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Why flowers in Van Gogh’s painting changed colour | | Scientists have identified why parts of the Vincent Van Gogh painting ‘Flowers in a blue vase’ have changed colour over time. A supposedly protective varnish applied after the master’s death has made some bright yellow flowers turn to an orange-grey colour. The origin of this alteration is a
hitherto unknown degradation process at the interface between paint and varnish, which studies at the
European Synchrotron Radiation Facility ESRF in Grenoble (France) and at Deutsches Elektronen-Synchrotron DESY in Hamburg (Germany) have revealed for the first time. Van Gogh
painted ‘Flowers in a blue vase’ in 1887 in Paris , and in the early 20th century, the
painting was acquired by the Kroller-Muller Museum . The master usually did not
varnish his works, but this painting was later covered with a supposedly protective
varnish, like many other Van Gogh paintings in the first half of the 20th century.
“A conservation treatment in 2009 revealed an unusual grey opaque crust on parts
of the painting with cadmium yellow paint,” paintings conservator Margje Leeuwestein
from the Kroller-Muller Museum said. The cadmium yellow used by Van Gogh was a
relatively new pigment, of which it has recently been discovered that in unvarnished
paintings, it oxidizes with air making the pigments lose colour and luminosity.
“We identified this process a few years ago, and the observation that instead
of a slightly off-white, transparent oxidation layer, the pigments in this painting
were covered with a dark, cracked crust intrigued us very much,” Koen Janssens,
study leader from the University of Antwerp, said. “The removal of the orange-grey
crust and discoloured varnish was not possible without affecting the very fragile
original cadmium yellow paint on these parts,” Leeuwestein said. To identify what
had happened, the museum took two microscopic paint samples – each only a fraction
of a millimetre in size - from the original painting and sent them to Janssens
for a detailed investigation. The scientists studied the samples using powerful
X-ray beams at the ESRF and at DESY’s PETRA III, revealing their chemical composition
and internal structure at the interface between varnish and paint. To their surprise,
they did not find the crystalline cadmium sulphate compounds that should have
formed in the oxidation process. “It emerged that the sulphate anions had found
a suitable reaction partner in lead ions from the varnish and had formed anglesite,”
DESY scientist Gerald Falkenberg said. Anglesite (PbSO4) is an opaque compound
that was found nearly everywhere throughout the varnish. “The source of the lead
probably is a lead-based siccative that had been added to the varnish,” Falkenberg
said. “At the interface between paint and varnish, the cadmium ions together with
degradation products from the varnish itself also formed a layer of cadmium oxalate,”
ESRF scientist Marine Cotte said. Together with the anglesite, the cadmium oxalate
(CdC2O4) accounts for the opaque, orange-grey crust disfiguring parts of the painting
on a macroscopic level. “The research into this hitherto unknown degradation process
of varnished cadmium yellow oil paint allows to better understand the current appearance of the painting,” Leeuwestein said. The study has been published in Analytical Chemistry.
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