Ore and Smelting Slag MARCOS MARTINÓN-TORRES University of Cambridge, UK Ores and smelting slag are, respectively, the main raw materials and by-products of metallurgy. As such, they are one of the major research foci of archaeometallurgical studies. Te analysis of archaeological ores and slag can yield information about the metals being produced and underlying engineering parameters such as temperatures and redox conditions, the selection and manipulation of natural materials, and the efciency of ancient technologies. Trough large-sample comparative approaches it is possible to use slag analyses to reconstruct knowledge transmission, technolog- ical traditions, and their evolution in time and space in response to diferent environmental and cultural constraints. With comparatively limited curatorial concerns impeding scientifc analyses, ores and slag can be among the most revealing material categories in the archaeological record. Teir study has progressively become mainstream since the 1980s, afer decades of neglect owing to their “industrial” nature and allegedly limited museological potential. Ores are rocks that contain metals in such condition and quantity that they can be extracted in a cost-efective manner. Given the fuctuations in technology and in the value of metals in time and space, the attribution of a rock or geological deposit to the “ore” category is culturally contin- gent. Metal-bearing minerals are typically dense and colorful, and as such they have also been used as pigments and medicines or for lapidary work. In these nonmetallurgical contexts, the use of the term “ore” can be misleading and should be avoided. Tere are as many ore types as there are rock types, each with their own constraints and poten- tials. For example, sedimentary ores (hydroxides and carbonates) tend to be near the surface and easier to exploit but smaller, whereas massive hydrothermal sulfde deposits are typically larger Te Encyclopedia of Archaeological Sciences. Edited by Sandra L. López Varela. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc. DOI: 10.1002/9781119188230.saseas0426 but require higher investment and technological knowledge for mining and smelting. Te analysis of ores in archaeology ofen seeks to identify the nature of the rocks exploited, their grade (i.e., richness in metal content), and the types of metal-bearing (ore proper) and nonmetal-bearing (gangue) minerals that are geologically associated. Tese analyses can include a combination of mineralogical (e.g., X-ray difraction (XRD)), chemical (e.g., X-ray fuorescence (XRF), inductively coupled plasma (ICP)), and microstructural analyses (i.e., by microscopy), together with a broader assessment of the geological context. In addition, ore analyses are useful to facilitate mass balance calculations (see below). Teir chemical and isotopic compo- sition can be used as a reference for provenance studies, providing a signature against which those of slag or metals may be compared. A key challenge for ore analyses in archaeol- ogy is that of sample representativity. Ofen, the richest ore deposits will have been exhausted in the past, and hence those lef behind at mines or geological formations are not representa- tive of those that would have been used by the communities being studied. When broken ore fragments are found at archaeological sites, one can be more certain that these were extracted by past people, but the fact that they were discarded rather than smelted may be due to their not being of the desired standard; thus, they may not be representative as samples for grade assessment or mass balance calculations. Tis is in addition to problems such as the natural variability of geolog- ical deposits, meaning that ofen many samples are required to obtain a reliable characteriza- tion. Modern economic geology maps can be useful to identify potential ores used in the past, but archaeologists should be aware that these are likely incomplete: small and rich deposits exhausted in the past and/or lacking economic value in the present (and hence not included in modern maps) may be archaeologically relevant. Mining was usually followed by benefciation, which involved the mechanical preparation of the ore by crushing and sorting according to