THE HISTORY OF IRON
There is little doubt that the first human being to forge iron used a lump of meteoric iron which had plummeted fortuitously from the skies. Ancient iron artefacts dating back as far as five and a half thousand years have been found and there are compelling arguments to suggest that none of this early iron was the result of any sort of smelting process.
Firstly, where written records do exist the names given by different civilisations to the material used for these ancient artefacts generally make reference to the iron having fallen out of the sky; names such as ‘stone or metal from heaven’ or ‘star metal’.
Secondly, a chemical analysis of the material often reveals a high nickel content which is comparable with known meteoric iron which can have a nickel content between 7% and 15% and even as high as 30%.
Thirdly, there are many examples from relatively recent times of primitive peoples making useful implements from meteoric iron which they chipped laboriously from the remains of meteorites which still lay where they had fallen.
It is also undoubtedly true that at some time in pre-history man learned how to extract iron from iron-ore although the exact processes used are not known. It does not take a quantum leap of the imagination to conclude that the aim was to produce an iron which would be as pure and as malleable as meteoric iron, the material with which he was most familiar and which he had grown to revere. It could further be argued that the iron smelters of the more recent past were striving for this same goal and that any intermediate results such as pig iron, cast-iron and wrought-iron were simply stages in this process which happened to produce materials which had their own peculiar properties and proved to be invaluable in their own way.
However, without delving too deeply into the subject of metallurgy and raising issues upon which many experts fail to agree, it is possible to compare the various ferrous materials available today by concentrating on two fundamental attributes. Firstly their composition and secondly their structure. These are the two factors which make the material what it is and determine how it will behave.
Before any useful comparisons can be made it is also important to clarify exactly what we are talking about. All the ferrous materials used by the blacksmith of today are alloys of iron and could be described as mild steels, some of which of course are more mild than others.
The most misused word is probably ‘wrought-iron’ which in the public perception is a piece of ironwork with scrolls rather than a raw material. Even if we discard this misuse it is still a rather confusing name. When it was in production in this country the manufacturing methods relied heavily on the skills of the operators and therefore it was not the exact science which is employed in today’s iron and steel works. Quality varied considerably and led to the introduction in 1910 of a British Standard for wrought-iron used on the railways. In 1931 its scope was extended to include wrought-iron for general engineering purposes and by 1959 there were four specified grades with the introduction of a fourth, lower grade specifically for fencing. The last mill to produce new wrought iron closed in the 1970s and so the only wrought-iron available in this country today is re-rolled scrap, the provenance and quality of which may not be known.
This early material was called wrought iron because it had to be ‘wrought’ or worked by ‘puddling’ and hammering to rid it of impurities. Technically it was a very mild steel, but it had a fibrous structure caused by repeated hammering and drawing out. This structure gave it tensile strength which was desirable for civil engineering projects but it had a tendency to delaminate, expand and flake away as rust formed.
IRON-MAKING IN THE EIGHTEENTH & NINETEENTH CENTURIES
The problems involved with creating massive amounts of charcoal for the furnaces had confined the smelting process to areas where woodlands and water power for the machinery were located together. It was not until Abraham Darby I (1678-1717) substituted coke for charcoal that production began to increase. However, the smelted pig iron was only suitable for foundry castings and not for forges to convert into wrought iron.
It was Abraham Darby II (1711-63) who improved the quality of coke-smelted pig iron and made it suitable for forging into wrought iron. Wrought iron was the result of forging pig iron by continued hammering and re-heating in order to produce a material which was supple enough to be made into tools and implements.
The process of manufacturing wrought iron was speeded up in 1784 by a method of rapid coke-smelting developed by Henry Cort (1740 -1800). The molten iron was stirred with rods and then passed between rollers reducing the time taken to convert one ton of pig iron from 12 hours to just 45 minutes.
John Wilkinson (1728-1808) was the first businessman to exploit Abraham Darby II’s coke-smelting process on a large scale. He used Watt steam engines instead of water wheels to work the bellows of his furnaces enabling them to be moved to the coalfields. He not only built the first iron bridge for Abraham Darby III but also the first iron ship, the first iron lavatory and the first iron coffin – in which he was finally buried.
Some key terms explained:-
- SMELTING – melting in a furnace to separate the iron from the ore which contained it.
- PIG IRON – the result of smelting – brittle (owing to high sulphur content) but suitable for casting into stoves, grates and cannon.
- WROUGHT IRON – the result of forging pig iron by continued hammering and re-heating – more supple than pig iron and suitable for tools, weapons, screws and nails. The term is often incorrectly used to describe ornamental ironwork.
- PUDDLING & ROLLING – a faster way of converting pig iron into wrought iron without heating and hammering at a forge.
IRON-MAKING IN THE TWENTIETH AND TWENTY-FIRST CENTURIES
Mild steel was the obvious successor to wrought iron because it has greater strength and hardness, it is much cheaper and easier to make and with the advent of the modern steel mill a consistent and reliable product could be guaranteed.
For the ornamental blacksmith wishing to forge metal by hand the advent of steel was not such good news. It takes longer to heat than iron and is harder to work. It is not surprising that many blacksmiths have equipped themselves with power hammers to help with the work. Wrought iron production ceased in this country in the 1970’s and since then the only available material has been reclaimed salvage.
For forge-work and the restoration and replication of antique wrought ironwork, pure iron is the closest new material available today and in many ways a superior successor to the original wrought iron.
Pure iron is a term used to describe new iron produced in an electric arc furnace where temperatures sufficient to melt the iron can be achieved. The result, also known as ‘butter iron’ is typically around 99.8% pure with an approximate carbon content of 0.005% and a manganese content of 0.005%. There are also traceable amounts of a number of other elements some of which add certain properties or characteristics to the material and others which are of no significance. Each batch is tested and given its own technical analysis certificate showing its chemical composition. A typical certificate can be accessed by clicking hereXXXXX.
Wrought iron has a higher carbon content of up to 0.5% and mild steel has a carbon content of 0.8 to 1%. Carbon makes the material harder which is why mild steel is so widely used in manufacturing but why it is not universally popular with blacksmiths who want to hot work it. The iron with the highest carbon content and therefore the hardest is cast iron. This is so hard that it is brittle. Differences in hardness can also be achieved by altering the physical structure of the metal by quenching or tempering, techniques which are familiar to the working blacksmith.
The other attribute mentioned earlier was structure. Mild steel and pure iron are both homogenous i.e. they are the same all the way through whereas wrought-iron has a laminated structure. This made wrought-iron a very suitable material for civil engineering projects such as the Eiffel Tower because it has longitudinal strength unlike cast-iron which is only strong in compression. One of the disadvantages of this laminated structure is the way in which it corrodes. Whereas a homogenous material will rust from the outside, wrought-iron has a tendency to form layers of rust between the laminations (crevice corrosion) which can eventually lead to the material literally blowing itself and its surroundings apart. Evidence of this can be seen in many of our cathedrals and churches where wrought-iron fixings have split stonework apart necessitating expensive repairs and renewals. Also many park railings and gates which managed to survive the war are suffering from the same syndrome. Mild steel rusts more readily than wrought-iron but as this happens from the outside it can be seen and dealt with when it happens. Pure iron also forms rust only on its outer surfaces but does not rust as readily as mild steel. Pure iron is 22% more corrosion resistant than wrought iron. Click here to see the results of an independent corrosion test carried out by Keighley Laboratories. Corrosion is a continuing problem for conservators – see Conservation
A regular maintenance program is recommended for all ferrous metals to ensure that the protective paint layers are maintained.