Thursday, 1 December 2011

Iron Production

A smith needs a supply of iron before he can make anything. Luckily, in the Anglo-Saxon period England was quite rich in easily obtainable iron-ores both of low and high phosphorus content...

 These came from three sources, the first two being mined by simple open cast methods :  
  • Siderite, mainly in the form of ferrous carbonate [FeCO³] which occurred as nodules in the Wealden Sandstone and sedimentary deposits and found principally in the Cleveland Hills, Lincolnshire and Northamptonshire.
  •   Limonite, hydrated iron oxide, [Fe²O³.H²O] found in Lincolnshire and Northamptonshire and [FeO.OH] in the Forest of Dean.
  • The third source, most significant, was so-called Bog Iron *.  
(* the word ‘bog’ is of Gaelic origin. The Old English term might have been fen-isen, the Norse term fen-járn.)
Lepotothrix in ditch

Bog Iron is found in moorland regions with iron-bearing groundwater. It occurs principally in the North and West of England and in Denmark. Certainly in the early Anglo-Saxon period, this was the main source of iron ore.
Bog iron is principally Goethite [FeO(OH)]. It is found by probing the ground with a rod, then cutting the sod with a turf knife to reveal the hard, heavy-feeling masses. This miraculous material may well have been perceived as a gift from the chthonic powers. In fact, the nodules form naturally as a result of two processes.
Oily sheen indicating presence of iron

Firstly, in the acidic and hypoxic environment of the bog, dissolved iron undergoes slow chemical reactions tending to cause insoluble iron to form and precipitate out. More importantly, however, anaerobic bacteria such as Gallionella and Leptothrix growing in the fen concentrate iron as part of their metabolism. Their presence is indicated on the surface of the bog by a characteristic iridescent oily sheen. This film is called járn-brák in Old Icelandic, literally ‘iron slick’. These bacteria obtain energy by oxidizing dissolved ferrous iron to the (less soluble) ferric form, which precipitates out.

Bog iron had a high phosphorus content, so could be smelted at a relatively low temperature, although the resultant iron was quite soft.

The Bloomery Process

Having obtained a supply of ore (Old English ora, Old Norse málmr) the smith needed to convert it to useful elemental iron. Firstly, the crushed ore had to be heated to drive off excess water. This roasting also converts the ferrous carbonate to oxides, which are easier to smelt. The ore then needed to be smelted.

The modern term is of Scandinavian origin but the Old English term was ‘mieltan’ meaning ‘to melt, refine or purge’. In the Anglo-Saxon period, the usual bloomery furnace was of the so-called shaft furnace design, which, using either natural draught or bellows to raise the temperature, was more efficient than the earlier bowl furnace. These were made from clay, or rarely stone. Crushed iron-ore was layered with charcoal in the furnace and lit. At the high temperatures in the furnace, carbon is partially oxidized to carbon monoxide. This then typically reacts with the iron oxide releasing the elemental iron. The reaction can be summarized thus :

        FeO + CO = Fe + CO²

At about 1,200ºC most of the slag (Old English sinder) became fluid and separated from the iron. It is important to note that this process never melted the iron, despite several amateur accounts I have read! As iron only melts at 1535ºC and a bloomery hardly ever reached above 1,250ºC. When most of the slag had been removed, the smith was left with a spongy lump of solid iron mixed with the remaining slag; the bloom (Old English bloma). This slag could then be further reduced by reheating and hammering [using the 'bietl' hammer], resulting in a lump of raw iron containing fine slag strings. This type of iron is termed 'Bloomery Iron'. Further smith-craft [ using the 'slecg' hammer] would produce a useful bar / billet of "Wrought Iron" called, by the Anglo-Saxons, a 'Fyrbend.'

The iron in the bloom was relatively pure ferrite - a malleable and ductile metal. However, iron can exist in a number of structural forms depending on the presence and quantity of other elements, notably carbon. Iron containing a certain amount of carbon becomes steel. (Old English Style)

Steel in this period was considered to be iron with a minimum of 0.5% carbon content.
The Anglo-Saxon smith usually created steel by introducing carbon into the ferritic iron  in a process called carburisation. This involved heating either a strip of iron (or rarely a completed item) in a carbon-rich environment. The carbon was gradually absorbed but the surface would always be more carbon-rich than the core, so the smith would then carefully re-heat the bar to welding heat and fold, weld and re-fold the metal with the idea of producing a piece of steel with a uniform carbon content. This was rarely totally successful, producing so-called ‘piled’ steel with alternating bands of high and low carbon content. Care had to be taken, as if the iron absorbed too much carbon the resultant steel would be very hard but too brittle to be useful.
The hot metal was then quenched : cooled rapidly in a fluid, usually water. After quenching, the workpiece would be gently re-heated or tempered, allowing the stresses in the structure of the steel to relax and produce metal of optimum quality.

Unfortunately, high phosphorus iron produced from bog ore will not carburize easily, so steel could not be produced from it.

It is now known that the smið could also produce steel in another way. By choosing a certain type of manganese-rich iron-ore, which absorbed carbon easily, the bloom produced by the bloomery would be of steel rather than iron. Again, forging would be necessary to remove the slag.

Ore was often transported considerable distances before smelting. Often a favoured iron smelting site would depend more on a good nearby supply of charcoal rather than convenient access to ore. It has been calculated that 7-10lbs of charcoal are needed to produce a single pound of iron. Due to the expense of producing it, there would have been a brisk trade in bar and scrap-iron.
Iron would have been distributed to local smiths as bars and each smith would have had a stock of  varying types.

1 comment:

JAS said...

Thanks for very straight forward info. The article has informed me greatly with regard to my iron finds.