The Pietralba archaeological site (Pieve S. Stefano, Arezzo, Italy): a mineralogical and biochemical study of ancient manifactured stones

A mineralogical and biochemical study of some manufactured rocky block found in the archeological site of Pietralba (Pieve S.Stefano, AR, ITALY) has been carried out aiming to clarify the use of these enigmatic objects. According to the pottery found nearby the structures, these latter could be attributed to the Middle Age. Even though similar manufactured rocks have wide diffusion (e.g. Vicchio, Massa Marittima, Montefeltro; Cherici, 1989, 1990), their use and their age are still unclear though several hypotheses have been put forward. The most credited ones concern uses as water places, as plant-mills or for tanning. This research represents the first work trying to shed light on this “mystery” by using both traditional and innovative chemical methodologies. The Pietralba site is located on the right slope of the deeply incised valley of the Tignana Creek, a tributary of the Tiber River. Along this slope several blocks of sandstone occur as the product of ancient rockfalls. Some of these blocks are carved and despite they are highly weathered, the chisel signs are still visible. The blocks, clearly worked “in situ”, display different shapes resembling, for instance, a tub, the letter L, a throne, a pyramid, and were probably used for different purposes. In this work we focused on the tub and on the adjacent L-shaped carved block and report the chemical results obtained on the tub block). The tub, characterised by an open side downslope, has a depth of few decimetres; this feature suggests that only small volumes of unknown substances were temporarily stored in the tub. Beside this volume limitation, we argue that any stored substance may have had a significant economic value to justify its manipulation in a so remote location. Furthermore, the absence of any source of water at a suitable distance suggests that any treatment would be possible with few or no water solution. The location of the two handicrafts along the slope, with the L-shaped carved block resting downslope the tub, suggests that the hypothetical productive process exploited the morphologic gradient to drain fluid phases eventually extracted and accumulated in the tub. The present lack of physical connection between the two blocks may have provided during the productive operation by mobile and perishable (wood) linking artefacts. Rock samples collected in the tub were analysed in order to obtain the sandstone mineralogical (XRD analyses) and chemical composition (XRF analyses). Successively, analytical instruments such as Fourier Transform Infrared Spectroscopy (FTIR) and Gas Chromatography-Mass Spectrometry (GC-MS) were used with the aim of investigating the possible organic components. Very interestingly, oily organic substances have been found and, in particular, vegetable lipids, both diterpenic and triterpenic molecules which are often contained in the bark of plants. These results are in agreement with the hypothesis that the tub was used for maceration or beaten of cortex or berries, acorns and/or leaves of oak or chestnut. Furthermore, according to this hypothesis, is possible to comprise the importance of both the topographic position and morphology and the vegetable cover, besides to understand the characteristics of the handicrafts: resistant water tight tanks, located in a dry place, and capable of withstanding the weight of such a type of working. Work is still in progress and the chemical analyses will be performed on several rock samples. Even though preliminary, our data seem to suggest that the treatment of berries and/or other vegetable substances could have been carried out in the tub to obtain particular medicated ointments.