MEDIEVAL IRON · The Stanley Grange Medieval Iron Project: Production, Exchange, and Technology Transfer in Medieval Europe

The Stanley grange Medieval iron project: production, exchange, and technology transfer in Medieval Europe

In 1997, an area of circa 1.5 ha associated with medieval iron smelting was investigated by Trent and Peak Archaeology at Stanley Grange, Derbyshire, UK. Excavation recovered the remains of at least eight furnaces, ore crushing areas and deposits of smelting slag, as well as features possibly related with the forging of the iron produced (bloom smithing). Although only the base of each furnace survived, they appeared to be large and of novel form. The furnace was circular in plan, had an internal diameter of 50-70 cm, 50-60 cm thick walls and a 50 cm wide tapping arch at its western side. The furnaces have been dated by archaeomagnetic techniques to the 13th or 15th century AD, but only the earlier date is supported by artefacts, principally medieval ceramics.% LStanley Grange is one of only a handful of medieval iron smelting sites known from excavation within Britain and is contemporary with the rapid and dynamic technological developments that characterised European iron smelting between the 11th and 14th centuries. In this period a gradual transition took place from direct produced iron in low shaft furnaces or bloomeries to indirect produced iron in blast furnaces. The terms direct and indirect refer to the subsequent treatment of the produced iron. In a bloomery malleable iron is won in a solid state extraction process, in a blast furnace fluid cast iron is produced that has to be decarburised in a subsequent fining process to make it malleable. Such technological developments had a significant impact on European society and economy. The objectives of the Stanley Grange medieval iron project incorporate two broad components: a full understanding of the technological basis of the iron production at Stanley Grange, and a contextualisation of this evidence within our current archaeological, historical, cultural and environmental understanding of medieval Western Europe. To meet these objectives, the site archive, consisting of context information, colour slides, excavation plans and profiles, has been digitised to facilitate linking between finds, context and location. In a second stage, all finds, which mostly consist of iron slag, were cleaned and analysed optically (shape, vesicularity, colour, etc.) and physically (weight, magnetism, and density). The total number of fragments counts nearly 15 000, their weight is close to 660 kg. In a third stage, slag samples were taken from two furnaces with adjacent slag tapping pit and prepared for compositional (40 samples) and mineralogical (33) analyses. A third context, which might be associated with bloom smithing, was sampled as well. In this way slag types identified during the optical analysis could be compared in regard to their chemical and mineralogical composition, and the smelting temperature and yield of the smelting process established. The detailed analyses of the slag led to a deeper understanding of their formation. Two main groups stood out: slag that solidified within the furnace and slag that solidified when it ran out of the furnace. The latter group consisted of two extremes: dense and vesicular slag. In addition, a large amount of slag with a consistency between these two extremes was found. The dense slag resembled bloomery slag as known from for example Roman iron smelting sites. The vesicular slag, on the other hand, suggested operation temperatures higher than applied in a traditional bloomery furnace thus indicating a more evolved smelting process. Interestingly, vesicular slag occasionally ran on top of dense slag. This phenomenon can be explained only by assuming higher temperatures in the furnace towards the end of the smelting process as the chemical composition for both dense and vesicular slag is more or less the same. The higher temperature would cause a reduction of the viscosity of the slag and result in an increased vesicularity because trapped gasses in the slag would expand correspondingly. An intriguing question is whether forced draught or natural draught was used at Stanley Grange. High temperatures can be obtained by increasing the air supply, which was usually done by means of bellows. In particular water powered bellows played a crucial role in the development of the blast furnace. In Stanley Grange, however, the furnaces might have run by using natural draught. Slag tubes found on the site might point to multiple perforations through the furnace wall in order to obtain an air flow strong enough to raise the temperature to at least 1 200-1 300 degrees Celsius (the approximate smelting temperature of the slag). Similar tubes have been found at several medieval smelting sites in the Lahn-Dill area in Germany. The slag and furnaces from these sites resemble those from Stanley Grange. In short: the increased size of the furnaces, the higher smelting temperatures and the possible saving of man power by using natural draught, suggest the use of an evolved type of bloomery furnace at Stanley Grange that shows strong similarities with furnaces found on the Continent. Therefore, it is likely that the introduction of the blast furnace did not succeed the simple bloomery furnace as hitherto assumed, but that Britain followed the technological developments in iron smelting on the Continent. Iron smelting probably took place at Stanley Grange because of the availability of the raw materials needed: iron ore, wood and clay. Sideritic iron stone nodules served as ore and can be found within the coal measures. They might have been found near Stanley Grange, possibly at the site of the later colliery immediately north of Stanley Grange. Wood for charcoal will have been available as well as many parks and forests are known from this area in the Middle Ages. And clay for building furnaces was probably dug on site as possible clay quarry pits found during the excavation imply. The distribution of the furnaces suggests that iron was not produced continuously, but rather annually or at longer intervals. The knowledge for iron smelting could have come from itinerant smelters as is known from medieval written sources on amongst others the Forest of Dean. Stanley Grange was built to serve Dale Abbey, a house of Premonstratensian canons founded in about 1200 AD. It must have been built after 1241. The earliest record of the grange is from the second quarter of the 14th century. Therefore it is not clear if smelting took place under the auspices of Dale Abbey or that Dale Abbey acquired the land after smelting activities had ceased. It is however likely that the production at Stanley Grange stopped as a result of wood shortage, either because all suitable trees had been used or because the land had been cultivated.

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