Microsamples from art masterpieces moulded in Plexiglass plates ready for investigation with synchrotron X-rays. The historic paint tube at the bottom is from the personal collection of M. Cotte. Credit I. Montero/ESRF. |
With
a sophisticated X-ray analysis scientists have identified why parts of
the 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.
The results are published in an upcoming issue of Analytical Chemistry,
the first author of which is Geert Van der Snickt, who received a PhD
in Conservation and Restauration from the University of Antwerp
(Belgium) for this work. The research team was led by Koen Janssens from
Antwerp and also comprised scientists from TU Delft (Netherlands), the
French CNRS, the Kröller-Müller Museum in Otterlo (Netherlands), the
ESRF and DESY.
Vincent
Van Gogh (1853-1890) painted “Flowers in a blue vase” in 1887 in Paris,
and in the early 20th century, the painting was acquired by the
Kröller-Müller 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,” says
paintings conservator Margje Leeuwestein from the Kröller-Müller Museum.
The
cadmium yellow (cadmium sulphide, CdS) used by Van Gogh was a
relatively new pigment, of which it has recently been discovered that in
unvarnished paintings, it oxidizes with air (to cadmium sulphate;
CdSO4) 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,” says Janssens.
“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,” adds Leeuwestein.
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,” explains DESY
scientist Gerald Falkenberg. 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,” adds Falkenberg.
“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,” says ESRF scientist Marine Cotte. Together with the
anglesite, the cadmium oxalate (CdC2O4) accounts for the opaque,
orange-grey crust disfiguring parts of the painting on a macroscopic
level.
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“The
research into this hitherto unknown degradation process of varnished
cadmium yellow oil paint allows to better understand the current
appearance of the painting,” explains Leeuwestein. Joris Dik from TU
Delft adds that “it also provides information on how later-applied
varnish layers may contribute to the decline of certain pigments of a
painting. In the future, this degradation process can hopefully be
inhibited or even prevented thanks to novel preservation and
conservation techniques.” Whether removing the varnish and crusts from
paintings with this type of degradation is possible and appropriate is
not yet fully understood. Leeuwestein adds that “in every similar case
of a possible varnish and crust removal, it should always be considered
that this varnish and crust contain original material from the cadmium
yellow oil paint. The possible removal of original material from a
painting during a conservation treatment is of course undesirable.”
After this discovery, conservators in many museums will have to newly address the question of restoring Van Gogh paintings.
“This
study on the deterioration of cadmium yellow is an excellent example of
how collaboration between scientists and conservators can help to
improve our understanding of the condition of Van Gogh’s paintings and
lead to better preservation of his works,” says Ella Hendriks, head of
conservation of the Van Gogh Museum in Amsterdam, who did not take part
in the study. “Many of Van Gogh’s French period paintings have been
inappropriately varnished in the past and removal of these non-original
varnish layers is one of the challenges facing conservators on a
world-wide basis today. The type of information provided by Janssens and
his team is vital to support the difficult decisions that conservators
often have to make regarding such complex cleaning treatments.”
“Once
again, we find that paintings by Vincent van Gogh are not static
entities for decades and centuries to come. Over a period of 100 years,
they can actually be considered a fairly reactive cocktail of chemicals
that behaves in unexpected manners,” concludes Janssens. Geert van der
Snickt adds that “especially the presence of sulphides puts the
durability of the paintings at risk.”
In
the next four years, Janssens’ research group plans to study how museum
indoor conditions and air pollution affect cadmium yellow and related
sulphide-containing pigments used by artists.
