**2. History of Obsidian**

The modern word "obsidian" comes from the name of a Roman explorer, Obsius, who saw extensive geological quantities in Ethiopia [1], although obsidian sources on Italian

**Citation:** Tykot, R.H.

Non-Destructive pXRF on Prehistoric Obsidian Artifacts from the Central Mediterranean. *Appl. Sci.* **2021**, *11*, 7459. https://doi.org/10.3390/ app11167459

Academic Editor: Marco Martini

Received: 11 July 2021 Accepted: 11 August 2021 Published: 13 August 2021

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**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

islands, including Lipari and Sardinia and on the Greek island of Melos, had been used for thousands of years and were already well known from their wide usage.

Obsidian is an igneous glassy rock, formed during volcanic eruptions during the past 20 million years, which fractures conchoidally. It is typically an extrusive rock formed along the edges of viscous lava flows, in a volcanic dome, or when it cools while airborne. Some sources are intrusive, formed at the edges of dikes or sills. Obsidian may be found in many parts of the world, but only in certain geological regions where the magma composition was rhyolitic (or in some cases basaltic). Overall, the production of insufficient sizes to make stone tools (at least a few centimeters), the presence of gas vesicles trapped during formation (e.g., pumice), or the breakdown over time of unstable glass with phenocryst or spherulite crystalline formations limit the number of sources used in antiquity to certain mountainous terrestrial areas and volcanically formed islands (Figure 1).

**Figure 1.** Map showing many of the obsidian source locations used in antiquity.

Obsidian was widely used for cutting and scraping tools during the Old and New Stone Ages, starting in the Lower Paleolithic by early *Homo* (ca. 1.75 million years ago) and continuing until recently in some parts of the world [2]. Due to its higher level of sharpness, compared to chert (flint), quartzite, and other stone tool material, it was intentionally acquired, flaked to produce broken edges, transported, and traded over distances of 1000 km or more within the central Mediterranean [3]. The preparation of cores was generally done near the geological source, with blades and other tools produced by trained lithic workers at many of the distant archaeological sites (Figure 2). Findings of obsidian artifacts at mainland sites in Greece and Italy, from island geological sources in the Mediterranean, infers the development of maritime travel by the late Upper Paleolithic [4] of at least simple crafts, to more complex vessels by the Early Bronze Age (3rd millennium BCE); the earliest remains found in the Mediterranean of open-water boats (or drawings) are only more recent. Obsidian is still used today for surgical procedures (including eye and heart), due to its greater sharpness and faster healing process, compared to stainless steel scalpels (just search online for a commercial scalpel vendor). Obsidian was also used for polished mirror surfaces, and for jewelry in certain cultures even today.

**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).

In addition to the size of natural obsidian blocks, and the quantity produced in a geological source area, the visual characteristics of obsidian were important, especially when there was a variety in color, luster, transparency, and the presence of phenocrysts. Most obsidian is in the black-to-gray color range, but there are some sources with brown, tan, red, orange, yellow, or blue, often mixed with black, caused by some inclusions in the magma or by trace elements. The orientation of any patterns may be indicative of the geological formation process (e.g., lava flow).

The ability to identify the geological origin of obsidian artifacts found at archaeological sites allows the reconstruction of cultural interaction and trade patterns, including the likely movement of other materials (e.g., pottery, domesticated animals, clothing, food products) as well.

## **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

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 19th and early 20th 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].


**Table 1.** Analyses of obsidian artifacts in the central Mediterranean.

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 [11,13–15]. 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 [17–19], and especially portable XRF spectrometers, has revolutionized the analysis of obsidian in many parts of the world [20–26]. 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. After about 2005, commercially produced hand-held portable XRF (pXRF) instruments were available from several companies. Their small size makes them transportable in a backpack and as carry-on luggage on planes.

**Figure 3.** Most recent pXRF instrument (Bruker Tracer V*g*) used in these studies. Mounted upright using a home-made plastic stand.

In traditional XRF instruments, samples are placed within a vacuum chamber so that secondary X-rays are not absorbed prior to reaching the detector. However, a vacuum is not necessary for quantitative measurements of elements above potassium, so that obsidian objects need not be contained in a vacuum chamber for measurement of K-shell electron replacements of elements from Ca to La. For hand-held XRF instruments, 50 kV is the highest energy setting for primary X-rays, thus limiting K-line energy measurements of only a few elements in periodic table row 6. For pXRF analyses, elements specifically used for obsidian source identification include major elements Ca, Ti, Mn, Fe, and trace elements, including Rb, Sr, Y, Zr, and Nb (Figure 4). The use of a filter (12 mil Al, 1 mil Ti, 6 mil Cu) reduces the background for these elements, with detection limits for trace elements in single digit ppm [27,28].

**Figure 4.** K-line energy peaks for two obsidian samples. In blue is from Monte Arci (Sardinia) SC; note the high strontium, zirconium, and barium peaks when compared to SA (in red). The blue lines represent the multiple peaks for barium.

As with all XRF instruments, the specific energy peaks for different elements, especially the L-lines for elements in period table rows 6 and 7 (e.g., Cs, Ba, La, Ce, Nd, Th, U) overlap with the K-lines for lower Z elements, thus limiting precision for those trace elements with similar energy levels to major elements (e.g., Fe and below) when significantly present. Other elements, such as those used for compositional analysis of metals and other materials (Ti, Cr, Fe, Co, Ni, Cu, Zn, As, Pd, Ag, Cd, Sn, Sb, Pt, Au, Hg, and Pb), may also be detected, and other filters used to minimize background effects [29].

In most cases, the highly precise measurements of the K-lines of just a few of these trace elements is sufficient to distinguish obsidian sources in any part of the world, while multi-variate statistics of 5–7 trace elements may be used for identifying pottery production groups [27]. The beam size of the X-rays is typically about 5–8 mm in diameter, with some having options for smaller beams reaching the sample. The length of time necessary for analysis has decreased with newer pXRF models, which use silicon drift (SDD) rather than silicon PIN (Si-PIN) detectors; with the first Bruker Tracer model used in 2007–2012 (III-V+), each spot for trace element analysis was run for 180 s; for 2013–2016 (III-SD), 60–90 s; and for 2017 to the present (V*i*, V*g*), just 30 s. Running for longer times does not go beyond limitations of the detector and increase the precision or sensitivity limits. For homogenous obsidian, one analysis per artifact was sufficient and only in a few cases were reruns needed to confirm the results and source assignment. In some of those cases, the samples were of irregular shape, or were small bladelets just a few mm in width and fewer in thickness, with lower than usual total counts. Incorrect source assignment is avoided in such cases by using trace element ratios.

Museums and government agencies in many countries are open to international collaboration and access to their archaeological collections, with analytical research facilitated when done without necessary permissions for destructive sampling, and movement (of intact artifacts or samples) to laboratories, even within the same country. The portability, easy operation, and low maintenance for hand-held XRFs also enabled many archaeologists to conduct analyses themselves, without depending on a geoscientist or laboratory staff to prepare and run their samples. The homogeneity of obsidian, the relatively flat areas on stone tools that may be tested, and little if any surface weathering make it a perfect material for non-destructive X-ray fluorescence analysis.

For a number of years, however, there were concerns raised about the integrity of analyses being conducted and, in particular, the production of data with actual concentration values [23,30]. How do we deal without calibrated quantitative results for major elements silicon (typically 65–75% for SiO2 in obsidian), aluminum, sodium, potassium, and magnesium? How do we deal with matrix effects on secondary X-rays? What standard reference materials may be used for inter-laboratory comparisons? These issues were mostly for archaeology users who needed to compare their analytical data for artifacts with those of other scholars' analyses of geological source samples in their geographic region. Obsidian calibration software was developed and shared, however, by 2008 for the Bruker pXRF instruments, based on 40 geological obsidian samples analyzed by INAA, LA-ICP-MS, and XRF [22,31]. Separately, pXRF users who analyzed sufficient geological samples of known origin could also make a direct comparison of the uncalibrated raw data with that from the archaeological samples that they analyzed with the same instrument. Nevertheless, the use of calibrated data is expected for publications in many journals. For archaeological obsidian artifact studies, nearly all research is conducted by scholars with geological samples from their region of interest.

#### **4. Obsidian Sourcing in Europe and the Mediterranean**

There are four obsidian sources in the Central Mediterranean, from volcanic formations on the Italian islands of Sardinia [11,13,32–35], Palmarola [36], Lipari [37,38], and Pantelleria [3,39–42] (Figure 5). People with agriculture lived on both Sardinia and Lipari from the start of the Early Neolithic (ca. 6000 BCE), with the use of obsidian for stone tools starting at the same time. No one settled on the tiny island of Palmarola, while it appears that Pantelleria was not occupied until about 3000 BCE [43]. There is also Melos in the Aegean with two subsources [44], and one Carpathian source that was used, in southeastern Slovakia [45,46]. Geological surveys and analytical research were conducted on each, assessing the quantity and quality of obsidian from multiple outcrops and the ability to distinguish between these sources. For the four Italian islands, analyses were conducted by the author, using INAA, LA-ICP-MS, and EDS-XRF, defining multiple subsources for each island, which is important for the study of archaeological artifacts and our interpretation of prehistoric access and collection of obsidian [2,3,35,47].

Non-destructive analyses by pXRF of obsidian artifacts are also able to distinguish Monte Arci (Sardinia) subsources (Sardinia A, Sardinia B1, Sardinia B2, Sardinia C), as well as for Lipari (Gabellotto, Canneto Dentro, Monte Guardia) and Pantelleria (Lago di Venere 1 and 2, Balata dei Turchi) (Figures 6–9). At least with the elements currently measured and calibrated, we cannot distinguish the three Balata dei Turchi subgroups, nor the three subsources on Palmarola. Given the circumstances in which obsidian would have been obtained on these two islands during the Neolithic period, in particular by visitors rather than residents, these distinctions are not considered important.

**Figure 5.** Map showing obsidian sources in Europe and the Mediterranean.

**Figure 6.** Distinguishing European obsidian sources with selected trace elements. Not all geological samples tested are shown.

**Figure 7.** Separation of multiple subsources for Monte Arci (Sardinia) using a selection of geological samples collected by the author.

**Figure 8.** Two prehistoric Lipari obsidian groups used for tools, using geological samples collected by the author.

**Figure 9.** Pantelleria is separated by Balata dei Turchi (BdT), and both Lago di Venere (LdV) 1 and 2.

#### **5. Applications of pXRF on Obsidian Artifacts from Central Mediterranean Sites**

With a non-destructive pXRF, the author expanded his research on obsidian, from a focus on Sardinia and Corsica [13,33,42,48–50] to throughout Italy as well as to Malta and Croatia [3,51–54]. In the past ten years, more than 12,000 artifacts have been analyzed, with ≥ 25 from each of the > 100 archaeological sites (Figure 10).

## *5.1. Sardinian Obsidian in Continental Italy*

The transportation and trade of obsidian from Monte Arci in Sardinia to Corsica and beyond was realized in the early study by Hallam et al. [7], which identified it at several sites in Southern France and Northern Italy. Since then, Monte Arci obsidian has been identified at many sites in Northern Italy, which also is a great distance from the other central Mediterranean sources, with changes over time in proportion to Lipari obsidian [55,56]. In addition, social network analysis supports hypotheses of different obsidian pathways, including open-water north-bound from Corsica to Southern France [57]. With the very large number of non-destructive analyses conducted in this millennium, we now see that Sardinian obsidian also reached central Italy as a significant percentage of assemblages and made its way to southernmost Italy and even Sicily in very small numbers, supporting an interpretation of a down-the-line type exchange during the Neolithic [3]. A total of just 23 artifacts of Sardinian obsidian (out of nearly 1300 obsidian artifacts analyzed) were identified at sites south of Rome, indicating that travel routes were not directly across the Tyrrhenian, but from Sardinia to Corsica, then through the Tuscan archipelago and southward, on or along the Italian peninsula (Figure 11). Undoubtedly, other materials were exchanged and traveled in opposite directions, including domesticated animals (sheep, goat, cattle, pig), produce (from wheat, barley, other plants), clothing, tools, wood, etc. [58].

**Figure 10.** Map of central Mediterranean showing archaeological sites with ≥10 source analyses of obsidian artifacts. Sites in red analyzed by the author.

**Figure 11.** Map with sites in southern Italy with Sardinia obsidian artifacts. From north to south, in red circles: Poggio Olivastro 20/100 (20%); Casale del Dolce 1/35 (3%); Venafro 1/132 (<1%); Pulo di Molfetta 1/37 (2.7%); M. Di Gioia 12/12 (100%); Ausino 3/21 (14%); Saracena 4/842 (<1%); Bova Marina 1/200 (<1%); Valdesi 1/41 (2.5%).
