Abstract
During the conservation treatment of Memling’s Christ with Singing and Music-making Angels, three panel paintings that are among the most monumental works in early Netherlandish art, the conservators came across insoluble surface layers containing calcium oxalates. A very thin and irregular layer of this type, hardly visible to the naked eye, was spread across the surface of all three panels. A much thicker layer forming an opaque and highly disfiguring crust that obscured the composition (Figs. 15.1 and 15.7) was locally present on areas of dark copper-containing paint, where multiple layers of old discolored coatings and accretions remained in place before the most recent cleaning.
This article describes the application of a wide range of analytical techniques in order to fully understand the stratigraphy and composition of the crusts on the Memling paintings. FTIR spectroscopy in transmission and reflection mode, micro-ATR-FTIR imaging and macro-rFTIR scanning, SEM-EDX, mobile XRD, and SR-μXRD showed that the crusts contained two related Ca-based oxalate salts, whewellite and weddellite, and were separated from the original paint surface by varnish, indicating that they did not originate from degradation of the original paint but from a combination of microbial action and a thick accumulation of dirt. Supported by the results from these different analytical techniques, which when used together proved to be very effective in providing complementary information that addressed this specific conservation problem, and aided by the presence of the intermediate varnish layer(s), the conservators were able to remove most of the crusts with spectacular results.
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Notes
- 1.
“Llegó a Madrid, donde, tras su limpieza, se intentó que lo comprara la Reina Maria Christina,” (Gómara 2013). Many thanks to Sara Mateu for providing the article.
- 2.
Flaking paint was noted in February and May 1901, in different areas. It was mentioned that the panels were covered by a layer of dust that adhered to the surface and diminished the brightness of the colors. Consolidation and surface cleaning with bread crumbs was proposed, but the charge made for the work was only for consolidation.
In 1952, 1977, and 1980, the panels occur in lists among other paintings that were treated, but details were not specified.
Although nothing is mentioned about the cradles, judging from their appearance, it is thought that they were attached in the 1950s or 1960s when the panels were already in the museum. In addition, a synthetic varnish (isobutyl acrylate) was found on the surface which must also have been applied in the museum.
- 3.
Infrared imaging was carried out twice: at the beginning of the project by Adri Verburg, using a phase I scanning back (4 × 5 inch) with spectral range 1000–1100 nm, and during the project by Claudia Daffara and Mattia Patti, using a multispectral scanner, spatial resolution 4 pt./mm (250 micron), InGaAs with spectral range 800–1700 nm, C.N.R.–I.N.O.A (Istituto Nazionale di Ottica Applicata).
- 4.
X-radiography was undertaken by Guido van de Voorde, Royal Institute for Cultural Heritage, Parc du Cinquantenaire 1, B-1000 Brussels.
- 5.
SEM-EDX detected copper in the blue and green paints, suggesting the use of azurite and a copper-containing green pigment (most probably originally verdigris). The grey paint layer depicting the clouds consists mainly of a mixture of lead white and a black pigment (probably a carbon-based black), mixed with a little azurite, identified by SEM-EDX and FTIR microscopy.
- 6.
http://www.esrf.eu/computing/scientific/FIT2D/. Accessed February 2017.
- 7.
http://www.bruker-axs.de/eva.html . Accessed February 2017.
- 8.
http://xrdua.ua.ac.be/. Accessed February 2017.
- 9.
- 10.
MOLAB is a mobile laboratory composed of a unique collection of portable equipment which is available to cultural heritage researchers across Europe, including art historians, conservators, and conservation scientists. The MOLAB analyses presented here were funded under the EU CHARISMA project (FP7, grant no. 228330). MOLAB can currently be accessed through the Horizon 2020 IPERION project (www.iperionch.eu, Grant No. 654028).
- 11.
In principle, whewellite and weddellite can also be distinguished on the basis of their FTIR spectra (although there is some disagreement in the literature about precise band positions), but in practice this is difficult, particularly when the salts are present in mixtures or in combinations with other compounds, see, for example, Garty et al. (2002), Conti et al. (2010), and Leroy (2016).
- 12.
Although the conservators were of the opinion that the copper-containing paint on top of the varnish layer was probably overpaint, it was left on the surface during cleaning as no certain proof for this could be found in the cross sections or on the surface of the paintings. It was not present as a continuous layer but more as local spots.
- 13.
The location of the copper oxalates suggests that some degree of migration of copper ions into adjacent layers has occurred but not all the way up into the surface crust. These results clarify why only calcium oxalates were detected at the surface of the paintings in the scrapings and using the MOLAB non-invasive reflectance FTIR equipment. It should be noted that calcium oxalates may also be present in those layers containing copper oxalates, but this cannot be confirmed because of the overlap of the spectral bands and the 900 cm−1 cutoff for the micro-ATR-FTIR imaging equipment.
- 14.
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Acknowledgments
A conservation project this large involves many people. Many thanks to Ineke Labarque, Régine Wittermann, Jean Albert Glatigny, Sara Matteu, Madeleine ter Kuile, Greta Toté, the advisory committee, and all colleagues from the conservation studio. We are indebted to the staff of the P06 beamline of the PETRA III facility and to DESY for providing the beam time for the synchrotron radiation-based measurements. KJ, GvdS, FV, and SL acknowledge financial support from the University of Antwerp Research Council via SOLARPAINT and other projects, the Fund Baillet Latour, and the Fund for Scientific Research (FWO), Brussels. CH and MS acknowledge the assistance of their colleague David Peggie. The conservation of the panels was supported by the Friends of the KMSKA and Interbuild NV, and the associated research was supported by the Baillet Latour fund.
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Klaassen, L. et al. (2019). Characterization and Removal of a Disfiguring Oxalate Crust on a Large Altarpiece by Hans Memling. In: Casadio, F., et al. Metal Soaps in Art. Cultural Heritage Science. Springer, Cham. https://doi.org/10.1007/978-3-319-90617-1_15
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