The Use of Hand-Held XRF for Investigating the Composition and Corrosion of Roman Copper-Alloyed Artefacts

Article excerpt

Authors: Ricardo Fernandes (corresponding author) [1,2]; Bertil JH van Os [3]; Hans DJ Huisman [3,4]

Introduction

Hand Held X-ray fluorescence devices (HH-XRF) and other portable XRF (pXRF) devices are regularly used in industry and are gradually being introduced also for archaeological/historical applications [1, 2, 3, 4, 5]. Within archaeometric research, previous studies have also employed pXRF devices in the measurement of the elemental composition of bulk and corrosion layers of copper-alloyed artefacts [1, 4]. Given that this technique has the potential to provide high precision and fast results, allowing for non-destructive measurements to be made in situ, it becomes ideal for many archaeological applications.

In this study, a set of copper-alloyed archaeological artefacts from the Roman Limes military fort of Fectio, in the vicinity of Vechten (The Netherlands), were analysed using a HH-XRF device. The artefacts were recovered during a metal detector survey. The preservation status of the artefacts was established in two ways; macroscopically, by describing and classifying visible evidence for surface damage, and chemically by comparing elemental content (Cu, Pb, Sn, Zn) of each object?s corroded surface and uncorroded core. These criteria provide information on two different corrosion effects. The first, surface damage represents a semi-qualitative criterion but that provides relevant information for archaeological research. The second, elemental variability is a quantitative criterion although not necessarily linked with loss of archaeological information.

The main goal of the present study was to provide an illustrative application of the use of a HH-XRF device in an archaeological context. The material selected for analysis contributes to the existing knowledge on the composition of Roman artefacts from a military Limes site occupied between the 1[sup.st] and 3[sup.rd] centuries CE. Finally, a simple assessment was made of the relationship between the preservation status of collected artefacts and observed variations in elemental composition due to the formation of corrosion layers.

Materials and methods

Artefacts

Metallic artefacts, located using a metal detector, were collected from the top 30 cm of soil at the ancient site of Fectio; a Roman military settlement in the vicinity of modern day Vechten located approximately 5 km to the southeast of the city of Utrecht in The Netherlands [6, 7, 8]. The settlement was established during the Augustan period on the southern bank of a former Rhine bed, probably close to the spot where the river Vecht diverged from it [7, 9]. The site was abandoned in the 3[sup.rd] century CE when the river channel silted up [10].

The site had previously been the target of a baseline study to establish the preservation of copper-alloyed artefacts and its relationship with soil aggressiveness parameters [11]. The soil at the site is a thick and fairly impermeable clayey anthropogenic soil rich in lime and organic matter known in the Dutch soil classification as a ?tuineerdgrond? (loosely translatable as garden plaggen soil). The topsoil consists of loam and sandy or silty clay. Due to intensive bioturbation and anthropogenic mixing, the profile is fairly homogeneous, with a black to black brown colour [11].

A total of 61 copper-alloyed artefacts were collected from the site (Additional file 1). The majority of the collected artefacts were assigned to the Roman period (44). However, some more recent objects (e.g. brass buckshot shell casings) were also collected. Although some easily recognizable artefacts were found (Figure 1), most of collected objects were in a fragmentary state. The results here presented refer only to the archaeological artefacts assigned to the Roman period.

Additional file 1: Chemical composition and preservation status of copper-alloyed artefacts collected at Vechten.

Figure 1: Examples of metallic artefacts located at Vechten. …