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Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Spectromicroscopic Methods. Part 5. Effects of Nonoriginal Surface Coatings into the Nature and Distribution of Chromium and Sulfur Species in Chrome Yellow Paints
journal contribution
posted on 2014-11-04, 00:00 authored by Letizia Monico, Koen Janssens, Frederik Vanmeert, Marine Cotte, Brunetto Giovanni Brunetti, Geert Van der Snickt, Margje Leeuwestein, Johanna Salvant Plisson, Michel Menu, Costanza MilianiThe
darkening of lead chromate yellow pigments, caused by a reduction
of the chromate ions to Cr(III) compounds, is known to affect the
appearance of several paintings by Vincent van Gogh. In previous papers
of this series, we demonstrated that the darkening is activated by
light and depends on the chemical composition and crystalline structure
of the pigments. In this work, the results of Part 2 are extended
and complemented with a new study aimed at deepening the knowledge
of the nature and distribution of Cr and S species at the interface
between the chrome yellow paint and the nonoriginal coating layer.
For this purpose, three microsamples from two varnished paintings
by Van Gogh and a waxed low relief by Gauguin (all originally uncoated)
have been examined. Because nonoriginal coatings are often present
in artwork by Van Gogh and contemporaries, the understanding of whether
or not their application has influenced the morphological and/or physicochemical
properties of the chrome yellow paint underneath is relevant in view
of the conservation of these masterpieces. In all the samples studied,
microscopic X-ray fluorescence (μ-XRF) and X-ray absorption
near edge structure (μ-XANES) investigations showed that Cr(III)-based
alteration products are present in the form of grains inside the coating
(generally enriched of S species) and also homogeneously widespread
at the paint surface. The distribution of Cr(III) species may be explained
by the mechanical friction caused by the coating application by brush
that picked up and redistributed the superficial Cr compounds, likely
already present in the reduced state as result of the photodegradation
process. The analysis of the XANES profiles allowed us to obtain new
insights into the nature of the Cr(III) alteration products, that
were identified as sulfate-, oxide-, organo-metal-, and chloride-based
compounds. Building upon the knowledge acquired through the examination
of original paint samples and from the investigation of aged model
paints in the last Part 4 paper, in this study we aim to characterize
a possible relation between the chemical composition of the coating
and the chrome yellow degradation pathways by studying photochemically
aged model samples covered with a dammar varnish contaminated with
sulfide and sulfate salts. Cr speciation results did not show any
evidence of the active role of the varnish and added S species on
the reduction process of chrome yellows.