Obtaining metallic iron involves three main processes: extraction, smelting and forging. In the Roman period extraction involved opencast and bell-pit mining of the ore; smelting was the principal stage in reducing this ore to metal and forging was both the final stage in producing the metal and the primary means of fabricating useful artefacts from it. However, dating a particular iron working site as 'Roman' requires more than just evidence of these processes; as the Ordnance Survey paper on Field Archaeology observes:-
"Romano-Britons were very active....and many smelting sites exist, but they can easily be confused with those of later date, and definite evidence in the form of pottery, &c. is required before a site can be accepted as Roman, for in other respects the methods of work did not vary much over the centuries." [OS,1966,p95]
Iron ores of various types are found throughout Britain and "traces of Roman iron workings have been found all over the country" [Birley,1964,p124]. In most places such workings were on a very small scale; the main Roman iron-fields being in the Sussex Weald and the Forest of Dean. The index to the Ordnance Survey Map of Roman Britain lists 33 iron mines: 67% of these are in the Weald and 15% in the Forest of Dean [OS,1991]. Other sources identify more than this number, but still mainly in the Weald and Forest of Dean where there was "the conjunction of large quantities of convenient wood fuel with sufficient supplies of ore" [OS,1966,p95].
Initially, in pre-Roman times, suitable masses of iron ore might have been just picked up from the surface of the ground. Later it was "extracted by opencast methods in bowl-shaped pits, up to 60ft wide and 45ft deep. They were filled upon abandonment but many can be seen today as waterlogged depressions" [Bradshaw,1991,p35]. The proximity of such depressions to proven Roman smelters indicates that such opencast methods continued to be used in the Roman period.
Opencast mining was, however, incapable of meeting all the demand for iron in the Roman period. As Frere has observed:-
"The consumption of iron in Roman Britain soon greatly exceeded that of the preceding Iron Age; iron tools became more common, and a greater weight of metal was employed in some of them." [Frere,1967,p296]
Underground mining therefore began, using:-
"a similar approach to exploiting ore to that used by the Romans elsewhere...large shafts were sunk down to the ore to expose it...[and it] was hollowed out from the shaft bottom once its attitude and value could be seen." [Barnes,1979,p75]
Two underground Roman iron mines exist at Lydney Park [NGR SO616027] in the Forest of Dean; one of these can still be visited. The site also contains the earthworks of a fort and a late Roman bath and temple complex. The mine which can be visited "extends 15.2m underground and reaches a depth of 4.6m. There are original pick-marks on the side of the gallery.." [Dyer,1973,p129]. These pick-marks are not sufficient to date the mine for, as Barnes notes, "mining methods changed remarkably little until the advent of machine mining. Even the tools used are often similar: a Roman miner's pick looks very like one from modern Cornwall" [Barnes,1979,p45]. However at Lydney Park the mine "shafts cut through the earlier earthworks" [Catling,1990,p85] and were "probably made in the third century AD when Roman activity inside the hill-fort prior to the building of the temple seems to have been at its height" [Wilson,1980,p32].
Having extracted the ore the next stage prior to smelting was to remove as much of the gangue (the matrix of clay, sand, limestone &c. in which the ore was found) as possible. "To achieve this the ores were pounded to reduce them to as small a particle size as possible and then washed, often in running water, to remove the less dense gangue. Following this the ores were roasted to drive off the water...and .to render the ore porous" [Hodges,1964,p82]. Hence, one might expect to find a hard "stamping floor" and stone "mauls" [hammers] for breaking up the ore and gangue, together with a water supply, a roasting hearth and a tip of waste gangue. Such features are insufficient by themselves for dating but at Bardown in the Weald [TQ6629], for example, a Roman date was obtained from "some broken flagon necks found in the debris of one of the hearths...these might have been employed as tuyeres [forced air inlets]" [Cleere,1985, p35].
Smelting followed the preparation of the ore. The process, known as the bloomery process, involved heating iron ore in the presence of carbon (for which the Romans used charcoal) to a temperature of 1,100-1,500oC. "A small charcoal fire was kindled in the bottom of the [smelting] furnace and a mixed charge of ore and fuel placed on this, and the temperature of the whole charge raised by the use of bellows" [Hodges,1964,p83]. The products of this process were a spongy mass of metal iron ,the 'unworked bloom', and an iron silicate slag. Evidence of this stage might include the 'charge' (ore and charcoal), smelting furnaces and the products (bloom and slag). In fact the largest amount of evidence which has been found is of the unwonted product: the slag.
"Slag was produced in such large quantities in Sussex that it was used to surface the local Roman roads for many miles." [OS,1966,p95]
And yet there are still enough sizeable slag heaps for them to serve as the main guide to field workers looking for the sites of iron workings. They have also often been the source of shreds of pottery &c. which confirmed (see above) a Roman date for their site. [Details of 67 bloomeries identified as Roman by these means feature in a gazetteer in Cleere,1985,p295-305.]
Pottery has also helped Cleere identify a distinctly Roman type of smelting furnace in the Weald. A smelting furnace is basically "an enclosed combustion chamber equipped with a means of supplying an air blast and with or without provision for the molten slag to be tapped off; there must also be an aperture to enable waste gases to escape to the atmosphere" [ibid,p39]. Based on a study of 70-odd early furnaces in the Weald Cleere concluded that the Romans replaced what he described as "the domed slag-tapping furnace" with "the shaft slag-tapping furnace". After "the end of the Roman period,...the non-slag-tapping furnace" was employed [ibid.]. Twelve examples of the Roman shaft slag-tapping furnace were found at Holbeanwood [TQ 6630] [ibid.].
As noted above, the 'air blast' for smelting furnaces would doubtless have been provided by bellows. No remains of actual bellows have been found in Britain; being made of wood and leather they would not easily survive. However there is evidence of the tuyeres (see above), into which they would have been inserted, being incorporated in furnaces found at Bardown, Beauport Park [TQ7814] and Chitcombe [TQ8121] [ibid,p46].
The final stage in producing useful metallic iron was the forging process. The unworked bloom was reheated to red heat and hammered whilst still hot on an anvil. The cycle of heating and hammering was repeated until a satisfactory 'worked bloom' was produced. "For Wealden iron makers in the ...Roman period the final product was probably not an artefact but a worked bloom" [ibid,p48]. An example of such a bloom was found at Little Farnham Farm and a handful are known from throughout Britain [ibid.]. At smaller, local iron workings the production of the metal and the fabrication of the artefacts, which involved the same forging process, was probably done by the same people in the same place.
According to Hodges [1964,p84] "the forge itself need not be a very elaborate affair; any reasonably hot [i.e. bellows blown] charcoal fire will suffice". Cleere [p47] states that "common remains [are] a hard-burnt patch of clay 0.3-0.6m in diameter, usually slightly concave in section". However, "for the handling and hammering of iron very different tools are required from those used in working metals which can be hammered cold...it is essential to have tongs...to handle the metal...and hafted iron-headed hammers...[and] the anvil itself would...have been far more substantial" [Hodges,1964,p84 orig. emphases]. Whilst differentiating between iron- and other metal-working sites, finds of such tools do not help with dating; Roman iron working tools (as displayed in the Museum of London, for example) would be totally familiar to a modern blacksmith.
As well as worked bloom there is another, waste, product of the forging process: cinder. This may contain unreduced ore and gangue, charcoal debris and, particularly, "black iron oxide (magnetite) [which] flakes away from the surface [of the worked bloom] in small scales...[it] may be a very good indication of the site of an old forge" [ibid,p85].
Finally, as well as these process related features that one might expect to find on a Roman iron working site, there might also be an associated infrastructure. Frere suggests that the Roman government "provide[d] roads to facilitate marketing the produce [i.e. iron]...This is especially noticeable in the Weald" [Frere,1967,p296]. The metalling of some of these with slag has already been noted (see above). Similarly good roads are known in the Forest of Dean. The Roman road from Weston-under-Penyard [ARICONIUM, SO 6423] to Lydney "is unusual in being paved with stone slabs....[and] is carefully built with kerb-stones on either side" [Wilson,1980,p132].
As well as transporting iron by road, boats may also have been used. In the Weald either the River Brede or River Rother might have provided routes from inland to the sea. No port structures have yet been found but Cleere [1985,p65] suggests Bodiam on the Rother [TQ 7826] as one possible site. Certainly the Roman's British fleet, the Classis Britannica, seems to have been involved with Wealden iron working. This is demonstrated by "the discovery of over 1,300 tiles stamped CL BR at Beauport Park (as well as on other Wealden sites)" [Wilson,1985, p43]. On this site there is an "excellently preserved bath-house on the slope above the slag bank...built in the last quarter of the second century and...abandoned...c250". There is also "a settlement of some twelve acres...indicated by surface finds across the stream from the bath-house. This of course was only a small mining village..." [ibid.].
In conclusion, although one might hope to find on a Roman iron working site direct evidence such as mine pits, smelting furnaces and forging hearths, evidence from pottery &c. would also be required to date them as specifically Roman. These might also be an associated transport and habitation infrastructure. However the most likely finds are the sort of remains by which Cleere has generally identified Wealden iron working sites: "the waste products of iron making - ore refuse, charcoal refuse, slag and furnace debris." [Cleere,1985,p49]
References
Barnes, J.W. (1979) 'The First Metal Workings and their Geological Setting' in Crawford,H.(ed) 1979 Subterranean Britain - Aspects of Underground Archaeology, John Baker (Publishers) Ltd, pp44-84.
Birley,A. (1964) Life in Roman Britain, B.T.Batsford Ltd
Bradshaw,J. et al [Kent Underground Research Group] (1991) Kent and East Sussex Underground, Meresborough Books.
Catling,C. and Meary,A. (1990) Gloucestershire and Hereford & Worcester, New Shell Guides.
Cleere,H. and Crossley,D. (1985) The Iron Industry of the Weald, Leicester University Press.
Dyer,J. (1973) Southern England: An Archaeological Guide,
Faber & Faber.
Frere,S.S. (1967) Britannia, a history of Roman Britain, Routledge and Kegan Paul.
Hodges,H. (1964) Artifacts, John Baker Publishers Ltd.
OS (1966) Field Archaeology, Ordnance Survey Professional Papers, New Series No.13.
OS (1991) Historical Map and Guide - Roman Britain, Ordnance Survey.
Wilson,R.J.A. (1980) A Guide to the Roman Remains in Britain, Constable & Co. Ltd.