Is it possible to integrate the p-XRF data from several sources? Method application to large collections of Iron Age glass

The INGOT-EL project (INvestigation of Glass Origin and Technology in Etruscan Lands) aims to bring new evidence on glass production and circulation by analysing a large set of glass beads uncovered in many Iron Age burials in central Italy. One of the first steps of the investigation accounts for a non-invasive in-situ approach with portable- X-Ray Fluorescence (p-XRF) equipment operating in several archaeological museums involved in the research. The aim of this preliminary step is to obtain a first overview on the elemental composition of the beads and guide the selection of representative samples that will be included in the following analytical steps with micro-invasive techniques, overcoming some limitations of the p-XRF approach such as the lack of data on light elements. The p-XRF survey is therefore crucial to enhance the sustainability of archaeometric investigations in large archaeological collections, as the preliminary non-invasive step reduces the number of the samples that will be subjected to micro-invasive approaches, without reducing the quality of the overall information. It is in fact mandatory that analytical procedures guarantee the general accuracy of the compositional data, so that they can be treated (and re-treated in the future) in the broader perspective of the archaeometric investigation of glass. There are many factors affecting the results of p-XRF analyses, therefore comparing data obtained with different spectrometers is far from obvious [1]. Configuration of the equipment (in terms of energy of the primary radiation and the angles of take-off and incidence), density of the analysed material and energy of the element's characteristic fluorescent radiation may influence the intensity of the signals and, therefore, lead to case-study dependant data, without a possibility for comparison with other datasets. To manage these issues, we performed a dedicated study using four p-XRF spectrometers with different sources of incident radiation, geometry of measurement and analytical spot sizes. Data were systematically collected from Corning glass reference materials that represent different compositional groups of historic glasses and several archaeological glass pieces with known composition. All spectra were treated with the free XRF toolkit PyMca [2]. The procedures for data treatment are presented and discussed in terms of statistical bias and precision of the data. Three of the considered spectrometers – which allow the analyses of small areas thanks to their focussed incident ray - have been then employed in the museums involved in the INGOT-EL project to analyse archaeological glass beads.