MINERALOGICAL AND CHEMICAL CHARACTERIZATION OF ROMAN SLAG FROM THE ARCHAEOLOGICAL SITE OF CASTRA (AJDOVŠC ˇ INA, SLOVENIA)* S. KRAMAR, 1 † V. TRATNIK, 2 I. M. HROVATIN, 3 A. MLADENOVIC ´ , 1 H. PRISTACZ 4 and N. ROGAN ŠMUC 5 1 Slovenian National Building and Civil Engineering Institute, Dimicˇeva 12, 1000 Ljubljana, Slovenia 2 Institute for the Protection of Cultural Heritage of Slovenia, Centre for Preventive Archaeology, Poljanska 40, 1000 Ljubljana, Slovenia 3 Via Biancospino 30/3, 34151 Opicina (Trieste), Italy 4 University of Vienna, Institute of Crystallography and Mineralogy, Althanstrasse 14, 1090 Vienna, Austria 5 University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geology, Aškercˇeva 12, 1000 Ljubljana, Slovenia This study deals with the mineralogical and chemical characterization of archaeometallurgical material from the Roman archaeological site of Castra (Ajdovšcˇina, western Slovenia). Samples were initially analysed via optical microscopy and X-ray powder diffraction, with the composition of individual phases then determined using scanning electron microscopy coupled with electron-dispersive spectroscopy, and Raman microspectroscopy. Chemical investigation was carried out by using inductively coupled plasma atomic emission spectroscopy to measure the major element content. The results showed that the slag originated from iron smithing operations, with calcium-rich olivines, as well as wuestite and leucite, the most abundant phases. The investigated slags were also found to be characterized by significantly high lime levels, which probably promoted the formation of the calcium-rich olivines. KEYWORDS: ARCHAEOMETALLURGY, IRON SLAG, ROMAN SLAG, CALCIUM-RICH SLAG, CASTRA ARCHAEOLOGICAL SITE (SLOVENIA), RAMAN MICROSPECTROSCOPY INTRODUCTION By-products of the metallurgical process, slags are often the only relicts of ancient pyrometallurgy and as such represent a valuable source of information regarding ancient metal- lurgical technology and economy (Pleiner 2000, 2006; Buchwald 2005). Slags vary in size, shape and chemical/mineralogical composition as a consequence of the technological process involved (Bachmann 1982; Heimann et al. 2001; Manasse and Mellini 2002; Buchwald 2005; Paynter 2006), although two main types are generally distinguished: smelting slag, which represents the gangue material discarded from the ore, and smithing slag, which is formed in the smithing hearth. The range of both smithing and smelting technologies available results in the production of numerous types of silicate slag; however, the chemical/mineralogical compositions of smithing and smelting slags are often indistinguishable (McDonnell 1991). For instance, the composition and properties of smelting slags are influenced by the ore type, the fluxes added, the furnace construction material (lining), the use of charcoal and the cooling conditions (Bachmann *Received 27 June 2013; accepted 17 March 2014 †Corresponding author: email sabina.kramar@zag.si Archaeometry ••, •• (2014) ••–•• doi: 10.1111/arcm.12116 © 2014 University of Oxford