# Non-Destructive pXRF on Prehistoric Obsidian Artifacts from the Central Mediterranean
## 3 Analytical Methods  
The chemical analysis of obsidian to look at trace elements began in the early 1960s with the use of optical emission spectrometry, with simple X-Y graphs, such as barium vs. zirconium, distinguishing many sources in the Mediterranean and Near East [5]. By the mid-1970s, additional methods, including X-ray fluorescence (XRF) spectrometry and instrumental neutron activation analysis (INAA), were able to produce even more distinctive results by using many trace elements with excellent precision [6,7]. In addition, fission-track dating was also used, discriminating the limited numbers of obsidian sources by differences in their formation ages [8]. During the 1980s, major and minor elements were also shown to be successful in distinguishing obsidian sources, using atomic absorption spectroscopy (AAS) [9] and scanning electron microscopy with an energy-dispersive  
Figure 2: Example of Neolithic obsidian blade tools from Sicily. Scale in centimeters. Most stone tools would have been mounted on wood or bone handles (rarely preserved). spectrometer (SEM-EDS) [10]. The homogeneity in the composition of molten lava, and the rapid formation of glassy obsidian, have led to significant differences between sources in composition for many major and trace elements. This allows a choice of simple X-Y graphs of a few elements to distinguish source groups. Despite the development of these many methods of successful analyses by the 1980s, little detailed research had been done on the geological obsidian sources from an archaeological perspective, particularly that discriminating between multiple subsources of usable obsidian within each island. The total number of central Mediterranean obsidian artifacts that had been analyzed in the 1960s and 1970s was less than 200, and by the end of the 1980s, this had reached only about 500 total (see table IX, pp. 66-68 in [11]). Many of those artifacts tested had come from museum collections dating back to the late 19 th and early 20 th centuries, not having archaeological contexts or chronology. Even by 1995, the total analyzed was fewer than 900, with only 26 sites having 10 or more artifact analyses and allowing statistical comparisons (Table 1). Nevertheless, a lot had been learned about obsidian trade by that time [12]. In the 1990s, however, the introduction of inductively coupled plasma (ICP), optical emission spectrometry (OES), and mass spectrometry (MS), often with laser ablation (LA), allowed the minimally destructive analysis of artifacts and increased the numbers tested [11, 13, 14, 15]. XRF also developed further, with some instruments allowing minimally destructive (or even non-destructive) analyses. The use of mounted microsamples of 1-2 mm wide solid pieces of obsidian, with as many as 16 samples on a single 1-inch disk, was developed for electron probe microanalysis (EPMA) for this reason [11, 13]. Similar analyses were conducted using a scanning electron microscope (SEM-EDS) on obsidian artifacts [16]. Overall, the use of automated analyses also reduced costs, which are always a limiting factor for archaeological studies. By 2010, more than 3000 obsidian artifacts in the central Mediterranean had been analyzed [13]. Starting in this millennium, the creation of desktop, non-destructive XRF analysis instruments [17, 18, 19], and especially portable XRF spectrometers, has revolutionized the analysis of obsidian in many parts of the world [20, 21, 22, 23, 24, 25, 26]. The homogeneity of obsidian and its high resistance to weathering are part of the success of non-destructive surface analyses. The pXRF may simply be placed adjacent to the cleaned object in the field, running on batteries and using a built-in computer, or within museums or other facilities (Figure 3). The main reasons for initial commercial production of pXRFs was for businesses and agencies with applications such as field testing of soils near factories; however, its utility for analyzing art and archaeology objects represented an academic market, too.