March 2023 ESRFnews
17
CULTURAL HERITAGE
Jon Cartwright
strongly suggests that Rembrandt used litharge as a drier
(Angew. Chem. Int. Ed. doi:10.1002anie.202216478).
But Gonzalez stresses that other hypotheses still have
to be considered. “We have to remain very cautious,” he
says. “The Night Watch has a very complex conservation
history, so we’re also looking into ancient restoration
treatments that could explain why lead formates are
present today.”
X-ray diffraction, as used in the study of The Night
Watch, has the longest pedigree among synchrotron
techniques applied to artworks. Familiar even to new
synchrotron users, being a refinement of a laboratory
technique, it is able to deliver information about local
elemental composition and crystallographic phases.
High-angular resolution X-ray diffraction (HR-XRD)
at the ESRF’s ID22 beamline can identify the substances
present in highly heterogeneous samples, for instance,
while μXRD, boosted by the brightness of the EBS,
involves X-ray beams ranging from a few microns to
tens of nanometres in diameter at ID16B, ID13,
ID01 and ID11. In 2019, a combination of HR-XRD
and μXRD allowed Cotte, Gonzalez and colleagues to
identify the ingredients that Rembrandt used to create his
famously textured “impasto” effect in three of his other oil
paintings (Angew. Chem. Int. Ed. 58 5619).
X-ray absorption to the fore
In recent years, however, X-ray fluorescence and absorp-
tion, primarily at the ID21 beamline (now being refur-
bished have proven to be equally useful techniques
as they are able to reveal the chemical state of elements
of interest In one of the first experiments conducted
after the EBS upgrade Cotte collaborated with Marta
Ghirardello of the Politecnico di Milano in Italy and oth
ers to understand the degradation of cadmium yellow in
Picassos 1907 portrait Femme Exploiting the height
ened symmetry of the new beam as well as ultrastable
ESRFdeveloped doublecrystal monochromators the
team was able to show via Xray absorption that the loss
of the yellow likely resulted from the presence of another
compound in nanocrystalline cadmium sulphide making
it more reactive Microsc Microanal 28 1504 In another
experiment, ID21 helped determine the origin of the
ultramarine blue that the Dutch artist Johannes Vermeer
used in his 1665 painting, Girl with a Pearl Earring (Herit.
Sci. 8 25). (See “Two portraits, two techniques”, above.)
While the ramifications of the Rembrandt, Picasso and
Vermeer studies for conservation are not yet clear, past
studies on cultural objects have provided indispensable
insights. Previously, a curator from Barcelona in Spain
approached Cotte wanting to know whether he could
remove blackish marks on a painting: the synchrotron
data revealed the marks to be not dirt but degradations
of an original red pigment, and so were best left as they
were. In another study, researchers from the University of
Antwerp and elsewhere wanted to know whether brown
manganese spots in stained-glass windows could be
removed effectively via the common application of a
reactant, hydroxylamine hydrochloride. ESRF data
showed that the procedure risked side effects, namely the
development of cracks in the glass.
It is results such as these that show the practical
importance of synchrotron science to conservation, and
fortunately they are set to become more frequent The
extra Xray flux of the EBS means that many experiments
can be conducted 1000 times faster than before Mean
while the newly shared access mode for cultural heritage
means that the time elapsed from idea to experiment is
drastically reduced and with much less paperwork
The shared access works very well and is fostering a
great collaboration between teams and notably students
from many European countries says Gonzalez And
with the new EBS source experiments that were done in
a few hours can now be performed in a few minutes
TWO PORTRAITS, TWO TECHNIQUES
X-ray absorption spectroscopy
Girl with a Pearl Earring (1665),
Johannes Vermeer
This portrait is notable not only for the
fleeting glance of its subject, but also
the (at its time, costly) ultramarine blue
of her headscarf rendering. Ultramarine
consists of the mineral lazurite, which
derives from the semi-precious stone
lapis lazuli. In 2020, X-ray absorption
spectroscopy at the ESRF revealed that
Vermeer’s ultramarine was extracted from
lapis lazuli via unusual heat treatment.
X-ray diffraction
Susanna (1636), Rembrandt van Rijn
A woman falsely accused of adultery in
the Book of Daniel in the Bible, Susanna
has been a common subject for painters
throughout the ages. Rembrandt brings
her humanity to the canvas with his thickly
layered “impasto” technique. High-angular
resolution X-ray diffraction and micro-
X-ray diffraction at the ESRF’s ID22 and
ID13 beamlines identified plumbonacrite,
Pb
5
(CO3)
3
O(OH)
2
, as the secret ingredient
that gives impasto oil its unique texture.
“The extra X-ray flux of the
EBS means that many experiments
can be conducted 1,000 times faster
than before”
M A U R I T S H U I S, T H E H A G U E M A U R I T S H U I S, T H E H A G U E