Long ago (3000BC), metal working developed through the bronze age.
Some metals are sufficiently inert that the pure native metal can be picked up and, being comparatively soft, can be easily worked.
The reduction of ores with charcoal will produce copper and tin from sulphides, oxides or carbonates. Bronze is an alloy of copper and tin.
It is possible that iron meteorites might have fallen into the hands of metal workers in ancient times. Picked up most easily in deserts or snowfields, this material might be beaten (forged) into high status, magical, objects. Such a knife was found in Tutankhamun’s tomb.
From 1200BC it was possible for ironworkers to smelt iron from its oxide ore. An ore will always contain impurities such as sandstone silicates. When smelting iron in a small bloomery, carbon in charcoal form can combine with the oxygen in the iron oxide to free particles of iron. (Actually, carbon monoxide from the carbon is a very effective reducing agent—finally converting to carbon dioxide.) The bloom will contain unreacted iron oxide and carbon and slag as well as the reduced iron. Reheating and subsequent hammering on an anvil will help the iron particles to cohere and express out much of the slag and other unwanted material.
The early ironworkers managed to achieve a temperature high enough to render the slag molten but not high enough to melt the iron and so it was by lucky chance that the iron billet which was eventually formed was a comparatively soft malleable iron of very low carbon content which could be easily fire welded.
The small amount of slag which still remained in the iron has some helpful properties in both resistance to weathering corrosion and as a flux during hot hammer welding. The working of the bloom gives us the name of the material ‘wrought’ iron.
Pure iron is so soft that one cannot make tools which will remain sharp and much experimentation took place in making knives, swords, axes etc. In fact a very small carbon content in the iron (say 0.5%) can produce a solid solution of iron carbide in the iron which we call steel. Wrapping an iron sword in organic (carbonaceous) material such as leather and cooking it in a charcoal furnace might well harden the surface so that it could be sharpened.
The development of early blast furnaces in the 1500s and 1600s allowed iron to be smelted in larger quantities.
A tall chimney-like structure loaded with layers of charcoal and crushed iron ore (and a little limestone) and furnished with a bellows-driven air blast, reduced the ore to free iron which then melted and was tapped off onto a sand floor producing pig iron (so named from the shape of the channels which guided the ingots). Glassy slag could be tapped off separately and perhaps broken up into crozzle – a crude water-impervious building material.
We know that carbon will dissolve in molten iron and pig iron contains about 4% carbon by weight. Pig iron is very brittle and totally useless for blacksmithing work. The brittleness is partly caused by carbon coming out of solution in the iron crystals as a graphite. Molten pig iron can be poured into sand/clay moulds to form cast iron objects which are strong in compression but weak in tension. Much effort went into refining pig iron to reduce its carbon content so that blacksmiths could work it.
Eventually a successful, efficient process was invented by Henry Cort at Fontley in Hampshire(1783 patent). In 1787 Cort agreed that Richard Crawshay should use the puddling and rolling process at the Cyfarthfa works in Merthyr Tydfil. The managing partner was James Cockshutt of Wortley.
The Bessemer process blew oxygen through molten pig iron to burn out most of the carbon to form steel. It was often easier to burn out all the carbon and then replace a controlled amount to give steel of a given carbon content. however, Henry Bessemer was not an experienced metallurgist and some problems had to be solved. The phosphoric iron ores in Britain gave poor results until the converters were lined with alkaline dolomite which converted the phosphates to basic slag. A problem of excess oxygen occluded in the metal was solved by Robert Mushet by adding a proportion of manganese ore to the melt. Manganese oxide passed out into the slag.
It is interesting to note that David Mushet (Robert’s father) had found that a manganese steel alloy was very hard wearing and was used by Sir Robert Hadfield for railway wheel tyres and heavy duty railway points.
Later (1869) Siemens open hearth steel making began using a hot blast alternating between two hearths.
Carbon will dissolve in molten iron and will make it harder (steel 0.5% C) and more brittle (pig iron 4% C).
The early ironmakers smelted iron from iron ore and charcoal in a small bloomery, producing a spongy porous bloom containing particles of iron, unreacted iron oxide and charcoal and some stony slag. The iron did not actually melt and so carbon was not absorbed. When this bloom was reheated and worked with hammers, the iron was consolidated and the impurities mostly squeezed out.
This ‘wrought’ iron, containing less than 0.1% carbon, is comparatively soft, can be forged by blacksmiths and easily hammer-welded.
From the 16th century, iron could be smelted in the newly developed blast furnaces but at a higher temperature which allowed carbon to dissolve in the molten iron. This pig iron can be used to cast iron objects in a foundry but cannot be forged. It was necessary to burn out the dissolved carbon in a finery forge to make ‘wrought ‘ iron for blacksmiths.
In the 1790s Henry Cort’s more efficient puddling process was introduced at Wortley Low Forge and the wrought iron was also rolled into bars. These bars were used in the local wireworks and in many small nail forges.
The wrought iron still contains about 2% of stony slag which gets rolled within the bars. It is thought that this glassy slag helps as a flux during hammer-welding and gives some resistance to rusting at the surface. Old rolled wrought iron bar usually exhibits slag lines on the surface giving a wood-grain fibrous appearance.
The puddling process works because pure iron has a higher melting point than iron with dissolved carbon and wrought iron can be raked out of the puddle as the carbon content is reduced by the furnace.
Puddling efficiency was greatly improved in 1830s when the reverberatory furnace was lined with iron oxide to supply oxygen to oxidise the carbon in the molten pig iron instead of just depending on air at the surface.