the use of a furnace, through appropriate draft.
In the case of copper minerals, the temperature can be around 800°C, even if pure copper melts at 1083°C. Carbon in the form of charcoal or wood was the most used reducing agent in ancient metallurgy, the chemical reaction between carbon monoxide at a stable temperature and the oxides from which oxygen was removed led to the primary production of all metals used in ancient times and, secondarily, for the production of gas and slag containing waste materials.
Reaction scheme:
Mineral, flux and fuel; Type of furnace; Operating mode; Temperature; Slag composition; Viscosity; Slag/metal separation; Metal produced.
The absence of oxygen is necessary, which can be obtained by mixing and covering the mineral with coal, so to create a reducing atmosphere. The oxygen bound to the metal in the mineral combines with CO (carbon monoxide) emitted by the coal: this forms CO2 (metal and carbon dioxide) which, being in a gaseous state, is dispersed in the air. Where it was not possible to remove unwanted material with enrichment, it was necessary to add thinning or slagging substances; in the very frequent case of quartz gangues the fluidifying agent was made up of iron oxides. The furnace for extracting metal from ore required a place where to introduce the right proportions of raw metal and fuel, and to have a limited heat dispersion. The suitable place consisted of a room where the chemical transformation took place to convert the concentrated ore into metal.
Therefore, the dynamics of fusion and the way to achieve it require some fundamental and quite complex factors, such as:
- The insulating capacity which influenced the thermal efficiency of the oven which was influenced by the fuel used which had to lead to the achievement of an adequate temperature (for example in Western Europe charcoal is found) and consequently create an appropriate reducing environment (carbon monoxide). Heat loss was produced by conduction and radiation, which is why many ovens were buried in the ground and were built with refractory material, such as, more frequently, a clay lining. The volume of the kiln was often connected to the ability to raise and maintain the desired temperature and to the quantity of material treated;
- The use of melting materials such as fluxes (quartz or iron hydroxides) to facilitate the formation of a slag fluid enough to separate from the molten metal by gravity;
- Adequate ventilation which could be achieved either through natural draft obtained through the circulation of air with holes opened in specific points of the oven wall or, in an induced manner, through leather bellows which ended with clay tubes called tuyeres created specifically to blow air, a method used since the 3rd millennium BC;
- Combustion through which the thermal energy necessary to start the reduction process of metallic minerals and the reducing gas and CO was produced, this led to the formation of the metal after the interaction with the mineral. The aim was to increase the contact surface of the mineral and favours the reduction reaction by decreasing the size of the mineral itself. The reduction of the ore was probably carried out near the mine: from a commercial point of view it would have been uneconomical to transport the raw ore. It has been estimated that in Jordan the Feinan mine, dating back to the Early Bronze Age, produced approximately 5000 tons of copper slag;
- The base metal smelting operation; the melting metal formed an ingot at the bottom of the furnace or a series of aggregates of metal and slag to be subsequently subjected to mechanical separation through crushing.
The last phase of the metallurgical process is that of refining or alloying, also possible between two or more metals, carried out after the smelting phase. The ingot of a metal produced, such as copper, was partially impure and was therefore refined by melting it in an open crucible with the aid of a rod to stir the metal. Lead and tin, low-melting metals, did not require this type of treatment.
A different refining process was the “cupellation” of silver: the processing of argentiferous galena consisted of desulfurization by roasting (heating to approximately 1000°C in an oxidizing atmosphere) followed by the reduction of the product obtained, litharge. While the sulphide volatilized in the form of sulphide dioxide, the litharge, lead sulphate and residual galena reacted to form lead which settled at the bottom of the furnace leaving a lead-silver alloy containing many impurities.
The subsequent manufacturing processes were essentially two even if, on the same object, multiple manufacturing cycles could have been carried out: the casting of the metal or metal alloy in the casting moulds, and the hot processing through annealing or cold processing with hammering, hot and cold working.
In general, the type of operation carried out relating to the creation of a metal artefact consisted of melting, for example, a bronze ingot in a crucible placed in a charcoal furnace with the bellows providing the necessary oxygen to reach the high temperatures required to dissolve and make the alloy liquid, in this case copper and tin. With tools such as wooden rods or metal tongs, the crucible was taken and the now liquid metal was poured into the melting mould which was kept near heat sources to prevent the metal from cooling. The temperature variation would have caused a lower fluidity of the metal in the matrix, putting the production process at risk. The surface of the bronze that emerged from the casting was substantially spongy and porous, the artefact therefore had to be finished with scraping or hammering, and sanding to eliminate the fusion ridges.
The artefact could be the result of different processing techniques highlighted by the compositional and textural study of the object itself.
In the Near and Middle East all the metals that were used in ancient times are found: copper and its alloys (arsenical copper; bronze, made up of copper and tin; brass, union between copper and zinc), lead, tin, silver, gold, iron, antimony and mercury.
We know that metals are scarcely present on the earth’s crust, therefore the deposits in which, thanks to complex genetic processes, metal minerals have naturally accumulated and concentrated are of great economic importance. Epigenetic deposits are those that originate as a result of the encasing rock, remaining in situ. The syngenetic mineral deposits were instead produced at the same time as the encasing rocks due to mechanisms linked to erosive phenomena; an example of this are the alluvial deposits of gold, native copper and tin, in which the mineral was deposited at the same time as the encasing sediments. In epigenetic deposits, mineralizations often appear in the form of veins that is tabular or lenticular bodies of variable thickness and length, formed by filling the cavities of pre-existing fractures or fissures in the rock masses. These bodies can be single or multiple, sometimes branched and intersecting; mineralization rarely fills the entire seam, since the useful minerals tend to concentrate in restricted areas, called mineral columns. These are linked to changes in the direction of the vein or its diving in depth.
The first mining activities cultivated only the superficial levels of the deposits with metal minerals easily identifiable due to their accessibility and bright colour which indicated the presence of native elements such as copper malachite, azurite, galena and gold. In the parts exposed to the external environment, minerals frequently tend to undergo alteration processes, chemically decomposing, creating new compounds or solubilizing. These processes are particularly interesting in mixed sulphide deposits, which have been the main object of exploitation.