1MOLAB is a unique collection of integrated mobile instruments based on non-invasive spectroscopic methods exploiting light from mid-infrared to X-ray and working in point or scanning mode1. Founded in 2001 at Perugia University (UNIPG) in collaboration with the Institute of Molecular Science and Technology of CNR (ISTM-CNR), MOLAB has been conceived as a mobile facility which allows scientists, conservators, art historians and archaeologists to carry out scientific studies on cultural heritage materials when sampling is prohibited or when the bad state of conservation or the dimensions of the examined object advise them against any transportation to a laboratory. Since 2004, MOLAB has been the first (and unique) mobile transnational access supported by the European Commission. Through the Eu-ARTECH and CHARISMA projects, providers as UNIPG/ISTM-CNR (Perugia), INO-CNR (National Institute of Optics of CNR, Firenze) and CNRS-C2RMF/LAMS (Centre of Research and Restoration of French Museums/Laboratory of Molecular and Structural Archaeology, Paris – a joint research unit with CNRS) made available to transnational users a set of 15 different portable equipments for the non-invasive in situ development of diagnostic/conservation work, with neither sampling nor contact with the examined work of art.
2Thanks to the use of the MOLAB facility, successful studies have been carried out in numerous projects by hundreds of users, carrying out measurements on different types of heritage objects, including paintings, sculptures, manuscripts, and ceramics1,2. In particular, regarding paintings, studies on execution techniques (e.g. recording of underdrawings, identification of original materials and mapping of their distribution), have been carried out on works of ancient, modern and contemporary masters, as Bronzino, Leonardo, Caravaggio, Renoir, Cézanne, Van Gogh, Munch, Picasso, Mondrian, de Stael, and many others2. Illuminated manuscripts have been also studied, as the Book of Kells, pre-Columbian Mexican codices3 and others, and ceramics, as Renaissance Italian lusterwares1 and early Meissen porcelains.
3Exemplary studies of underdrawings were carried out using a multi-band scanner operating in the near infrared range. The system simultaneously collects a set of images at different wavelengths, with a step of 100 nm in the 800-2300 nm range. The resulting dataset is an image cube that can be sliced in multi-band reflectograms free of any geometrical distortion. As an example, the study of multi-band reflectograms acquired on Munch’s paintings at the Munch Museum in Oslo, allowed B. Topolova-Casadiego and co-workers to visualize different traces of underdrawings4, especially in Puberty, 1894 (fig. 1), and in Vampire, 1895, where the painter, after sketching the figures with a charcoal or graphite pencil, refined the composition in wash, using a brush.
Fig. 1. Multi-band reflectograms acquired on Munch’s paintings at the Munch Museet in Oslo
The underdrawings are outstandingly evident in Puberty, where the position of the girl is defined by tracing several times the edges, as we can appreciate thanks to the numerous corrections in the placing of the head and of the knees. [From ref. 4]
© Archetype Publication in association with the University of Oslo.
4Wide characterization of pigments has been achieved non-invasively exploiting the MOLAB integrated analytical method based on XRF elemental analysis, followed by molecular investigations, namely by near- and mid-FTIR, UV-Vis absorption and emission, and Raman spectroscopy and/or X-ray diffraction. As an example of the molecular recognition of different pigments, we report some results concerning the painting The Ferry Boat from Antwerp by Jacob Jordaens (c. 1623). The artwork was studied at the Statens Museum for Kunst in Copenhagen (fig. 2) where one of the questions addressed by the users was the identification and mapping of blue pigments over the painting surface. Indigo and smalt were found on original areas such as on the woman’s skirt (fig. 2). Jordaens used the organic dye as a dark underpaint, highlighted by partially discolored smalt brushstrokes. He applied azurite to color both the sky and the sea, often mixed with malachite and lead white. Besides the original pigments, a further two blue compounds (cerulean blue and Prussian blue, not temporally compatible with Jordaens’s period) were revealed and assigned to early restorations, dating back to 1797-1884, probably carried out to disguise the discoloration of smalt.
Fig. 2. Study of blue pigments of The Ferry Boat from Antwerp (c. 1623) by Jordaens at the Statens Museum for Kunst in Copenhagen (a detail of the painting is shown in the middle)
(a) smalt has been identified on the basis of Co signals in XRF spectra (left) and near-FTIR (right) evidencing the typical shape of the d-d transition of Co(II) in pseudo-tetrahedral coordination; (b) indigo has been identified on the basis of mid-FTIR (left) features and the UV-vis fluorescence emission at 730 nm (right); (c) azurite has been identified by the presence of the Cu signal in XRF (left) and mid-FTIR combination bands (right); (d) cerulean blue has been distinguished from smalt by the signal of Sn in XRF (left) and by the different position and shape of the Co(II) transition in the near-FTIR (right); (e) Prussian blue has been identified by the strong CN asym. stretching in the mid-FTIR spectrum. Inset in (f) shows the optical image of a cross-section taken from the skirt of the woman on the boat. [From ref. 1]
© American Chemical Society.
5The MOLAB non-invasive in situ approach has been recently demonstrated to be suitable also for the discrimination among pigments sharing similar chemical compositions and structures, as those belonging to the following chemical classes: Pb2Sb2-xYxO7-x/2 (Y=Sn, Zn, Fe, Pb) of interest in studies on Renaissance glazed ceramics, or PbCr1-xSxO4, (1−x)PbCrO4·xPbO and Cd1-xZnxS and CdS1-xSex which are yellow, orange or red synthetic pigments widely used by modern and contemporary painters2. On this regard, the possibility to identify and map the distribution of different type of chrome yellows (CY, PbCr1-xSxO4) has attracted the interest of scholars dealing with issues related to the conservation of Van Gogh’s paintings. In fact, the darkening of CY, caused by the photo-reduction of original chromates to Cr(III)-compounds, is favored when the pigment is present in the S-rich form of PbCr1-xSxO4 (x>0.4, also called light sensitive chrome yellow = LS-CY)5. In fig. 3, Raman profiles of different CYs, as recorded on Van Gogh’s paintings (at the Van Gogh Museum in Amsterdam, VGM), and localization of different forms of chrome yellows and other pigments on the Sunflowers (VGM), as determined by Raman spectroscopy, are reported.
Fig. 3. (A) Distribution of different forms of chrome yellows and other pigments on Sunflowers (Van Gogh Museum, Amsterdam)
(B) Selection of Raman profiles acquired from yellow areas of the painting.
(A) LS-CY=light sensitive chrome yellow; LF-CY: lightfast chrome yellow; CO = chrome orange; V = vermilion; RL = red lead.
(B) V and RL indicate spectra recorded where chrome yellow is mixed with vermilion and red lead. [From ref. 5].
© With permission of John Wiley and Sons.
6In the context of diagnostics for conservation, a wide number of MOLAB access projects have been carried out by users interested to assess the distribution of alteration and/or migration compounds (e.g. oxalates, metal carboxylates and surfactants) at the surface of a painting. As an example, reflection mid-FTIR spectra indicating the presence of copper oxalates collected during different MOLAB projects6 are shown in fig. 4. As a further example, B. Ormsby and coworkers7 exploited portable mid-FTIR spectroscopy (fig. 5) and atomic force microscopy (AFM) for documenting relative differences in surface surfactant abundance on acrylic paintings with respect to: paint brand, pigment type, and changes induced by surface cleaning treatments, differentiating between the effects of aqueous and aliphatic solvent systems7.
Fig. 4. Identification and mapping of Cu oxalates
Photographs refer to MOLAB in situ measurements on: (I) a painted early Meissen Stoneware (18th C) at the State Art Collections, Dresden, DE , (II) a panel of the altarpiece painting Last Judgment (1465-1471) by H. Memling at the National Museum, Gdansk, PL; (III) a Silesian painted map of 17th-18th century at the Ossolinski National Institute, Wroclaw, PL. Corresponding in situ mid-FTIR spectra in pseudo-absorbance (a) and after Kramers-Krönig correction (b). [Rearrangement from ref. 6]
© Photo MOLAB.
Fig. 5. Reflectance mid-IR spectra from the surface of several paint passages on John Hoyland’s 25.4.69 (1969) at The Tate Modern, London, showing the presence of PEO surfactant (largest band at ca. 1110 cm-1) on the brown, red and green paints. [From ref. 7]
![Fig. 5. Reflectance mid-IR spectra from the surface of several paint passages on John Hoyland’s 25.4.69 (1969) at The Tate Modern, London, showing the presence of PEO surfactant (largest band at ca. 1110 cm-1) on the brown, red and green paints. [From ref. 7]](docannexe/image/592/img-5.jpg)
© Photo MOLAB.
7The MOLAB access program registered an outstanding impact on the heritage science community, promoting a large diffusion of the scientific approach to the study and conservation of artworks and stimulating discussions on results among scientists, scholars and professionals in conservation that strongly contributed to the creation of a common language between them, overcoming the barrier possibly imposed by the different disciplinary ground.
8The MOLAB platform has been recently upgraded in the current IPERION CH project with new instruments including advanced mapping/imaging multi/hyperspectral tools for 2D and 3D examinations. Five of the new MOLAB instruments are prototypes specifically developed within the CHARISMA joint research activities for measurements of Single-Sided NMR Depth Profiling (NMR-MOUSE), integrated Absorption-Emission spectroscopy and Fluorescence Decay, Terahertz spectroscopy and imaging (T-Hz), Digital Holographic and Speckle Pattern Interferometry (DHSPI) and Optical Coherence Tomography (OCT).
9The portable T-Hz Time Domain Spectroscopy system is capable to retrieve information in situ from different layers within stratified samples. It allows, for example, the characterization (identification/visualization) of hidden sub-layers to be achieved, as in easel and mural paintings. A 2D and 3D mapping on and in the inspected objects can be also carried out using the time of flight8.
10The new DHSPI device is an improvement of a preceding version suitable for non-destructive diagnosis of the structural condition of an object, by detecting cracks, detachments, or other defects. The revealed alterations can be located on the surface and in the interior of the object and are detected in the form of interference patterns. From the measurements, a risk-map is created, after the quantitative analysis of the fringe density per surface unit 9.
11OCT examination aides in resolving the structure of primary layers on a painting, provided they are transparent enough to near-IR radiation. In particular, it allows useful applications to be carried out for studies of deteriorated varnishes, glazes, over-paintings, and retouchings. Ancient glass artefacts can be profitably also investigated, as well as glazes on ceramics and semitransparent stones, as jades or other materials. As a result of balance between resolution and depth of examination, the OCT system developed within CHARISMA consists of a very compact spectral domain instrument equipped with a super-luminescent source (870 nm central wavelength, 200 nm bandwidth, 0.8 mW at the object) with 3.1 µm axial resolution (in air) and 13 µm lateral resolution. Using this device, successful studies have been carried out and, among several examples, we report the study of the varnish of the Leonardo’s Madonna dei Fusi at OPD in Firenze, where number, depth and dimension of retouchings were characterized in detail. Front OCT imaging at various depths (separated by coherent detection), made possible to establish the sequence of varnish layers in restored areas, as well as to recover the outline of the retouching and compare it to the contour of actual paint losses10. Exemplary slices of such a multilayer structure are shown in fig. 6.
Fig. 6. Results of the OCT examination of the painting Madonna dei Fusi (private collection), attributed to Leonardo da Vinci (16th C.), oil on wood (detail)
(a) OCT tomogram from the region indicated by red bars in pictures (b, c); (d, e, f) OCT front images at different depths under the surface (6, 18, 35 µm respectively). (1A, 1B) varnishes, (2A, 2B) two stages of retouching, (3) primary paint layer. [From ref. 10]
© With permission of Springer/Photo OPD.
12The enhancement of MOLAB TNA capabilities in 2D and 3D examination meets the current demands of advanced research in heritage science, not only to reveal the chemical composition of materials but also to map their spatial distribution. Thus, the introduction of the new cutting edge imaging systems currently extends the interest in MOLAB, widening the user groups to new potential users.
The MOLAB activities described in this work were possible thanks to the support of the European Commission, through the Research Infrastructures projects Eu-ARTECH (FP6 -RII3-CT-2004-506171) and CHARISMA (FP7- GA n. 228330).
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