Some slags can be very similar in appearance to certain rocks or other archaeological waste materials, such as over-fired ceramic, burnt brick, etc. Bulk chemical analysis is normally sufficient to identify metallurgical slag. Most shaft furnace slag is characterized by iron oxide levels between 40 and 80 wt.%, mostly less than 50wt.% and often less than 30 wt.% silica, and less than 10 wt.% alumina. Geological or clay-based materials have typically less than 20 wt.% iron oxide, more than 10 wt.% alumina, more than 50 wt.% silica, and higher levels of alkali and alkali earth oxides than slags. Glassy blast furnace slags may not fall within these brackets but have much lower iron oxide and higher silica concentrations; these are typically identified as metallurgical slag by their elevated base metal content and often lime-rich bulk composition, when compared to geological or clay-based materials.
Almost all pre-modern iron production is based on the bloomery process which produces a slag with 50-70wt.% combined iron and manganese oxide, and base metal concentrations of less than 0.1 wt.%. The main slag phase crystallizing from these slags is fayalite, Fe2SiO4. Minor oxides and trace elements, such as lime, titania, phosphate, arsenic, and nickel, can be indicative of specific ore types, but can also derive from fuel ash or absorbed furnace wall material; these need to be considered when interpreting the analytical data.
Most early copper slag is also fayalitic and can appear very similar to iron smelting slag, but contains higher levels of copper and other base metals, up to a few percent by weight. Further discriminating criteria are the redox condition as preserved in the oxidation state of iron (metallic, ferrous, or ferric), with copper slag being less reducing, and having higher sulfur levels if a sulfidic ore was smelted. The redox conditions in these iron-rich slags are best determined through RLM of the free iron oxide phase, wuestite (‘FeO’) in iron slag or magnetite (Fe3O4) in copper slag.
Lead and tin slags are often glassy and mostly characterized by elevated levels of their respective metal, often present also as metallic inclusions. Lead slags in particular can be very variable in their chemistry and mineralogy, ranging from fayalitic to extremely rich in barium oxide, calcium fluoride or oxide, and lead oxide, reflecting the wide range of gangue minerals associated with lead ores. A crucial question for lead slags is whether the smelting was for lead or for the silver associated with many lead ores; no clear criteria have yet been developed for this distinction.
Most secondary metallurgical processes leave their own specific waste materials; their comprehensive treatment however is beyond the scope of this article. They are on average richer in the metal(s) processed than the average smelting slag. The differentiation between iron smithing and smelting slag by chemical and mineralogical methods can be difficult for individual pieces, but morphological criteria together with the archaeological context often allow reasonably safe identification. The close association of most nonferrous secondary production waste with ceramics, typically crucibles, has already been mentioned. Only in very early periods (Early Bronze Age I and earlier in the Old World) was copper smelted in crucibles; copper-rich crucible slag in all later periods is almost certainly from secondary processes, such as alloying, casting, or refining.
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