Issue 98

January / February 2008

Contents

news

Major new galleries open in Cardiff

Popular scheme threatened: culture change needed

Cultural icon: Phil Harding or Jonathan Ross?

Roman governor in Scotland

Bones of our forefathers

Secrets of Silbury poet revealed

Medieval archaeology comes of age

In Brief & Phase 2

features

Drapers Gardens
First insights into striking discoveries from Roman London

Detecting the past
Let the rally begin: we consider new detecting developments

First iron age furnaces
Rachael Hall describes extraordinary remains from Corby

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Recommended websites
Fringe archaeology, and a new website from Heathrow.

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CBA correspondent

Campaigns, comment and communications from the CBA
Lynne Walker reports on the CBA's recent historic building casework

ISSN 1357-4442

Editor Mike Pitts

features

Putting the iron into Iron Age

A new metal was introduced to Britain in about 800BC, initiating the iron age. Yet iron production sites are rare. Excavation at Priors Hall, Corby, Northamptonshire has revealed the first substantial remains of Britain's prehistoric smelting.

Corby is synonymous with a heritage built upon iron and steel. The coming of the steel industry in the 1930s prompted rapid growth in the area and the steel works dominated the town and people's lives. This heritage does not however begin in the new industrial age following the great depression; it can be tracedmuch further back.

This earlier story began to be revealed in 2006, when excavations were undertaken in advance of a mixed development of housing and other infrastructure. Located in the Rockingham Forest region of Northamptonshire, Priors Hall comprised a 183ha open green space, on the eastern fringe of Corby overlooked by the Corus steelworks. Excavations were carried out by Archaeological Project Services for Wardell Armstrong LLP on behalf of Bela Developments.

The works began in the southern part of the site with the topsoil stripping of 12ha for limestone quarrying, adjacent to a scheduled Roman villa that had been excavated during the 1950s. The summer of 2006 was one of the hottest on record. Coupled with the local geology, this meant that not only did the ground have a concrete-like consistency, the dried soilsmade it very difficult to distinguish archaeological remains from natural deposits. Added to this was the curious phenomenon of patiently circling red kites, who would take their opportunities to visit the opened areas to remove nails, labels and string as they saw fit.

Despite such challenges, the soil strip exposed five burnt areas that even before excavation were readily identifiable as furnaces, along with many other archaeological features indicating a complex landscape of activity. At the time it was assumed that these represented the bases of iron smelting furnaces, similar to other examples recorded elsewhere in the county. Such remains were not overly surprising in the Rockingham Forest area. It is well suited for such an industry with local ironstone outcrops and extensive woodland to provide the fuel needed to smelt the ore. A recent survey has shown that at least 32 smelting sites are known, mainly undated but likely to be late iron age to medieval (from 100BC).

The first excavated example (furnace 1), in the northern part of the site, was exactly this: a simple bowl structure, with evidence of an adjacent possible ore roasting pit and other associated small pits backfilled with slag and fired clay. It was no surprise to find only the base of the furnace, as excavations typically reveal superstructures to have been truncated.

The next three furnaces we investigated, however, were better preserved, and proved hugely significant. These were what has been termed sunken shaft furnaces: the lower section of the structure is built below ground within a construction pit. The first of these to be excavated (furnace 2) was in what we thought at the time to be a really good state of preservation, better than indicated in plan, with the superstructure standing to a height of 35cm. It neatly fitted the previously described models of shaft furnaces, namely a clay superstructure with a tapping arch located towards the base of the furnace that opened into a tapping pit. Helpfully, late iron age pottery (100BC–AD43) was recovered from both deep within the shaft structure and the tapping pit itself, securely dating it.

A three-sided rectilinear beam-slot enclosed the furnace with no visible structural remains on the southern side, suggesting a shelter with an open front. Despite the considerable truncation of the site, a survey of furnace 2 and the surrounding area identified a concentration of hammerscale in the structure's northeast corner. This suggests that the wooden shelter probably also functioned as a smithy, with the processing of the raw materials creating scale from hammer blows against the furnace's newly smelted iron. If this is right, the structure would have had no windows: a lack of light would be essential for iron working, as the process of creating workable iron from the furnace bloom requires a precise knowledge of varying temperatures, something that would have been possible only through assessing the colour of the flame produced during the process. No great quantities of hammerscale were found elsewhere on the site or were recovered from the environmental samples, despite extensive investigations.

Prior to their excavation furnaces 3a and 3b had already been flagged as significant, as furnace 3b clearly truncated the tapping pit of furnace 3a, giving a sequence between the two. As excavation progressed, two fantastically preserved substructures were revealed. Both had been dug into a deep pit, whichmeant that the lower sections were completely intact and standing to an unprecedented height of 70cmand 60cm respectively.

The openings through the fronts of the furnaces were crude, with uneven shapes and rough edges, suggesting that they had been created after firing by breaking or "tapping" through the hardened clay structure, rather than made in the initial construction. The furnace fronts, known as the belly section, were a great deal thicker than the backs or sides, with the hardened clay quite uneven and patchy in colour and the surfacesmuch rougher than elsewhere. After the tapping through of openings to extract the pure iron and release the smelt by-products (slag), it seems the walls had been resealed with clay leaving the furnaces ready to be used again. This patching-up may have taken place straight after the extraction of the iron, thus retaining some of the heat fromthe last firing. Again, fortuitously, fragments of late iron age pottery were retrieved from within the shaft and the tapping pit fill of both furnaces.

Further excavation revealed a fourth sunken shaft furnace (furnace 4). Although considerably truncated by later agriculture (a plough furrow cut along the edge of the structure) this resembled the other three examples. Also investigated was a somewhat larger furnace that had suffered greater truncation than those sited further south. It was notably different from these, with at least nine relinings of the entire structure indicating many periods of use. This is more reminiscent of otherwise unique furnaces excavated at Laxton, dated to the Roman period.

As the site seemed to be exposing important iron working remains, we invited the University of Bradford Department of Archaeological Sciences to undertake a programme of archaeomagnetic dating. This confirmed the dating implied by the associated pottery, placing the onset of the furnaces' use in the late iron age, making themsome of the best preserved examples from this period. Moreover it had been assumed previously that the sunken shaft furnace was indicative of Romano-British and later technology.

During December 2006 an access road was stripped of topsoil immediately south of the summer excavation site and north of the scheduled site of Weldon Roman villa. Here a further three sunken shaft furnaces were revealed, including the most complete and probably finest example of this type to have been excavated in recent times. Furnace 6 stood to a height of 70cm and was constructed in the same method as those described previously, with its bottom tapering up into a cylindrical shaft. Again an opening had been tapped into the belly section of the furnace and once again there was considerable evidence of the resealing of the furnace following the extraction of the pure iron.

Two of the three furnaces found in the new area had late iron age pottery associated with them, and were truncated by Romano-British graves probably belonging to the villa just to the south. The third furnace, judging by its association and form, is likely also to be late iron age.

The excellent preservation of these furnaces, coupled with the scientific analysis, allow us to suggest a simple model for construction and smelting technology in the late iron age at Priors Hall.

1. All the furnaces are located in the lee of a gentle south-facing slope. Construction began with the excavation of a pit within which the furnace was to be built.There are traces of scorching in the base of some of the construction cuts, perhaps suggesting a fire was burnt to dry out the hollow.

2. The clay shaft structure was then constructed within the pit, with its bellied bottomsection tapering up into a cylindrical chimney. Although the Priors Hall furnaces survived only to the surface level of the pit, it is thought that the shaft would have extended perhaps 1–2mabove ground level. The lower belly has a slightly bell-shaped interior appearance with the walls of the substructure notably thicker. It is difficult to say how the superstructure would have beenmade, although it has often been suggested that the clay was moulded around a wooden frame which was then burnt out. Large extraction pits containing good quality clay were located just north of the furnaces.

3. Once the furnace had been constructed the outer edge of the pit was backfilled with good quality clay. This may have provided additional insulation and stability to the shaft. It is also likely that in the above ground section, although close to the belly, there would have been tuyère holes to which bellows would have been fitted to encourage the circulation of air within the shaft. Finds of tuyères are extremely rare, but a large section was recovered from within the shaft of one of the Priors Hall furnaces.

4. The furnace would then have been packed with charcoal which was heated to c1200°C to allow for a successful smelt, and the iron ore then added. Interestingly, there is little evidence for the preparation of charcoal nearby, although the samples are dominated by the presence of oak charcoal which must have fuelled the furnaces. There are several ore-roasting pits on the site along with plentiful roasted ore suggesting that there was essential preparation of the ore prior to its addition to the furnaces.

5. Once the smelting process was complete an opening was broken through the front of the furnace near the base of the belly. Through this the pure iron could be extracted and the waste slag cleaned out. The opening was then resealed with clay to allow the process to be repeated. It is likely that this occurred immediately to exploit the retained heat fromthe last smelt. The constant repair and resealing of the furnace served to produce a dshaped profile towards the furnace's base and accentuate the belly-like appearance of the lower section of the superstructure.

There is very little evidence of any activity other than industry taking place at the site during the late iron age, although occupation does continue through into the early Romano-British period with features including a cemetery and field systems associated with Weldon Villa which was occupied between AD70/80–400. During the excavation of the villa some 50 years ago, the bases of several possible furnaces were identified beneath the floor layers. Wall foundations were described as being built of stone and slag, suggesting the presence of several more furnaces in the landscape.

The excavations at Priors Hall paint a largely industrial scene in the late iron age, which we would not associate with religion or ritual. Our modern preconceptions howevermay be misleading. Those involved in the late iron age industry were likely to have been highly revered for their great skill and craftsmanship in producing a valued metal froman apparently futile material.

It has often been suggested than iron smelting took place away from settlements, perhaps because of taboos and the "magical" nature of the process. Ethnographic studies of small-scale metal production inmodern societies have shown a close link between the process and fertility. More than often the industry is male dominated, with females not involved in the workings of the site. Pot-bellied smelting furnaces that allow efficient air circulation have the appearance of a pregnant torso. Quite often ethnographic examples have clay breasts added to the exterior to further embellish the furnace's female form. No such evidence of this was seen in the surviving remains at Priors Hall! Once the smelting process was complete the furnace was broached low down below the belly, to extract the bloom in the manner of a birth.

What is undoubtedly clear is that the furnaces excavated at Priors Hall are some of the best preserved and earliest examples to have been excavated in recent times. Further analysis will no doubt provide us with exciting new evidence regarding Britain's late prehistoric smelting technology.

Rachael Hall is project officer at Archaeological Project Services. APS would like to thank Bela Partnerships Limited for funding this investigation, Anna Slowikowski for pottery dating, David Greenwood, Cathy Batt and Anthony Swiss for undertaking the archaeomagnetic dating and scientific analysis, Helen Martin-Bacon for co-ordinating the project on behalf of Wardell Armstrong LLP and Gerry Macdonnell for his help, advice and enthusiasm throughout. Thanks also to all those who took part in the excavations, which are being published by aps as a short monograph (Rachel@apsarchaeology.co.uk).

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