Synthetic chemistry meets with artwork restoration

How can synthetic chemistry be used as a tool to solve artwork restoration problems? This issue has recently been addressed through two case studies: stone material protection and blanching easel painting restoration.

Like cultural heritage in general, the survival of stone artworks and easel paintings is a challenge because of diverse degradations due to environmental, anthropogenic and biological conditions. In particular, water is commonly considered as one of the main factors causing deterioration of stone artworks because it can start and promote harmful physical, chemical and biological processes. For example, the devastating flood which happened in 1966 in Florence (Italy) severely and sometimes irreversibly damaged stone sculptures and monuments. Chemistry was then recognized as having an important role to play for recovering damaged artworks. Different problems of degradation and aging caused by the action of water were studied after this event. In particular, polymeric products, especially amide derivatives of perfluoropolyethers, were synthesized and investigated as water-repellents on different stone materials at the University of Florence and in specialized research units of the Consiglio Nazionale delle Ricerche (CNR). However, researchers are still looking for products which can ensure good water-repellency, effective adhesion on stone as well as environmentally friendly practices during restoration [1].

Concerning easel paintings, extra moisture is considered to be responsible for blanching phenomena. Blanching is an alteration, explained by common moisture-induced clouding alteration which affects varnish and paint layers of easel oil paintings. Blanching can appear after water damage, storage in a humid environment or moisture-based restoration. Depending on the degree of deterioration, the pictorial composition can be partially opacified or even completely concealed by whitish veil. Highly porous structures are observed on the blanching areas of easel paintings by scanning electron microscope (SEM). The Girodet Museum at Montargis (France) was severely damaged by a flood in June 2016. The museum provisional reserves were submerged. 5900 works have remained under water for 72 hours and almost all of the paintings now show significant blanching phenomena (fig. 1). The lack of effective and durable treatments to overcome the blanching of paint layers is frequently mentioned by restorers. Common restoration treatments for blanching paintings are usually recognized as “folklore”, like rubbing with egg or oil, treatment with solvents, etc. Those restoration treatments are not efficient enough and blanching reappears on the paintings. Recently, perfluoropolyether diamide was reported to be a promising restoration product for blanching paintings [2]. It raised the opportunity to set up restoration treatments for blanching paintings by using compounds containing perfluorinated chains.

Fig 1. Portrait d’Alexandre Dumeis, François-Hippolyte Debon, Musée Girodet, Montargis © C2RMF/P. Salinson

A PhD was launched in 2018 to study and identify innovative products for an adequate restoration of stone artworks and blanching easel paintings. Previous researches had shown that partially perfluorinated compounds (especially partially perfluorinated oligoamides) provided promising performance for those restoration issues. For example, they offer good water repellency as stone protection products and contribute to decrease blanching in easel paintings by filling the pores. Inspired by those promising results, two families of partially perfluorinated derivatives with hydroxyl groups were designed as restoration products for stone protection and blanching painting restoration in this PhD research project. In particular, partially perfluorinated derivatives including C-glycosides and oligoamides were proposed.

C-glycosides are carbon-linked analogues of naturally occurring sugars which have high hydrophilic properties due to the polar hydroxyl groups. Those hydroxyl groups could be expected to give good adhesion on polar stone substrates by hydrogen bonding. Hydroxyl groups can also be compatible with the porous structures appearing in blanching easel paintings and help the molecules entering into the pores. Meanwhile, hydrophobicity of the compound can be realized by introducing perfluorinated chain into C-glycoside. Thus, C-glycosides with perfluorinated tails could give good water repellency to protect the stone materials from water by pushing the fluorinated segments on the outer surface and achieve a good coating on the stone materials. In addition to the hydrophilicity and water repellency, C-glycosides themselves possess an improved stability towards acid, base and enzymatic hydrolysis due to their C-C bond between aglycone and sugar [3]. This feature can contribute to the good stability of the partially perfluorinated C-glycosides as restoration products, together with the high stability of C-F bonds in the perfluorinated chain. Eventually, the proposed C-glycoside with a perfluorinated tail could demonstrate good hydrophobicity and good adhesion for stone protection, and help filling the pores in blanching paintings. Starting from the unprotected carbohydrate, the natural, renewable and cheap D-glucose, β-C-glycosidic ketone as the direct C-glycosidation product was successfully obtained via Lubineau reaction with green-chemistry process. Target compound, partially perfluorinated C-glycoside (C-Glc-OH), was successfully obtained through the convenient one-pot reductive amination reaction with medium yield. Besides, the protected partially perfluorinated C-glycoside (C-Glc-OAc) was successfully obtained by the similar synthesis route. C-Glc-OAc was synthesized for control purposes, in order to investigate whether hydroxyl groups in C-glycosides can improve the restoration efficacy as expected. Different physical properties like physical states, solubility and color could indeed have an influence on the restoration performance.

With the aim to find suitable partially perfluoroderivatives, four new partially perfluorinated compounds with different structures and properties (i.e. solubility, hydrophilicity, different chain length, molecular weight, etc.) were obtained. Among them, two partially perfluorinated oligoamides contain also hydroxyl groups, which are named as DSTF and ESTF. In order to understand the roles of hydroxyl groups in the applicative performance, two partially perfluorinated oligamides (DSF, ESF) with no hydroxyl groups, but with the unchanged amine and succinate sources, were synthesized. In addition, a partially perfluoroamine (DF) was obtained in order to compare its applicative performance with the compounds containing amide functional groups.

The protection efficacy of those seven synthesized partially perfluorinated derivatives was tested on stone materials (Lecce stone, sandstone, and marble) by a series of measurements. Those measurements include solubility in environment friendly solvents, resistance to photodegradation, water repellency, chromatic effect after coating, contact angle, and vapor permeability. Partially perfluorinated oligoamide ESF was demonstrated to be a suitable protective agent for Lecce stone. DSF and DF could be potential protective agents for sandstone or marble if the chromatic change can be decreased after coating. Although C-Glc-OH showed low protective efficacy, the partial or full protection of the hydroxyls via -CO-NH- formation on the C-glycoside could improve its water repellency.

All the seven compounds were also tested for blanching easel painting restoration (fig. 2). Based on the results of the tests on blanching mock-up samples through pure-pigment test, colorimetric measurements, morphological study, and reversibility test, DSTF, DSF, DC6G900, DF, and ESTF could be considered as promising restoration compounds by acting effectively against blanching. Among those compounds, DSTF, DSF, and DC6G900 are recommended as restoration products for blanching paintings.

Fig. 2. Photography of treated raw umber samples by synthesized partially perfluorinated derivatives ©C2RMF/ Anne Maigret

In conclusion, we proposed and successfully synthesized partially perfluorinated derivatives for stone protection and blanching painting restoration. The synthesized compounds (ESF, DSF, DF, DSTF) show good efficacy towards different restoration issues. Further optimization of the molecular structure could be studied in the future by controlling the number of polar groups and molecular weight in C-glycoside and oligoamides as well as the concentration of solutions/suspensions for practical applications.

Yuqing Zhang

[1] Piacenti, F., “Chemistry for the conservation of the cultural heritage”, Science of the total environment, 143(1), 1994, p. 113-120.

[2] Genty, A., Baglioni, P., Eveno, M., Bastian, G., Uziel, J., Lubin-Germain, N., Menu, M., « Les chancis des peintures de chevalet : étude des traitements de restauration actuels et proposition d’une alternative innovante », Technè. La science au service de l’histoire de l’art et de la préservation des biens culturels, 46, 2018, p. 90-95.

[3] Rodrigues, F., Canac, Y. and Lubineau, A., “A convenient, one-step, synthesis of β-C-glycosidic ketones in aqueous media”, Chemical Communications, 20, 2000, p. 2049-2050.

Partenaires du projet :

Nadège Lubin-Germain, Florian Gallier, BioCIS – Biomolécules : Conception, Isolement, Synthèse, UMR 8076 (CY Cergy Paris University, Paris Saclay University, CNRS)

Antonella Salvini, Department of Chemistry ‘Ugo Schiff’, University of Florence

Mara Camaiti, CNR-IGG, Italy

Agnès Lattuati-Derieux, Myriam Eveno, C2RMF

Publications associées au projet :

YQ. Zhang, L. Vespignani, MG. Balzano, L. Bellandi, M. Camaiti, N. Lubin-Germain, A. Salvini, “Low fluorinated oligoamides for use as wood protective coating”, Coatings, 12(7), 2022, 927.

Crédit Photo à la une : Portrait d’Alexandre Dumeis, François-Hippolyte Debon, Musée Girodet, Montargis © C2RMF/P. Salinson