A combined experimental approach to the study of ancient coins and its application: the Venetian “sesino”

Sesino Doge Priuli rettoThe material study of ancient coins is very often rendered particularly challenging by several factors: the presence of surface alteration products; the occasional or deliberate, e.g., forgery, changes in the composition of the base alloy; the misleading information deriving from historical and literature sources.

We present herewith a multi-analytical approach to the study of ancient coins. The selected test samples are coins widely used in the Venetian Republic over a time span ranging from the second half of the 16th until the early years of the 17th century: the so-called “sesino”. The rationale of the study was to establish a model, taking into account the layered structure of the surface region of the coins, that once validated could be used for a fully non-destructive characterization of similar items. The specific interest of the results obtained from this investigation is twofold. First, the actual composition of the copper based alloy used for these specific type of Venetian coin, has been measured for the first time with direct measurements on the coin cross-section. Second, the detailed characterization of the coins provides an essential background knowledge for a fully non-destructive characterization of the same kind of coins.

Link to the complete text: https://doi.org/10.1016/j.nimb.2019.06.010

Combined XRD-XRF cluster analysis for automatic chemical and crystallographic surface mappings

X-ray diffraction-X-ray fluorescence (XRD-XRF) data sets obtained from surface scans of synthetic samples have been analysed by means of different data clustering algorithms, with the aim to propose a methodology for automatic crystallographic and chemical classification of surfaces.

Three data clustering strategies have been evaluated, namely hierarchical, k-means, and density-based clustering; all of them have been applied to the distance matrix calculated from the single XRD and XRF data sets as well as the combined distance matrix. Classification performance is reported for each strategy both in numerical form as the corrected Rand index and as a visual reconstruction of the surface maps. Hierarchical and k-means clustering offered comparable results, depending on both sample complexity and data quality.

When applied to XRF data collected on a two-phases test sample, both algorithms allowed to obtain Rand index values above 0.8, whereas XRD data collected on the same sample gave values around 0.5; application to the combined distance matrix improved the correlation to about 0.9. In the case of a more complex multi-phase sample, it has also been found that classification performance strongly depends on both data quality and signal contrast between different regions; again, the adoption of the combined dissimilarity matrix offered improved classification performance.

Link to the complete text: https://doi.org/10.1017/S0885715619000216

Characterization of the mistura alloy used for Venetian sesino coins (XVI century)

In this study a set of Venetian sesino coins, minted over a period ranging from 1554 until 1605, have been investigated in order to shed light on some aspects of the so‐called mistura (mixture) alloy. The widespread diffusion of these relatively low‐value coins of the Venetian Republic, commonly used in commercial transactions in the second half of 16th century, also outside the territories of the Republic, makes them an important proxy to be used in the reconstruction of the political and historical events of the period.

The specific issue of the actual composition of the mistura alloy is herewith addressed for the first time, using a combined approach based on X‐ray fluorescence and X‐ray diffraction nondestructively applied to the analysis of the samples.

It turns out that the mistura alloy, traditionally regarded as a Cu‐Ag two phase alloy, over the latest period of circulation of the sesinocoins, was actually made of copper only, still containing minor concentrations of lead, to be regarded as an impurity of the alloy and not as an intentional addition.

The link to complete article: https://onlinelibrary.wiley.com/doi/abs/10.1002/xrs.2979