ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 23 the Pyrenees with the Rock of Gibraltar at her feet and carrying in her hand an extended olive branch (Fig. 2). All the coins were magnificently engraved and signed by Luis Marchionni, who since 1861 held the position of main engraver of the Madrid Mint. On the reverse side, in silver, is the shield of Spain with a mural crown, and the legend with the coin’s value, the size (number of pieces in kilogram), the initials of the assayers and weighing judge, and the law of 900 thousandths in five pesetas (Fig. 3). To illustrate the importance of monetary reform and the intended survival of the new monetary standard, an international competition was called to decide the rates that should appear on the currency. The projects were awarded to José Esteban Lozano for gold, and to Luis Plañiol for bronze; the silver one was void and served as a model for Luis Marchionni’s definitive design. EXPERIMENTAL METHOD The piece under investigation is a 5-peseta silver coin from 1870 that belongs to the San Carlos Historical Fort Museum in the Uco Valley of Mendoza, Argentina. This is part of a collaboration agreement between the museum and the archaeometallurgy department from UTN Mendoza Regional Faculty (Fig. 4). When the piece was first received for study, it was covered with a dark gray corrosion product of an apparently waxy consistency. A little spot on the coin’s obverse was treated with 3 µm polishing paste and the attack reagent used was acidified ammonium hydroxide to obtain the metallography seen in Figs. 5 and 6. Initial observations show hexagonal grain boundaries and dark areas that are corroded due to chlorine. Grain boundaries that can be seen are straight lines and have not moved, which is in line with the age of this piece (154 years old). Numismatic data states the coin’s material is Silver Grade 900, which Fig. 5 — Micrograph of the 5-peseta silver coin, obverse side. Reagent: acidified ammonium hydroxide. Magnification: 100×. Fig. 6 — Micrograph of the 5-peseta silver coin (laurel’s zone on the obverse side). Reagent: acidified ammonium hydroxide. Magnification: 400×. Fig. 7 — SEM images from the 5-peseta silver coin. TABLE 1 — CHEMICAL COMPOSITION OF THE SILVER COIN %O %Al %Si %S %Cl %Cu %Ag 3.85 0.48 0.49 0.8 1.16 1.73 92.02 means a coin has a grade of 900 thousandths, or out of 1000 parts, 900 are silver (9 out of 10), and the rest is the metal used to alloy it. The studies carried out with SEM-EDS showed that the main constituent of this corrosion is silver chloride. The dark gray patina of waxy consistency was formed by oxygen and/or chlorine and induces loss of other elements of the base metal; that is, the matter is transformed and that is why the piece loses weight. According to data from numismatic catalogs this reduced weight is 25 g, and the coin analyzed was 24.6450 g. It is as important to study the aspects of the patina in addition to those of the chemical composition of the piece and the technological aspects of the coinage itself through optical and electron microscopy, which are the necessary tools for diagnosis. In this way, the studies carried out with SEM-EDS demonstrated that the main constituent of this corrosion is silver chloride, and it was confirmed that the material of the coin is mainly silver with a copper content of less than 2% (Fig. 7). Table 1 shows the chemical composition of the coin. CONCLUSIONS The peseta was the legal currency in Spain and its overseas territories from its appearance on October 19, 1868 until February 28, 2002, after the introduction of the euro. This experiment looked at a 5-peseta silver coin from
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