The production and working of metal is controlled by two main factors: technical constraints and cultural traditions. While there are certain fixed physicochemical conditions to be met for specific metallurgical operations such as smelting, alloying, refining, casting, and recycling, there are also many different configurations which may fulfill these conditions. The composition and quantity of the resulting materials, primarily metal and slag, reflect both factors. It is by identifying the fixed physico-chemical constraints that the culturally determined configurational factors can be revealed, producing archaeologically relevant information.
The most basic reason for slag analysis is to identify the type of metallurgical process, metal and ore type smelted or worked at a given site. This is mostly done through chemical analysis. Ore deposits comprise two complementary materials: the rich mineral and the gangue or host rock. Ore beneficiation mechanically separates the rich mineral from the gangue. Smelting then extracts the metal from the rich mineral through a series of chemical reactions while transforming remaining gangue to form the slag. In this process, unwanted components of the rich mineral are either transferred into the slag or escape as volatile components with the fumes. The type of rich mineral, such as oxidic, sulfidic, or complex, is broadly reflected in the composition of the smelted metal. The slag, however, contains all the gangue components as well as components of the rich mineral, effectively giving a more complete representation of the ore body. This picture is complicated through the addition, conscious or not, of further material to the slag, such as fluxes, eroded furnace wall material, and fuel ash. Alloying, refining, casting, and recycling all produce their own compositionally distinct types of waste material, typically in much lower quantities than smelting and often in close relationship to technical ceramics such as crucibles and hearths.
The second reason is to identify the nature of the operation. Metallurgical processes require elevated temperatures, typically in the range of 800-1400 °C, and a wide spectrum of redox conditions, spanning from highly oxidizing to strongly reducing. Each metallurgical process has its own characteristic combination of temperature and redox condition. Neither can be determined directly, but find their direct expression in the mineralogical makeup of the slag. Identifying these parameters is crucial for the basic identification of the process, as well as for identifying its particular configurational aspects, and relies heavily on mineralogical analysis.
Finally, the production remains are often well preserved and the best available indicator for the scale of operation of a given workshop or smelting site. Careful determination of total slag quantity and composition, in combination with an assumed or directly determined ore quality, can provide good estimates of metal production quantities by using mass balance calculations. Similar estimates can be made for workshop remains such as crucibles or smithing debris; quantities can be determined either for a site overall, or on an average annual basis if the lifespan of the site or workshop is known. Quantification is crucial for discussions of subsistence or surplus production, craft specialization, and trade relationships.
Of these three research fields, the first two are predominantly descriptive and require a good knowledge of ore geology, geochemistry, metallurgy, and petrology for their interpretation. The third is often speculative and based on assumptions concerning overall preservation or recovery rates of waste material, and the average or typical quality of ore or metal for which often no reliable data is available.
World History
