SR-XRD and SR-FTIR study of the alteration of silver foils in medieval paintings |
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Authors: | Nati Salvadó Salvador Butí Ana Labrador Gianfelice Cinque Hermann Emerich Trinitat Pradell |
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Institution: | 1.Dpt. d’Enginyeria Química. EPSEVG,Universitat Politècnica de Catalunya,Vilanova i la Geltrú,Spain;2.BM16-CRG Consorci Laboratori de Llum Sincrotró (LLS) c/o ESRF,Grenoble,France;3.Diamond Light Source,Chilton–Didcot,UK;4.BM01-ESRF,Grenoble,France;5.Dpt. Física i Enginyeria Nuclear, ESAB,Universitat Politècnica de Catalunya,Castelldefels,Spain |
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Abstract: | Altarpieces and polychrome carved wood from the fifteenth century AD usually exhibit golden and silvery areas by the application
of a very thin foil of metal. The metal foils were normally protected from the atmosphere by a varnish or resin which maybe
either preserved or absent. Moreover, they were glued to the background surface by adhesive substances (egg yolk, drying oil
or animal glue). The high proportion of the glueing substances often renders the development of reaction compounds. With time,
silver alters blacken or simply disappear completely. In this paper, we study the alterations to metal foils from a selection
of fifteenth century artworks showing different glueing agents, organic coatings and several degrees of conservation of the
organic coatings and metal leafs. The submillimetric layered structure and the high variability and low amount of most of
the compounds present in the different layers, as well as their differing nature (organic and inorganic) make the use of micron-sensitive
high-resolution techniques essential for their study. In particular, the high resolution, high brilliance and small footprint
renders synchrotron radiation most adequate for their study. SR-XRD was performed to identify the reaction compounds formed
in the different layers; μFTIR was used at to identify the silver protecting organic coatings, the metal foil glueing layers
and the corresponding reaction compounds. The results obtained suggest that atmospheric corrosion is the dominant mechanism,
and therefore that the degree of corrosion of the metal foils is mainly related to the conservation state of the protecting
coatings. |
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