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working processes

Unlike all the other metals used in establish the chronology. used to pump air into it. There is no the past, iron was produced and single furnace as worked only in the solid state. This All iron benefit from different reactions occur at different technology is called the 'Direct processing, which may include places. These range from initial Process' of iron manufacture, ie the washing, and crushing. reduction of the iron at about production of malleable iron direct Roasting was most important since it 800°C high up in the furnace to from the ore. By contrast, in the later broke down compounds and caused at over 1000°C near the 'Indirect Process', the micro-cracking in the ore lumps. base of the furnace. produced liquid pig iron which had This facilitated reduction in the to be refined into malleable iron. furnace, by allowing the reducing Two important reactions occur This datasheet only deals with the gases to penetrate the ore lumps. It during ; the reduction of direct process. was essential that the ore charged to iron to metallic iron, and the the furnace was as rich as possible, formation of a liquid slag. Carbon The direct or process over 70% iron oxide. Too much monoxide (CO) is formed by comprised several stages. Metal was (the non-metallic component reaction between in the air extracted from ore by smelting; raw of the ore, eg silica) would cause and carbon present in the fuel. This iron was refined by primary smithing large quantities of slag to form at the gas reacts with the oxygen atoms in and was then manufactured into expense of metal. the iron ore, reducing it to metallic artefacts by (secondary) smithing. iron: All these processes generated Furnace Structure 02 + 2C = 2C0 and residues as by-products. The basic furnace was a cylindrical Fe203 + CO = 2FeO + CO2 clay shaft between 1 and 2 metres in FeO + CO = Fe + CO2 Iron Ores and Ore height with an internal diameter of Processing between 0.3m and lm. The walls of The second reaction in the furnace is Iron (Fe) is the fourth most abundant the furnace were normally over 0.2m the formation of a slag from some of element in the earth's crust and many thick, to reduce heat loss from the the iron oxide (FeO) and the gangue geological processes concentrated furnace. The air inlets, called tuyeres (silica, alumina etc) in the iron compounds to form ore bodies. or blowing holes, were positioned ore. Separation of the metal from the The commonest ores are limonites, about 0.3-0.5m from the base. In the slag was achieved by the slag carbonates and hematites. They majority of furnaces an arch through liquating, dropping to the base of the occur as bedded or vein deposits and the wall of the furnace enabled slag furnace and ultimately being as nodules within other deposits eg to be removed, either cold or as removed. It was necessary for the clays. However, a major ore source tapped slag. furnace temperature to be in antiquity was bog iron ore, which sufficiently high for the slag to be formed by the precipitation of iron Fuel liquid (see Datasheet No 5). compounds in lakes and bogs. Iron The commonest form of fuel was Separation was not complete, and smelting sites are therefore not . Its availability was the lump of metal or bloom was a restricted to specific geological probably the most important factor mass of metallic iron mixed with regions; early (pre-16th century) in determining the location of slag. sites have been found in most furnaces, since large quantities were counties in Britain. needed and it could not easily be Different technologies use different transported great distances. methods of slag removal. With very Bog ores could easily be worked by rich ores, little slag was produced digging, but for other ores deeper Smelting and it could remain within the furnace. For leaner ores the slag open cast pits, bell pits and deep The furnace was charged with fuel needed to be removed from the were used. There is no and preheated. When hot, mixtures furnace. The commonest method was chronology of iron ore mining, as of ore and charcoal would then be by slag tapping, opening the arch there are few dated examples to fed into the furnace, and bellows

The Historical Society: Archaeology Datasheet No 3 Downloaded from hist-met.org and allowing the slag to run out of Welding joins two pieces of iron morphologies. the furnace. together by heating them to high and hammering them Smithing could either operate at a It should stressed that at no time together. Heat treatments improve simple level, producing (relatively) during the process was the metal the metal's properties. can be poor products or to a very high liquid. The bloom of metal produced made very hard by heating to about standard, manufacturing composite would often be heterogeneous, 880-900°C and then quenching by artefacts with superb cutting edges, varying in composition ranging from immersing the metal in cold water. such as knives. There is not yet ferritic iron (no alloying elements), Slight reheating (tempering) releases enough evidence to draw any firm phosphoric iron (containing up to the stresses in the metal, slightly conclusions about the development, 1% ) to carbon reducing the hardness but occurrence and usage of the different (containing up to 0.8 % carbon). The considerably improving the iron alloys in Britain. bloom grew until it started to toughness. interfere with the air blast, at which Although the broad picture of stage it had to be removed. There is The hearth ironworking is understood, most no evidence that the furnace had to Smithing can be done anywhere; it detail is absent. Therefore the be destroyed to remove the bloom. does not need a purpose built recovery, identification and analysis structure, but could use a domestic of iron working evidence and Bloom Smithing hearth. Modern forges are waist artifacts is essential for this vital The bloom from the furnace had to high, and there is documentary and material to be understood. be refined to remove excess slag, to artistic evidence for them back to the consolidate the iron and to either Roman period. Archaeological References homogenise the bloom or separate evidence for such hearths is very Crew, P (1991) The experimental the different alloys. poor, due to their above ground production of prehistoric bar position. The smith required fuel and iron. Historical Metallurgy The product of bloom smithing was an air blast to obtain high 25(1), 21-36. a billet of iron and some smithing temperatures. Charcoal was the main McDonnell, J G (1988) Ore to slags. These slags may have been fuel but from the Roman period artefact - a study of early very rich in iron and could have been onwards there is growing evidence ironworking technology. In E A recycled. The billet could be worked for the use of coal. Slater and J O Tate (eds) up into artefacts or traded off site to Science and Archaeology, another smith. By heating the metal the smith Glasgow,1987, 283-93. BAR increased the chances of the metal Brit Ser 196. Secondary Smithing oxidising and becoming useless. This McDonnell, G (1989) Iron and its Secondary smithing was the could be avoided by careful control alloys in the 5th to 1 lth manufacture and repair of artefacts. of the fire and also by fluxing the centuries AD in England. World Iron smiths had a number of metal surface with sand. This Archaeology 20(3), 373-82. different alloys available, including formed a thin slag layer which Salter, C J (1989) The scientific ferritic iron, (pure iron, relatively cleaned the metal surface and examination of the iron industry soft), phosphoric iron (harder but stopped oxidation of the metal. The in Iron Age Britain. In J more brittle) and steels (varying smithing process produced slags (see Henderson (ed) Scientific carbon contents, enabling very hard datasheet No 6); the most analysis in archaeology and its and tough edges to be produced). characteristic are piano-convex interpretation, 250-73. OUCA There were four main techniques hearth bottoms. The hammering of Monograph 19. used by the smith, cold working, hot the metal also scattered 'scale', the Scott, B G (1991) Early Irish working, welding and heat oxidised film of metal from the Ironworking. Ulster Museum. treatments. surface, around the working area. Gerry McDonnell In cold working the metal is worked Conclusion Dept of Archaeological Sciences at a low temperature, which distorts Early ironworking was a University of Bradford the crystal structure of the metal and sophisticated technological process. (slightly) hardens the iron. Hot There are a number of different April 1995 working (at over 600°C) enables the methods of iron smelting, smith to easily shape the metal. characterised by different slag

The Historical Metallurgy Society: Archaeology Datasheet No 3 Downloaded from hist-met.org