Abstract:
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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 highresolution
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. |