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Scientists
have identified a complex chemical reaction responsible for the
degradation of two paintings by Vincent van Gogh and other artists of
the late 19th century. This discovery is a first step towards
understanding how to stop the bright yellow colours of van Gogh’s most
famous paintings from becoming covered by a brown shade, and fading over
time. In the meantime, the results suggest shielding affected paintings
as much as possible from UV and sunlight. The results are published
online on 14 February 2011 in the journal Analytical Chemistry.
The
work was carried out by an international team of scientists from four
countries led by Koen Janssens of Antwerp University (Belgium), with
Letizia Monico, an Italian chemist preparing a Ph.D. at Perugia
University (Italy), taking the centre stage in the experiments. As an
Erasmus student, she worked for one year in Janssens’ research group in
Antwerp, and is also the lead author of the two papers. Scientists from
the CNR Institute of Molecular Science and Technologies (Perugia,
Italy), the CNRS C2RMF (Paris, France), TU Delft (Netherlands) and the
van Gogh Museum (Amsterdam, Netherlands) were also part of the team.
Uncovering
the secrets of the chemical reaction needed deployment of an impressive
arsenal of analytical tools, with synchrotron X-rays at the ESRF in
Grenoble (France) providing the final answers.
“For
every Italian, conservation of masterpieces has always mattered. I am
pleased that science has now added a piece to a puzzle that is a big
headache for so many museums” says Letizia Monico from University of
Perugia.
The
experiment reads like a crime scene investigation. The scientists
employed an X-ray beam of microscopic dimensions to reveal a complex
chemical reaction taking place in the incredibly thin layer where the
paint meets the varnish. Sunlight can penetrate only a few micrometres
into the paint, but over this short distance, it will trigger a hitherto
unknown chemical reaction turning chrome yellow into brown pigments,
altering the original composition.
Van
Gogh’s decision to use novel bright colours in his paintings is a major
milestone in the history of art. He deliberately chose colours that
conveyed mood and emotion, rather than employing them realistically. At
the time, this was completely unheard of and, without major innovations
in pigment manufacturing made in the 19th century, would also have been
impossible.
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It
was the vibrancy of new industrial pigments such as chrome yellow which
allowed van Gogh to achieve the intensity of, for example, his series
of sunflower paintings. He started to paint in these bright colours
after leaving his native Holland for France where he became friends with
artists who shared his new ideas about the use of colours. For one of
them, Paul Gauguin, he started painting yellow sunflower motifs as a
decoration for his bedroom.
The
fact that yellow chrome paint darkens under sunlight has been known
since the early 19th Century. However, not all period paintings are
affected, nor does it always happen at the same speed. As chrome yellow
is toxic, artists quickly switched to new alternatives in the 1950s.
However, Vincent van Gogh did not have this choice, and to preserve his
work and that of many contemporaries, interest in the darkening of
chrome yellow is now rising again.
To
solve a chemical puzzle nearly 200 years old, the team around Janssens
used a two-step approach: first, they collected samples from three
left-over historic paint tubes. After these samples had been
artificially aged for 500 hours using an UV-lamp, only one sample, from a
paint tube belonging to the Flemish Fauvist Rik Wouters (1882-1913),
showed significant darkening. Within 3 weeks, its surface of originally
bright yellow had become chocolate brown. This sample was taken as the
best candidate for having undergone the fatal chemical reaction, and
sophisticated X-ray analysis identified the darkening of the top layer
as linked to a reduction of the chromium in the chrome yellow from
Cr(VI) to Cr(III). The scientists also reproduced Wouters’ chrome yellow
paint and found that the darkening effect could be provoked by UV
light.
In
the second step, the scientists used the same methods to examine
samples from affected areas of two van Gogh paintings, View of Arles
with Irises (1888) and Bank of the Seine (1887), both on display in the
Van Gogh Museum in Amsterdam.
“This
type of cutting edge research is crucial to advance our understanding
of how paintings age and how they should be conserved for future
generations”, says Ella Hendriks of the van Gogh Museum Amsterdam.
Because
the affected areas in these multicoloured samples were even more
difficult to locate than in the artificially aged ones, an impressive
array of analytical tools had to be deployed which required the samples
travelling to laboratories across Europe. The results indicate that the
reduction reaction from Cr(VI) to Cr(III) is likely to also have taken
place in the two van Gogh paintings.
Background: The vacuum chamber of the synchrotron X-ray microscope with the sample stage illuminated. Foreground: Several microsamples from art masterpieces, moulded in Plexiglass plates ready for investigation. The paint tube contains historic yellow chrome paint from the personal collection of M. Cotte. Credit I. Montero/ESRF. |
The
reduction of chromium that had been observed in the artificially aged
sample from the atelier of Rik Wouters was finally confirmed in both
microsamples from the van Gogh paintings.
The
study was completed with a nanoscopic investigation of the discoloured
paint using electron energy loss spectroscopy at the University of
Antwerp, which confirmed the results and showed that the newly formed
Cr(III) compounds were formed as a nanometre-thin coating of the pigment
particles that constitute the paint.
Scientists
using a synchrotron have identified the chemical reaction in van Gogh’s
painting “Bank of the River Seine” that alters originally bright yellow
colours into brown shades. This is observed in many van Gogh paintings.
A microsample of the painting was analyzed with X-rays, revealing how
chrome yellow pigments can turn brown under the influence of sunlight.
Reference:
L. Monico et al., Degradation Process of Lead Chromate in Paintings by
Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy
and Related Methods. 1. Artificially Aged Model Samples and 2. Original
Paint Layer Samples, Analytical Chemistry 15 February 2011.
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The
microscopic X-ray beam also showed that Cr(III) was especially
prominent in the presence of chemical compounds which contained barium
and sulphur. Based on this observation, the scientists speculate that
van Gogh’s technique of blending white and yellow paint might be the
cause of the darkening of his yellow paint.
“Our
next experiments are already in the pipeline. Obviously, we want to
understand which conditions favour the reduction of chromium, and
whether there is any hope to revert pigments to the original state in
paintings where it is already taking place.”, summarises Koen Janssens
from University of Antwerp.
The crime scene investigation
The
techniques used by the scientists included X-ray diffraction along with
various spectroscopies employing infrared radiation, electrons and
X-rays at the universities of Antwerp and Perugia, and at two
synchrotrons (ESRF and DESY).
“I
am not aware of a similarly big effort ever having been made for the
chemistry of an oil painting”, says Joris Dik, Professor at Delft
Technical University.
In
the decisive step, two techniques were combined using a single X-ray
beam at the ESRF: X-ray fluorescence (XRF) and X-ray absorption
near-edge spectroscopy (XANES). For the XRF, the microscopic beam size
(0.9 x 0.25 µm²) made it possible to separate the study of degraded and
unaffected areas, and the XANES technique proved the speciation of
chromium, i.e. the reduction from Cr(VI) to Cr(III).
“Our
X-ray beam is one hundred times thinner than a human hair, and it
reveals subtle chemical processes over equally minuscule areas. Making
this possible has opened the door to a whole new world of discovery for
art historians and conservators,” says Marine Cotte, an ESRF scientist
also working at CNRS/Musée du Louvre.
The
reduction of chromium that had been observed in the artificially aged
sample from the atelier of Rik Wouters was finally confirmed in both
microsamples from the van Gogh paintings.
The
study was completed with a nanoscopic investigation of the discoloured
paint using electron energy loss spectroscopy at the University of
Antwerp, which confirmed the results and showed that the newly formed
Cr(III) compounds were formed as a nanometre-thin coating of the pigment
particles that constitute the paint.
Scientists
using a synchrotron have identified the chemical reaction in van Gogh’s
painting “Bank of the River Seine” that alters originally bright yellow
colours into brown shades. This is observed in many van Gogh paintings.
A microsample of the painting was analyzed with X-rays, revealing how
chrome yellow pigments can turn brown under the influence of sunlight.
Reference:
L. Monico et al., Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 1. Artificially Aged Model Samples and 2. Original
Paint Layer Samples, Analytical Chemistry 15 February 2011.