Caracterización electroquímica de metales nobles en objetos arqueológicos y en obras de arte

La presente Tesis Doctoral se dirige a la caracterización de materiales, técnicas de fabricación, tipos de alteración de metales nobles (principalmente de oro y plata) en objetos artísticos y arqueológicos mediante técnicas electroquímicas, incorporando además estrategias quimiométricas para el desarrollo de métodos de autentificación y datación. Para ello se parte de la aplicación de las herramientas electroquímicas (principalmente voltamperometrías de barrido lineal, de onda cuadrada etc.). La información obtenida se va complementando con otras técnicas como la microscopia electrónica (FIB-FESEM) o la microscopía de fuerza atómica (AFM). También se hace hincapié en la necesidad de utilizar metodologías de muestreo poco invasivas a causa del elevado valor cultural que se les presupone a este tipo de objetos, es por ello que se hace un uso extenso del protocolo “one-touch” en el que una barra de grafito se modifica con la muestra, al frotar esta barra sobre el objeto una pequeña cantidad de muestra (del orden de nanogramos) es transferida a la barrita que es utilizada posteriormente como electrodo. Toda esta metodología se engloba en la voltamperometría de micropartículas inmovilizadas (VIMP), técnica de estado sólido desarrollada por Fritz Scholz et al. Lo arriba expuesto se aplicó sobre materiales de referencia de plata y oro para poder ajustar los diferentes parámetros de medida para después pasar a conjuntos muestrales procedentes de colecciones museísticas: 1.Análisis de las primeras monedas polacas mediante VIMP. 2.Análisis electroquímicos de dorados de altares valencianos de los siglos XV a XX. 3.Análisis electroquímicos de hilos de oro procedentes de tejidos arqueológicos. 4.Estudio de objetos de oro arqueológico procedentes de los museos de Borriana, de Castellón y de Prehistoria de Valencia. 5.Datación de oro arqueológico mediante electroquímica de estado sólido. Además, se estudiaron muestras de oro de origen natural de nuevo mediante VIMP. Tras la consideración de los resultados obtenidos se elaboraron un conjunto de conclusiones sobre las respuestas electroquímicas obtenidas. En las muestras de plata esta respuesta se halla dominada por las señales de reducción de los productos de corrosión tanto de la propia plata como otros metales que suelen presentarse aleados (en especial cobre y plomo). A partir de estos datos se pudieron deducir la existencia de dos fuentes locales de materiales, así como que la acuñación se llevó a cabo en tres cecas distintas. Respecto a las muestras de oro, el contacto del electrodo con una disolución de HCl produce distintos procesos de oxidación y son particularmente interesantes aquellos que tienen lugar en defectos de la red metálica que pueden considerarse como centros activos (en comparación con la inercia quím. La aparición y desaparición de estos defectos puede relacionarse con la “historia de corrosión” de la pieza, su edad, composición y técnica de fabricación

Electrochemical analysis of gildings in Valencia altarpieces: a cross-age study since fifteenth until twentieth century, Journal of Solid State Electrochemistry

[EN] The application of the voltammetry of microparticles methodology to the study of gildings in paintings and architectural ornaments is described. Nanosamples from pieces from different churches of the Comunitat Valenciana (Spain) covering since the fifteenth century until nowadays were studied upon attachment to graphite electrodes in contact with aqueous HCl and H2SO4 electrolytes. Electrochemical measurements, combined with field emission scanning electron microscopy X-ray microanalysis (FESEM-EDX) and atomic force microscopy (AFM) data, denoted that a common manufacturing technique was used with minimal variations along time. The relationship between specific voltammetric features associated to bulk gold and active surface sites, however, changed monotonically with time, thus suggesting the possibility of age monitoring.Financial support from the MINECO Projects CTQ2014-53736-C3-1-P, CTQ2014-53736-C3-2-P and MAT2015-65445-C2-2-R, which are supported with ERDF funds is gratefully acknowledged. Likewise financial support of the Comunidad de Madrid and structural funds of the EU through Programa Geomateriales 2 ref. S2013/MIT-2914 is acknowledged. The authors thank the Seccion de Investigacion Arqueologica Municipal de Valencia for kindly authorizing sampling to carry out this research. The authors also thank Dr. Jose Luis Moya Lopez and Mr. Manuel Planes Insausti (Microscopy Service of the Universitat Politecnica de Valencia) for their technical support.Ferragud Adam, JV.; Piquero-Cilla, J.; Domenech Carbo, MT.; Guerola Blay, V.; Company Climent, J.; Domenech Carbo, A. (2016). 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Characterizing archaeological bronze corrosion products intersecting electrochemical impedance measurements with voltammetry of immobilized particles

[EN] Application of electrochemical impedance measurements to microparticulate deposits of copper corrosion products attached to graphite electrodes in contact with 0.10 M aqueous HClO4 electrolyte is described. The impedance measurements were sensitive to the applied potential and the amount of solid sample and were modeled taking into account the contribution of the uncovered base electrode. Several pairs of circuit elements provide monotonic variations which are able to characterize different corrosion compounds regardless the amount of microparticulate solid on the electrode. Application to a set of archaeological samples from the archaeological Roman site of Gadara (Jordan, 4th century AD) permitted to establish a grouping of such samples suggesting different provenances/manufacturing techniques.Financial support from the MINECO ProjectsCTQ2014-53736-C3-1-P and CTQ2014-53736-C3-2-P which are also supported with ERDF funds and Grants ES-2012-052716 and EEBB-I-16-11558 is gratefully acknowledgedRedondo-Marugan, J.; Piquero-Cilla, J.; Domenech Carbo, MT.; Ramírez-Barat, B.; Al Sekhaneh, W.; Capelo, S.; Doménech Carbó, A. (2017). Characterizing archaeological bronze corrosion products intersecting electrochemical impedance measurements with voltammetry of immobilized particles. Electrochimica Acta. 246:269-279. https://doi.org/10.1016/j.electacta.2017.05.190S26927924

Application of EIS for dating archaeological materials

Dating archaeological metals is in general a difficult analytical task. It is described the application of Electrochemical impedance spectroscopy (EIS) for this purpose, based on the measurement of impedance terms associated to the corrosion layers. EIS data were recorded for coins and other archaeological objects upon immersion of the pieces in mineral water and applying a bias potential for the reduction of dissolved oxygen. A generalized treatment, derived from previous analysis of lead and copper-based artifacts, is presented including theoretical expressions for the variation of impedances on corrosion time assuming uniform conditions of corrosion and a potential rate law

Archaeometric analysis of Roman bronze coins from the Magna Mater temple using solid-state voltammetry and electrochemical impedance spectroscopy

Voltammetry of microparticles (VMP) and electrochemical impedance spectroscopy (EIS) techniques, complemented by SEM-EDX and Raman spectroscopy, were applied to a set of 15 Roman bronze coins and one Tessera from the temple of Magna Mater (Rome, Italy). The archaeological site, dated back be- tween the second half and the end of the 4th century A.D., presented a complicated stratigraphic context. Characteristic voltammetric patterns for cuprite and tenorite for sub-microsamples of the corrosion layers of the coins deposited onto graphite electrodes in contact with 0.10 M HClO4 aqueous solution yielded a grouping of the coins into three main groups. This grouping was confirmed and refined using EIS experiments of the coins immersed in air-saturated mineral water using the reduction of dissolved oxygen as a redox probe. The electrochemical grouping of coins corroborated the complex stratigraphy of the archaeological site and, above all, the reuse of the coins during the later periods due to the economic issues related to the fall of the Roman Empire

Organic Matter Redox State Driven by Specific Sources in Mangrove Sediments: A Case Study from Peruvian Ecosystems

In order to determine the organic matter redox state in relation to specific sources in mangrove sediments, two 60 cm-long sediment cores were collected from mangrove-covered and mudflat zones within a mangrove forest in Peru. Sediment subsamples from these cores were analyzed to determine δ13C values and C:N ratios, whereas two redox indices, namely, electrochemical (fEAOM) and spectroscopical (A1650/A3400) indices, were taken from a previous study and correlated with the geochemical indices obtained from this work. These indices may provide accurate information on sedimentary organic matter diagenesis by oxidative processes through its redox state. The results show that the electrochemical index (fEAOM) and the spectroscopical index (A1650/A3400) for mangrove-covered sediments exhibited a positive correlation with δ13C values and a negative correlation with C:N molar ratios. These correlations suggest that the more labile sedimentary organic matter derived from non-terrestrial sources is in a more oxidized state than that derived from mangrove vegetation. However, this was not valid for mudflat zones, where non-significant correlations between geochemical indices were observed. Furthermore, the results suggest that the redox state of the organic matter deposited over time is dependent on source mixing influences, being better preserved in the presence of mangrove-derived organic matter