Slag Assemblage Analysis

Slag is the by-product of smelting or smithing iron. During smelting the slag forms from the gangue material in the ore, and combines with the furnace lining and fuel ash.  A range of furnaces have been used to produce iron in antiquity (Tylecote 1986: 132-141). The morphology of slag produced during smelting depends on the type of furnace used, which in turn varies both regionally and chronologically (Cleere 1972; Tylecote 1986; Bayley et al. 2001).

Smithing slag forms in the smithing hearth as the metal is worked, and consists of iron broken of during working, hammerscale, fuel and the clay of the hearth. These form smithy hearth bottoms, which have a distinct shape (McDonnell 1987; 1991).

The amount of slag which can be expected at a primary production smelting sites and secondary smithing sites varies considerably with the period. With prehistoric examples even a few kilograms of smelting slag can be significant. Whereas deposits at Roman and medieval iron-smelting sites can vary widely, up to thousands of tonnes (Bayley et al. 2001; 2008). Routine examination of slag aids the accurate identification of site function. Characterization of these residues may also provide information on methods, raw materials and equipment used (Bayley et al. 2008).

There are three possible models for the production of iron objects in any settlement (Figure 3.12; McDonnell et al. 2012). The first model is the self-sufficient model where the settlement makes and uses its own iron. The second option is the complex smithy model where the smith imports stock iron (iron bars, billets and strips) to use to make artefacts. The final model is the basic smithy model that repairs or recycles iron artefacts imported into the site. While it is possible that each of the models can be independent of the other models, it is equally likely that there could be a mixture of two or three models taking place at any one settlement.

Careful sampling, both of hand-recovered material and of soil samples for micro-residues, such as hammerscale, a by-product of iron-smithing. A further refinement to consider when dealing with soil samples is to use flotation and wet sieving to maximise the recovery of hammerscale, charcoal and other metallurgical residues though its cost-effectiveness on a range of site types has still to be demonstrated. If a magnet is run through the residues from sieving it is possible to remove the magnetic portion including hammerscale (Bayley et al. 2008). Using the distribution of hammerscale it is possible to identify the location of the smithy hearth and anvil (Mills & McDonnell 1992). The proportions of plate hammerscale to spheroidal hammerscale can be used to understand the nature of the iron-working operations on a site, the assumption being that spheroidal hammerscale is formed during primary smithing of blooms or during high-temperature welding operations. As a result, plate scale, formed during forging, is generally more heavily represented than spheroidal scale on sites where iron was worked rather than produced (Dungworth & Wilkes 2005).

• The main question asked of a slag assemblage is whether the industrial activity present indicates smelting, i.e. the production of iron, smithing to produce iron artefacts or a combination.
• If smelting evidence is present the next question to answer is what type of furnace or process was taking place. Slag is not datable, but consideration of the types which occur and their quantities may give some indication of the period. With prehistoric iron slag there can be difficulty in distinguishing smelting from smithing residues. However, in the Roman period and later, smelting slag are more readily distinguished, with tap-slag from bloomeries and glassy blast-furnace slag being characteristic.
• Often it is important to identify areas where the industrial activity is concentrated to identify the likely smelting furnace or the smithy. Examination of the slag assemblage, micro-residues, and context details should provide an indication of distribution across the site.
• Quantification of slag types could also provide information about the scale of the industrial activity, although chemical analysis of slag samples would also benefit this assessment.
• One question often raised is how the ironworking industry on the site relates to the settlement as a whole. By combining other artefactual evidence (particularly iron artefacts) it is possible to place the settlement into one of the three iron production models.
• Initial examination of the slag assemblage will also identify any further research questions and assess whether any further work needs to be carried out.

Like any other artefact type the slag residues should be examined and assessed. The cost of examination, classification and cataloguing of slag assemblages depends on several factors including assemblage size and site type. The approximate charge is £300 per 30kg of slag. Smaller and much larger quantities can be catered for. For a tailored quote for your site please send some site details (e.g. project design, post-excavation research design or excavation report), and the approximate quantities of material to eleanor.blakelock@archaeomaterials.co.uk. There may be discounts for charity/amateur groups or assemblages that fit within my research interest. Slag assemblages can either be sent to me for cataloguing or for large assemblages (or multiple assemblages) I can travel to the unit and do it in-house.

Bayley, J., Crossley, D. and Ponting, M. 2008. Metals and metalworking. A research framework for archaeometallurgy. London: Historical Metallurgy Society

Bayley, J., Dungworth, D. and Paynter, S. 2001. Centre for Archaeology guidelines: archaeometallurgy. London: English Heritage.

Cleere, H.F. 1972. The Classification of Early Iron-Smelting Furnaces. Antiquaries Journal 52: 8-23.

Dungworth, D. and Wilkes, R. 2005. Spherical Hammerscale and Experimental Blacksmithing. HMS Newsletter 59: 3.

McDonnell, G. 1987. The study of early iron smelting residues. In Scott, B.G., Cleere, H. and Tylecote, R.F. (eds) The crafts of the blacksmith: essays presented to R.F. Tylecote at the 1984 symposium of the UISPP Comitâe pour la Sidâerurgie ancienne. Belfast: UISPP Comitâe pour la Sidâerurgie ancienne. 47-52.

McDonnell, G. 1991. A model for the formation of smithing slags. Materialy Archeologiczne 26: 23-26.

Mills, A. and McDonnell, J.G. 1992. The identification and analysis of the hammerscale from Burton Dassett, Warwickshire. London: Ancient Monuments Laboratory Report 1992/47.

McDonnell, G., Blakelock, E.S., Rubinson, S.R., Chabot, N., Daoust, A.B. and Castagnino, V. 2012. The iron economy of Saxon Wharram Percy: modelling the Saxon iron working landscape. In Wrathmell, S. (ed.) Wharram. A study of Settlement on the Yorkshire Wolds, XIII. A History of Wharram Percy and its Neighbours. York: York University Archaeological Publications.

Tylecote, R.F. 1986. The prehistory of metallurgy in the British Isles. London: Institute of Metals.