Chemical and textural investigations on medieval slags from “ex-Monte di Pietà” archaeological site (Emilia-Romagna, Italy)

The aim of this work is to report chemical and textural analyses on archaeometallurgical slags from the medieval Forlì in north Italy. Between February 2004 and August 2005 the “Cassa dei Risparmi di Forlì” Foundation successfully exploited the “Ex-Monte di Pietà” archaeological site. Twenty-two slag specimens were collected from ancient rubbish dumps that accumulated scraps from craftsmen workshops over about four centuries (Guarnieri, 2009). Slags were analysed by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy and energy dispersive Xray spectrometry (SEM/EDS). The obtained results highlighted slags deriving from the production of both iron and copper; moreover, severely corroded iron nails was identified. In this paper, the chemical and textural characterisations of ferrous slags and nails will be discussed. Ferrous slags were glassy in aspect with quartz and charcoal fragments embedded into the matrix. The matrix was mostly formed by silicon, iron, calcium and aluminium; other elements that do not enter crystalline silicates, i.e. potassium from the charcoal and sulphur from the mineral charge were also detected. The most abundant phases in the glassy matrix were monticellite, wollastonite, fayalite, wüstite, sulphides, metals and metal alloys. The minerals probably underwent a roasting prior to smelting; moreover, melting agent was added to the mineral charge. Metallographic observations of the olivine habit and size distribution have provided information on the cooling process. In particular, olivines ranged from polyhedral-granular to acicular indicating a cooling rate of the melt lower than 60 °C/h (Ettler et al., 2009) The slag also contained large amounts of dendritic iron oxides as could be expected from the iron-rich bulk chemical compositions. Nails consisted of an iron-rich core about 0.5 cm in diameter. The elemental composition well evidenced the structure of the core, where crystallised magnetite was admixed with a greater amount of crystalline maghemite. The core lied beneath ferrous oxides, ferrous hydroxides and silicates/phosphates-enriched layers probably due to fluctuations in piezometric level of the acquifer. XRD analyses enabled wustite, goethite, ferrosilite, lazulite and quartz to be distinguished (Booth et al., 1962).