Mineralogical Characterization of Slags from the Oiola Site (Biscay, Spain) to Assess the Development in Bloomery Iron Smelting Technology from the Roman Period to the Middle Ages

Abstract

The Oiola archeological site, located in the mining complex of La Arboleda (Biscay, North Spain) was an important iron smelting center from the Roman Period to the Middle Ages and even in more current times (19th–20th centuries). Tap-slags and some plano-convex slags were identified as smelting slags. Samples were analyzed by optical microscopy, X-ray powder diffraction, scanning electron microscopy coupled with electron-dispersive spectroscopy and Raman microspectroscopy to perform a mineralogical and textural characterization. Additionally, thermogravimetric and thermodiffraction analyses were carried out to determine furnace operating temperatures. The mineral assemblage reflects furnace temperatures and cooling rates and the addition of quartz as the main flux to decrease the melting temperature of the iron ore. The comparison of slags from the Roman Period and the Middle Age reveals changes in the pyrometallurgical process through time.

XRF and powder X-ray diffraction analysis of ancient iron slags from the “Sladak Kladenets” and “Malko Dryanovo” archaeological sites, Bulgaria

“…The fayalite formation in slags shows enough quartz in the furnace to provide its crystallization (Portillo-Blanco et al, 2020). The close concentration of Si in all studied samples (Table 1) suggests no flux was added in the furnace, as the ore was self-fluxing.…”

“…In reduction furnaces, iron oxides react with CO emanating from the carbon fuel (charcoal). The charcoal is also a reducing agent in the system (Portillo-Blanco et al, 2020). Flux (limestone, quartz sand, sandstone) is often added to furnaces to lower the temperature of the iron reduction process and increase the rate of iron extraction.…”

“…Flux (limestone, quartz sand, sandstone) is often added to furnaces to lower the temperature of the iron reduction process and increase the rate of iron extraction. The reduction of iron started from about 800 °C but never reached the temperature of iron melting (Portillo-Blanco et al, 2020). This method of iron production determines the mainly ferro-silicate composition of the slag, which is close to fayalite, formed by the reaction of FeO from the ore and Si from quartz (Friede et al, 1982;Portillo-Blanco et al, 2020).…”

“…The reduction of iron started from about 800 °C but never reached the temperature of iron melting (Portillo-Blanco et al, 2020). This method of iron production determines the mainly ferro-silicate composition of the slag, which is close to fayalite, formed by the reaction of FeO from the ore and Si from quartz (Friede et al, 1982;Portillo-Blanco et al, 2020). The slag phase composition was controlled by the raw ore composition, furnace lining, addition of flux, but also by the design of the furnacethe way of air supply.…”

“…In other constructions of furnaces for air supply ceramic tubes were inserted. This reduction method of iron extraction is known as the direct method of iron production (Portillo-Blanco et al, 2020;Roberts et al, 2014).…”