Geoarch Report 2012/08

GeoArch Report 2012/08 Evaluation of archaeometallurgical residues from Ardreigh, Co. Kildare, [00E0156] Dr Tim Young 8th April 2012 Evaluation of archaeometallurgical residues from Ardreigh, Co. Kildare, [00E0156] Dr T.P. Young Abstract The assemblage from Ardreigh comprised approximately 67kg of material collected from 127 contexts. Well over 60% of the assemblage was identifiable as residue from iron-working (smithing), with just 4% positively identifiable as having been produced during iron smelting. The remainder of the assemblage was strictly indeterminate; there was no evidence for the working of non-ferrous metals. The iron-working slags are diverse, suggesting a variety of processes were undertaken – probably both primary and secondary smithing – together, perhaps, with other techniques. The smithing slags were accompanied by ceramic tuyères, the main means of blowing smithing hearths in the early medieval and medieval periods in Ireland. The iron smelting slags are few, but are indicative of smelting in a non-slag tapping furnace, the typical technology of iron smelting in medieval Ireland. The stratigraphic distribution of the residues shows that iron smelting and working had commenced in the early phase of the site, prior to development of the burial ground. The vast majority of the residues were recovered from contexts interpreted as being medieval in age. None of the material recovered possessed what might be considered typical later post-medieval characteristics. The spatial distribution of the residues shows no particular concentration suggesting an original area of activity – rather the residues are widely distributed at low concentrations, indicating that the deposition of waste occurred sparsely over the site and that the centre(s) of metalworking were probably outside the excavated area. In contrast to the interpretation given in the prelim report, it seems unlikely the burial ground was a focus of metalworking. Preservation of substantial and fragile pieces of tuyère may indicate those deposits containing the least reworked residues – and these mainly occur in Cutting 6 area 5, in features associated with the southern side of the burial ground and the northern edge of the hollow-way. Contents Methods Abstract .............................................. 1 All materials were examined visually with a low- powered binocular microscope where required. As an Methods .............................................. 1 evaluation, the materials were not subjected to any high-magnification optical inspection, not to any form of Results .............................................. 2 instrumental analysis. The identifications of materials in this report are therefore necessarily limited and must Description of residues ................................... 2 be regarded as provisional. The summary catalogue of Iron smelting slags ...................................... 2 examined material is given in Table 1. Iron working slags ....................................... 2 Tuyères and lining slags ............................. 2 This project was undertaken for Rubicon Heritage as Other materials ........................................... 3 part of the R417 Athy-Carlow re-alignment scheme. Distribution of materials Interpretation .............................................. 3 Evaluation of potential ........................................ 4 References .............................................. 4 Table 1: Summary Catalogue ............................ 5 Table 2: Residue classes by context ................. 27 Table 3: Smithing hearth cake weights .............. 33 Table 4: Comparison of SHC weights ............... 34 GeoArch Report 2012/08: Residues from Ardreigh Results an intervening void space. In addition, low density, prilly, variants of the ‘ conventional SHCs ’ may grade into a low-iron slag type with a smooth, glassy lining- Description of the residues influenced upper part above a prilly, more iron rich base, that have been designated as ‘ pro-tuyère The catalogue of the residues is presented in Table 1, tongues ’ (Young 2008). with a summary of the distribution of residue classes by context in Table 2. The Ardreigh assemblage includes examples of all of these varieties of SHC, suggesting (irrespective of the current poor understanding of the genesis of the Iron smelting slag different types) that several different processes were being undertaken by the smiths in the town. Just 2.4kg of iron smelting slag was identified (4% of the assemblage). This material represents just a small The sizes and contexts of those SHCs that were fraction of the likely slag waste from one smelt – and sufficiently complete to be able to measure or estimate indicates that smelting is very unlikely to have been their original size is given in Table 3 and a comparison undertaken close to the excavated area. of the weights of the SHCs in the assemblage with those of other similar assemblages, in Table 4. The examples of smelting slag include flow slags and parts of the main ‘furnace bottom’ (FB) slag mass, from The number of complete (or almost complete) SHCs is non-tapping furnaces of the slagpit variety – in which quite low, probably as a result of a high level of the slag produced during the smelt descends into a pit residuality. Of the 32 measurable examples, 69% below the shaft. This slag can be cleared from the pit weighed less than 500g, 88% less than 850g and 9% to allow the furnace to be re-used. Slagpit shaft weighed more than 1kg. The largest example was furnaces were the main type of furnace employed in 2.67kg (although larger examples were probably Ireland for smelting iron from the onset of the Iron Age, indicated by more fragmentary material). The mean probably into the early post-medieval period. A wide weight was 495g (Table 4). variety of furnace designs are becoming recognised, but the present material does not permit identification The maximum weight and mean weights are higher of the furnace employed in medieval Ardreigh. (Table 4) than for sites (of high medieval or post- medieval age) that are interpreted as having been Before smelting commenced, the slagpit of such a solely concerned with the end-use of iron (i.e. were furnace was packed with an organic material. The only blacksmithing; e.g. Ballykillaboy, Gorteens, Ardreigh slags clearly indicate a packing with large Garryleagh, a site on the edge of Clonmacnoise and wood pieces, the most common approach in Ireland Collamurry). They are more closely comparable with (e.g. Carlin 2008; the alternative use of grass or those of sites (e.g. Gortnahown, Carrigoran and cereals is much less common, e.g. Camlin 3, Young Parknahown) where smelting is known to have been 2011). undertaken (thereby increasing the need for primary smithing or refining processes producing large cakes). The residue materials from smelting were all fairly Some sites (e.g Coolamurry, Blackchurch and stout and strong fragments, suggesting that this Ballyellin) have high maximum and mean SHC weights material is largely residual in the contexts in which it despite having no evidence for smelting; these have was found. been interpreted as indicating that iron was moved from source to end-user smith, in the earlier medieval period at least, in the form of incompletely refined Iron working (smithing) slag metal, so that there was requirement for smiths to Slags from iron-working were the dominant type of finish the refinement of the metal at the point of end- residue on the site, with 62% of the assemblage being use. This means that the evidence does not slags identifiable as being from smithing hearth cakes necessarily indicate that a large amount of smelting (SHCs). Much of the 22% of residue from the site that was undertaken at Ardreigh, particularly given the very was slag that was strictly indeterminate, was probably small amount of smelting residue. By the late medieval also either comminuted SHC debris or other forms of period it would appear that movement of fully refined hearth slag. iron was more common, so that average and maximum SHC sizes on late- and post-medieval blacksmithing Smithing hearth cake is a generic term for a wide sites are typically low. Late- and post-medieval variety of broadly plano-convex slag cakes that form in smelting sites (e.g. Ballykilmore) continue to show high smithing hearths. Whilst these were originally thought SHC weights. to be produced during forging activities (the heating of iron to permit forming and welding) it is increasingly One observation on the Ardreigh assemblage was that being recognised that other processes, most notably the proportion of specimens of SHCs for which various remelting techniques, may also be involved in corrosion suggested the former presence of large iron their generation. clasts seemed very high compared to that on other sites. However, such an observation is extremely Several distinct forms of SHC are now recognised (e.g. subjective and any suggestion that this medieval Young 2009a, in press), but are currently the subject of ironworking may have incurred more metal loss than in on-going research. These are currently loosely termed the early medieval assemblages that form the majority ‘conventional SHCs ’ (with variably vesicular and/or of the comparative material, must remain a very charcoal-bearing low dense sections and upper often tentative and speculative suggestion. glassy sections), ‘ thin crust SHCs ’ with a dominant skeletal texture of large olivine crystals forming a Besides the SHCs a slightly unusual,

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