## 1 Introduction  
Obsidian is a volcanic glass, naturally occurring when lava extruded from an eruption cools rapidly with minimal crystal growth, and it is usually related to volcanic regions of young geological age. In particular conditions, the lava silicate ions cannot reach the ordered formation of a crystalline lattice and assume a chaotic arrangement, giving origin to obsidian flows. The sources of this material are few and well known, and obsidian artifacts, also due to their resistance to environmental deterioration, are considered some of the earliest evidence of commerce [1]. This is especially true for those periods which predate the manufacture of pottery, the material of choice for many archaeological studies due to its durability and diffusion. Actually, obsidian has been known since antiquite times, and it was already used at the end of the Paleolithic period to make knives and blades because of its durability. In the Neolithic period, its trade played an important role in the Mediterranean area: samples of polished obsidian have been found hundreds of kilometers from extraction sites, thus being of particular interest for tracing prehistoric trading patterns. Indeed, many published works about obsidian provenance refer to the Mediterranean basin [2; 3; 4; 5; 6; 7; 8; 9], as this geographic area was the most important supplier in ancient times, even if sources were present also in the Carpathians, Anatolia, the Caucasus, and the Arabia peninsula. Obsidian that was used for artifact production is usually chemically homogeneous both in its major and trace elements, and it is a good representative of the original source. Characterizing the sources thus allows to study the provenance of the artifacts. The primary constituents in obsidian are SiO\({}_{2}\) (up to 80%), Al\({}_{2}\)O\({}_{3}\) (up to 15%), Na\({}_{2}\)O, and K\({}_{2}\)O (up to 5%) together with many minor and trace elements which largely vary from one source to another [10]. When dealing with a high number of geological obsidian samples, major and trace element detection and quantification can even distinguish specific sub-sources [11; 12], and this permits archaeological considerations about access to obsidian sources, territorial control, craft specialization, and mobility of populations. Several chemical analyses have already proved their ability to distinguish sources of obsidian since the first studies in the 1960s [13; 14; 15; 16; 17; 18]: the earliest measurements used emission spectrometry [19], atomic absorption spectrometry (AAS), flame photometry [20], and proton inelastic scattering [21]. Later, laser ablation-inductively coupled plasma mass spectrometry (La-ICP-MS), particle-induced X emission (PIXE) [4; 22], and energy-dispersive spectroscopy with a scanning electron microscope (SEM-EDS) were applied [23] as well. One of the most used techniques is still neutron activation analysis (NAA) [2], a nuclear technique that has sometimes been coupled with particle-induced gamma emission (PIGE) [18] or X-ray fluorescence (XRF) [23]. This peculiar analytical technique guarantees high precision and a good range of elements detected, but it requires the irradiation of the samples with a neutron flux. Throughout the years, Raman micro-spectroscopy [22; 24], Mossbauer spectroscopy [15], and optical emission spectroscopy (OES) [19] were also used for the same purpose. Among the techniques applied for obsidian provenance studies, energy-dispersive X-ray fluorescence (ED-XRF) plays a leading role [5; 25; 26; 27; 28; 29; 30; 31; 32], as it in principle allows the performance of element quantification in a non-destructive manner with relatively low detection limits on samples of any size. Moreover, both portable and laboratory equipment are available for elemental quantification. Regardless, due to suboptimal detection ability for low Z elements, specific calculation and calibration software are required, resulting in the necessity of customizing data elaboration [10; 33].