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.