Rehren, T; (2008) Metals: Primary production studies of. In: Pearsall, D, (ed.) Encyclopedia of Archaeology. (pp. 1616-1620). Elsevier: Netherlands.

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Abstract

The origin of many metals was from natural occurrences but the supply of metals increased dramatically with the inception and spread of mining and extractive metallurgy, as did the quantity of waste or slag. This article considers the analysis of waste material from primary production sites of the metals and alloys known before c. AD 1500. There are three main types of evidence left by metallurgical activity: metal objects and waste metal; slag and other associated products; the remains of installations such as furnaces and hearths. Of these, slags are typically the best-preserved, most abundant and most informative. Slags can provide information on metallurgical processes and ore types, production technologies and scale of production; slags contain gangue components and components of the rich mineral and therefore give a more complete representation of the ore body than the smelted metal. This article therefore focuses on slag. Three research fields are identified: the identification of the type of metallurgical process, metal and ore type smelted or worked at a given site; the identification of the temperature and redox condition of the process; and the elucidation of the scale of the operation at a given workshop or smelting site. Bulk chemical analysis such as XRF or ICP can be used to identify a material as metallurgical slag. For example, shaft furnace slag has characteristic oxide levels of iron oxide (40-80 wt%), silica (<50 wt%) and alumina (<10 wt%), whereas glassy blast furnace slags are typically identified by their elevated lime and silica content, and lower iron oxide. A particular problem with lead slags is to identify whether the smelting was for lead or the silver associated with the ore. The best method for estimating process temperatures depends on the complexity of the system. For a relatively simple system (three or four major oxides add up to 95 wt%) the bulk composition can be compared to the appropriate phase diagram. For more complex systems, experimental determinations of melting temperatures are more reliable. Redox conditions can be determined through a combination of Moessbauer spectrometry and microscopic analysis. If full recovery - or at least full quantification - of slag is possible at a site then it is possible to make an estimate of the amount of metal worked. With knowledge of the average ore composition and average slag composition it is possible to estimate the amount of metal produced per unit of slag. The article emphasizes that a good knowledge of ore geology, goechemistry, metallurgy and petrology are essential for accurate interpretation of the evidence and that data from certified reference materials should be published alongside the unknown samples, in order to be able to compare data from different laboratories.

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