IRON FOR THE ROMAN EMPIRE
The only pure iron to be found naturally on the surface of the earth arrives in the form of meteorites from outer space. The earth’s crust initially contained no iron but over millions of years of volcanic action iron was gradually brought to the surface and deposited as iron ore (rocks which contain varying amounts of iron). For thousands of years people have attempted to extract the iron from its ore with varying degrees of success. It is now known that the Romans had the technology to mass produce pure iron which they used to make their armour. When the Roman Empire collapsed the technology was lost and it was not until the advent of the Industrial Revolution and a better understanding and control of the processes involved in smelting iron that something approaching pure iron could be produced in anything like commercial quantities.
A PRACTICAL EXPLORATION OF THE FORGING TECHNIQUES REQUIRED TO PRODUCE VIABLE QUANTITIES OF HOMOGENOUS, PURE IRON FROM A BLOOM USING ONLY A FIRE, ANVIL AND HAMMER.
By Don Barker FWCB
There are many examples of Roman armour which have been found to be made from a very pure homogenous iron but it is not known how this was produced nor how it was possible to produce it in the huge quantities required by the Roman armies.
There are also examples of surviving iron artefacts, most notably the pillar in Delhi which is known to be made from a very pure iron but it is not known how the pillar was made.
The purpose of my experiment was to try to determine how it was possible for pre-historic and later civilisations to produce usable quantities of pure iron from iron ore without the aid of the modern furnaces used to produce iron today.
It is relatively easy to smelt iron ore using a clay furnace charged with alternate layers of iron ore and charcoal. Draught can be achieved by clever design of the furnace and judicious siting in a windy location and the addition of hand-operated bellows and a plentiful labour force. The resultant bloom is a spongy mass consisting of iron particles trapped in silica and other metals and contaminants which must be removed if a usable, homogenous pure iron is to be produced. Past experiments have involved repeated heating of the bloom in the fire and hammering on the anvil which tends to squeeze out the siliceous slag. This is a time consuming and laborious task which results in an iron which is more like cast iron than pure iron. The more the iron is heated and hammered the purer it becomes but it also reduces in size as increasing amounts of iron are lost along with the unwanted slag. This would appear to make this method unlikely because of the wastage of iron and forge fuel involved and it would not be possible to produce sufficiently large quantities economically.
I began by using the above method to rid the bloom of the majority of the slag and to shape it into a lump which could be further refined. I then discovered that if the iron is heated to a particular temperature it becomes extremely easy to forge and refine into pure iron as all the contaminants rise to the surface of the iron and can be brushed off leaving behind the homogenous block of pure iron. The critical factors in this forging process are the size and temperature of the fire which must be constantly managed to maintain the correct parameters, the position of the iron in the fire and the temperature of the iron for forging. This process could only be attempted by a highly experienced smith because it requires a thorough knowledge of and familiarity with the fire and great precision in its use.
As long as the correct temperature is achieved, this secondary refining process is a very quick way of refining the iron into a usable material. It can be reasonably assumed from this experiment that large quantities of iron in billet form could be produced relatively quickly. Further, the small billets could be fire welded together to form larger billets. A continuous fire-welding process using something like a “pot forge” could be used to form large masses of iron to achieve results similar to the Delhi pillar. This would require a large labour force including many skilled blacksmiths working in shifts on one large furnace and several smaller forges. It would be perfectly feasible for a group of blacksmiths to undertake a similar but smaller project today if sponsors could be found to provide the necessary funding.