atw International Journal for Nuclear Power | 04.2020

atw Vol. 65 (2020) | Issue 4 ı April
| Fig. 3.
High-resolution digital radiography (section) of a French impressionist
painting by Frédéric Cordey,1854-1911, showing the structure of the
canvas, part of its fixing on the stretcher with nails on the right, various
damages in the form of losses, and the brush stroke structure which can be
considered characteristic for the painter.
Since Wilhelm Conrad Röntgen
discovered in 1895 that X-rays exhibit
a high penetration, even in materials
which are opaque for visible light, the
new radiation was an opportunity
for many applications, starting from
medicine, but soon in material
analysis, and also in the study of art
objects and archeological artefacts.
As a helpful tool for the questions
rised by art historians, archeologists
or conservators, the classical X-rays
were introduced systematically in
radiography studies of paintings by
the radiologist A. Faber in 1914 [9]
and it is well known that non-destructive
imaging with X-rays was a very
powerful tool to learn more about
mummies in the early 20 th century.
Already in 1924, the Pinakothek in
Munich and the Louvre in Paris
operated own X-ray devices for the
radiography of paintings [9]. In the
1930’s dedicated X-ray tubes for investigations
of painting were available
and although there was a period of
severe doubts and rumors about possible
radiation damages to valuable
art objects, radiography of paintings
with X-rays became more and more
accepted [9]. The thesis of C. Wolters
[10] on X-ray studies in art history is
one of the basic documents, which
comprised the knowledge so far and
worked out the important factors for
the interpretation of X-ray images of
paintings.
In the radiography of paintings,
the technique uses the contrast
achieved by different absorption of
X-rays in the pigments, very similar
to the contrast effect of different
tissues in medical X-ray imaging.
Heavy elements clearly dominate this
effect, like lead (in the form of the
pigment Lead-white) or – far less
important – mercury (red Cinnabar
pigment) or barium (Lithopone).
Before the mid of the 19 th Century,
Lead-white was used in every white
paint layer and blended with other
pigments to brighten these up. In the
case of paintings, high-resolution
X-ray images exhibit the distribution
of lead white pigment with excellent
accuracy and are well suited to analyse
the overall composition, the brushstrokes
of the painter (Figure 3) and
to discover changes (Pentimenti) and
overpaintings.
In an interesting project to be
mentioned here [11], of the French
Centre de Recherche et de Restauration
des Musées de France (C2RMF) in
cooperation with museums, several
important thought-to-be-lost paintings
of the French painter Frédéric Bazille
were rediscovered below the paint
layers of other paintings by Bazille –
which means that he was probably
very critical with his paintings and
overpainted several works. Similar
cases have been reported quite often.
X-ray radiography gives a pro jection
of the thin layers of a painting. Such
projections are similarly applicable to
other objects in cultural heritage like
sculptures or archeolo gical artefacts,
within the limits of the penetration of
denser materials. In the classic X-ray
tube, the radiation is emitted from a
small focal point in the tube, so
geometrical effects when analyzing the
projections have to be taken into
account. These are, how ever less
limiting for flat paintings in large focal
distances.
Later and up-to-now developments
comprise 3D X-ray tomography and
optimized X-rays sources, digital
recording instead of film material
and computed analysis. By these
| Fig. 4.
Measurement of X-ray fluorescence radiation (XRF)
with mobile equipment and professional data analysis
reveals the elemental composition of the pigments in the painting.
techniques, 3-dimensional X-ray imaging
with computed tomography became
standard, also in art and cultural
heritage studies. However, as in the
case of reactors and synchrotrons
already mentioned above, such
methods rely on stationary equipment
and are therefore not applicable when
an object shall not be moved from the
conservatory environment.
Moreover, X-rays are capable to produce
X-ray fluorescence in materials
that are irradiated, and this X-ray
fluorescence (XRF) is widely used for
the identification of characteristic
elements in pigments or for the analysis
of metallic alloys. The principle is
based on the fact that X-rays from
an X-ray source entering the surface
layers of the object (they will usually
not enter very deep) excite atoms in
the material which then emit their
characteristic K- or L-X-rays. These
characteristic X-rays are detected with
an energy dispersive detector. The
method is therefore capable to detect
the elemental composition of the
irradiated spot and thereby support an
assignment of these elements to the
pigments used. The X-ray source for
the excitation will be chosen adequately
and may be a synchrotron with
brilliant, monochromatic X-ray beam
in a research center or a small, dedicated
X-ray tube.
In their simple form with irradiation
spots adjustable between 1 and
8 mm, these instruments are small
and fully mobile, although they contain
the X-ray source and the detector.
They can therefore be brought directly
to the object and mounted on tripods
for accurate measurements, see the
typical setup in Figure 4.
Modern XRF instruments dedicated
to art and cultural heritage are
RESEARCH AND INNOVATION 227
Research and Innovation
Radiation in Art and Cultural Heritage ı Frank Meissner and Andrea Denker