The application of non-destructive techniques for the study of corrosion patinas of ten Roman silver coins: The case of the medieval Grosso Romanino
Introduction
The study of ancient coins involves aspects related to minting technologies, economic history and archeology of a specific historic period [[1], [2], [3], [4]].
The knowledge of the alloy composition is important to reconstruct the monetary policies and to understand the relationship between political and economic events.
Moreover, the identification of the corrosion products can reveal the conditions of burial processes and provides indications about restoration and conservation [2,3,[5], [6], [7], [8]].
The aim of this research is to investigate the near-surface composition and the corrosion patina of medieval Roman coins called Grosso Romanino. This denomination is of particular relevance as it was minted in one of the few periods in which the popes lost the control of Rome and other institutions tried to govern the city. Indeed, the Grosso Romanino was minted by Senatus Romanus and not by pope [9,10] as other denominations.
The original composition and technology of production of the Grosso Romanino are poor known as the official documents of the Mint were completely lost and the issue is little know [9,11]. The Grosso Romanino is made of a Ag-Cu alloy and, as often happens for metallic artifacts, it probably has a more or less homogeneous core and a surface with different composition [12]. Indeed, it is well known that Cu may be depleted from coin surface, depending on production techniques, severe environmental conditions, corrosion process and so on, occurring during the use of coin and conservation treatments [13,14]. All these conditions can modify the superficial Cu content and obstruct the reconstruction of the original composition of the alloy. Numerous studies have been made on ancient silver coins, using both destructive [7,8,15] and non-destructive techniques [3,[16], [17], [18]]. In this work, non-destructive techniques were applied to provide information on production technology and surface conditions. During analytical process, we aim to preserve the symbolic and historic value of the coins.
The coins were characterized by a combined use of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM-EDAX), Raman spectroscopy and voltammetry of microparticles (VIMP). XRD and SEM-EDAX techniques allowed providing information on the nature of the alloy in the external surface of the sample without any manipulation. In particular, the application of SEM investigation coupled with EDAX is a method for the study of metal artifacts, useful to investigate the chemical composition and microstructure of the surface, providing morphological details that allows to explore the technological production of the coin [5,14,15,19].
Moreover μ-Raman spectroscopy and XRD were used to obtain mineralogical information about new-formed phases of corrosion [14,15,[19], [20], [21], [22]]. All Grosso samples were also analysed using VIMP, a technique developed by Scholz et al. [23,24] to investigate the electrochemistry of minerals. This methodology was then refined and extensively applied by Doménech-Carbò et al. [[25], [26], [27], [28], [29], [30], [31]] in conservation and archaeometric studies. This technique allows obtaining an electrochemical fingerprint of the patina composition due to the voltammetric peaks of each compound. Moreover, we applied VIMP to characterize the composition of the surface of the coins.
Section snippets
Historical context
In 1252 CE Brancaleone D'Andalò became civic magistrate of Rome in a period of violent clashes inside the society. With the help of the Senatus Romanus, Brancaleone D'Andalò challenged the power of the higher nobles and the pope. And worked for the restoration of the city to its former position of power.
To achieve this aim, Brancaleone introduced the Grosso Romanino, a new silver-rich denomination, of high quality. Grierson [9] reported that this denomination was minted by Senatus Romanus for
Material and methods
Ten silver Grosso Romanino coins belonging to Private Collections (Fig. 1) were analysed. Numismatic analysis shows that these coins were minted in different years, in a period from 1252 CE to the end of 14th century.
Commonly, the observe of Grosso Romanino shows in the centre a lion walking to the right and along the edge a circular inscription with the minting institution, i.e., Senatus P.Q.R (or S.P.Q.R. or other formulas with similar meaning). The reverse of Grosso Romanino presents a
XRD and Raman spectroscopy
A representative number of samples were analysed by μ-Raman Spectroscopy and XRD aiming to explore the nature of the corrosion products. Results are displayed in Table 2 with an estimation of the abundance of the different mineral phases. The analysis was conducted both in the corrosion layers and in the patina free areas.
The acquisition of Raman spectra was problematic, probably due to the thermal sensitivity and fluorescence of the Ag-Cu alloys and corrosion products [40].
XRD spectra allow
Conclusion
Archaeometric analyses by means of micro-Raman spectroscopy, XRD, SEM-EDAX and VIMP techniques allow characterizing the external surface of ten silver Grosso Romanino coins, belonging to Private Collections and for which any invasive techniques were applied.
The multi-analytical approach permits to solve archaeological problems, concerning the composition of the alloy, and identifying the corrosion products, as well as to hypothesize the application of methods for Ag enrichments in the surface.
Acknowledgements and authors contributions
Financial support was provided by Sapienza University of Rome (Ateneo funding, 2014–16). Prof. F. Mazzei and Prof. F. Favero are thanked for the availability of electrochemical instrumentations whose analysis has been carried out in the Biosensors Laboratory of Chemistry and Drugs Technologies Department of Sapienza University of Rome.
L.F. performed and interpreted XRD data. L.M. acquired and interpreted micro-Raman data. F.D. acquired and interpreted electrochemical data. F.C. acquired the
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