Non-invasive analysis of samian pottery assemblages from frontier contexts: The benefits and limitations of portable X-ray fluorescence

We report on the applicability of portable X-ray fluorescence (pXRF) as a rapid, in situ , cost-effective and non-invasive analytical technique to identify sources in Gaul of samian pottery ( terra sigillata ) from fifteen Roman military sites and one Iron Age hillfort in Scotland and South Shields Roman fort, northern England, using data gathered from three phases of investigation. Building upon the considerable samian composition database created by others using destructive techniques, notably WD-XRF, we systematically tested the viability of pXRF on this material using both published collections independently identified through traditional methods and less well understood or unpublished assemblages; over three hundred sherds were analysed. The combination of its fine texture, standardised production from spatially separate regions and recovery in plentiful quantities makes samian ware an excellent candidate for pXRF analysis. Recognising some limitations, largely associated with the semiquantitative character of the data, we conclude that pXRF is effective in assigning a common, if not consistently defined, regional production source to samian sherds based upon chemical composition. Products of Central Gaul, principally the Lezoux workshop are the most reliably identified, followed by probably two workshops in South Gaul: La Graufesenque and Montans; Rheinzabern and other workshops in East Gaul are poorly represented. The ramifications are important insofar as they are not reliant on traditional criteria. We establish a methodological framework that will prove invaluable to researchers and commercial units in view of the diminishing number of specialists with expertise in this important class of material.