his is the next in our series of posts on metal-working written by Dr. Kevin Leahy, PAS National Finds Adviser. The articles were first published in The Searcher magazine and are reproduced here with kind permission of Harry Bain, editor for The Searcher.
We all love iron, don’t we? Well perhaps not, many detectorists set their machines to discriminate against iron finds – out of 1.4 million finds on the PAS database, only 6,353 (less than half a percent) are made of iron. There are reasons for this: iron is usually poorly preserved, difficult to date, and there is an awful lot of it out there. If you dug every iron signal, you’d get nowhere fast. However, iron can be interesting and, like everything else, has its story to tell.
Iron was used in Egypt to make small beads around 3,200BC but their high nickel content showed them to be made from meteorite iron, a source used at an early date by other peoples. An iron dagger found in the tomb of Tutankhamun, who died in 1,323BC, has also been found to have come from a meteorite. Iron was first smelted from its ore in Anatolia, Turkey, around 2,200BC and by 1,400BC was in common use, at least amongst the aristocracy.
Iron first appeared in Britain around the 7th century BC and there was a period overlap when both iron and bronze were used. This is not surprising as early iron objects were pretty useless. Iron is a weak, soft metal and was inferior to bronze. There is an account of Celtic warriors at the battle of Telamon (224BC) having to break of the action to stamp on their swords and straighten them! It was only with the development of steel (see below) hat iron became a strong and useful metal.
So, if iron was so useless, why did it replace bronze? Well, it was more readily available – while copper is found only in the north and west of Britain, and tin only in Devon and Cornwall, iron is widespread. There are deposits of iron ore in the Weald, Lincolnshire, Northamptonshire, the Cleveland Hills, Cumbria and the Forest of Dean. In addition to these, there are many deposits of “bog iron”, easily smelted, which was an important resource in early times.
After mining, the ore had to be dressed to remove as much of the non-ore material as possible, before it was roasted to around 500-800ºC, driving off water and turning it into oxides. The roasted ore was broken up and put into the furnace for smelting to convert rock into iron.
Smelting is based on the affinity that carbon has for oxygen at a high temperature: the carbon monoxide (MO) gas from the burning charcoal desperately wants another oxygen atom to become the more stable carbon dioxide (CO2) and grabs an oxygen atom from the iron oxide leaving the iron alone as metal. The reaction, CO + FeO –> Fe + CO2, creates the iron metal and caron dioxide gas.
Finding fuel was perhaps a bigger problem than getting the ore as the smelting process used charcoal in enormous quantities. 16 pounds of charcoal were needed to make one pound of iron, and as charcoal is very light, this represents quite a bag full. The process was slow, calling for six to twelve hours pumping the bellows to maintain the blast of air.
During this process the iron never melted but changed from ore to metal while remaining solid. While iron melts at 1,536ºC, smelting starts at around 800ºC and iron could be extracted at this temperature. However, iron is not the only thing to come from smelting. As well as iron oxide, ores contain other minerals which form slag and a temperature of 1,150ºC is needed to melt the slag so that it separates from the iron.
The final product from smelting was a spongy lump of iron known as a “bloom”, as well as large amounts of slag waste which still contained a lot of iron – early smelting was inefficient. After removal from the furnace, the bloom was consolidated by heating and hammering which forced out most of the slag to form wrought iron. Blooms were small and to make a large iron object such as anchor or a furnace bar it was necessary to forge together a large number of blooms.
Although iron smelting was a slow and arduous task, large amounts of iron were made in the past. During the excavation of the Roman fort at Inchtuthil, Perth and Kinross, a hoard of iron weighing ten tons was found, including 875,000 iron nails weighing seven tons. These had been buried when the Roman army moved out in AD86 to prevent the local tribesmen using them to make interesting things.
Iron was worked in a markedly different way to bronze. Bronze could be hammer-worked to produce sheet metal objects like shields and buckets. This was done while the metal was cold but with frequent reheating to relieve the stresses and prevent cracking. Iron on the other hand was worked as white-heat and, unlike bronze, was not cast in antiquity. New types of object were devised to take account of the different way in which iron had to be worked. For example, complicated shapes like a socketed axe could be easily cast in bronze but difficult to fabricate in iron.
The properties of iron offered advantages: heated to white-heat, it becomes highly malleable; it can be shaped by forging; and, unlike bronze, pieces of it can be welded together. If two pieces of wrought iron are made white-hot and hammered together, they will fuse making a strong permanent joint. Iron is a poor conductor of heat making it possible to heat just a single area; the middle of a bar could be heated so that is could be bent or expanded or hammered from its ends.
It was the development of steel that made iron important. Steel is made by introducing carbon into the iron by a process known as “carburisation” in which iron bars were kept at around 900ºC in a charcoal fire, blown by bellows with charcoal being added. After about six hours the carbon would have penetrated 2mm into the iron producing steel. By “piling” – repeatedly folding the carburised bar along its length and hammer welding – the carbon was distributed in layers throughout the bar.
Steel is harder than iron and offers the great advantage in that it can be heat-treated – heated until it is red-hot and plunged into water to rapidly cool it. This gives a hard but brittle steel and it is necessary to “temper” it by heating it to a lower temperature and allowing it to slowly cool, giving a tough, usable steel. Making steel was a slow process and alloy (steel is an alloy of iron and carbon) was used carefully. Often a strip of steel was welded along the edge of an iron tool for economy, or a layer of steel was incorporated into the core of a knife. It’s sometimes possible to see the line of the steeling on a corroded iron knife.
In the late 15th century a new type of iron was introduced into Britain in the form of “cast iron”. This was produced in a blast furnace which, unlike the earlier method, produced liquid iron that could be poured into a mould or cast into “pigs”. Cast iron picks up carbon from the charcoal or, in later periods coke, used in the blast furnace. While carbon is added to iron to make steel, cast iron contains too much carbon – more than 2.5% – which has a dire effect on its properties: cast iron is brittle and will snap if bent or placed under tension. Cast iron could be converted into the workable wrought iron in a “finery” where the carbon was oxidised from the molten iron. The development of the “puddling process” in the 1780s allowed cast iron to be converted on a larger scale.
Henry VII introduced blast furnaces and cast iron from the continent to produce cannon balls, guns still being made by the “stave and hoop” method. Cast iron took quite a while to really catch on. Later, guns were cast, as were fire-backs, but it’s not until the 18th century that things really took off with the large-scale use of cast iron in building, machinery and household goods. Cast iron is capable of taking incredible detail, and be cast into thin and intricate shapes. Most of the iron we see today is “mild steel”, a cheap but entirely satisfactory material.