pubs.acs.org/Langmuir © 2009 American Chemical Society
Gels for the Conservation of Cultural Heritage† Piero Baglioni,* Luigi Dei, Emiliano Carretti, and Rodorico Giorgi Department of Chemistry and CSGI, University of Florence, via della Lastruccia 3 - Sesto Fiorentino, 50019 Florence, Italy, Received March 19, 2009. Revised Manuscript Received April 8, 2009 Gels are becoming one of the most important tools for the conservation of cultural heritage. They are very versatile systems and can be easily adapted to the cleaning and consolidation of works of art. This perspective reviews the major achievements in the field and suggests possible future developments.
It is not an easy task to establish when a new perspective of human knowledge was born. Science advances continuously, and discontinuities, the hallmarks of new and fundamental discoveries, are rare. Modern conservation science originated from the tragic flood that devastated Florence and Venice in 1966, imposing the search for new conservation methods.1,2 The Ferroni-Dini method is an important example of the philosophy underlying the science of conservation that has two major branches. One of these is the search for new scientific methods for conservation, including gels. Originally, the use of gels was confined to aesthetics rather than preservation (i.e., cleaning procedures necessary to remove unwanted layers of varnish, grime, stains, and dirt). Today, gels are a fast growing area of conservation, and their use includes both cleaning and stone preservation. Although the use of sol-gel systems for stone preservation can be traced to the 18th century, George Wheeler revitalized the field, making the use of alkoxysilanes a “standard” in stone conservation.3,4 This method has been extended to the use of nanoparticulate inorganic sols (nanosols). An example is the impregnation of wood with silica sols5 by the sol-gel technique. Because of their high surface-to-volume ratio, nanosols are metastable and usually hydrolyze to form 3D xerogel networks that improve the mechanical properties and resistance to water, fire, and microbial or insect attack of wood or stone objects. The major contribution of gels is related to the cleaning of works of art. The common cleaning procedure for painted surfaces was based on mechanical action or organic solvents to remove surface dirt from works of art. Unfortunately, solvent penetration into the paint layers can lead to swelling6 and leaching of organic structural components7,8 (mainly polymeric substances), causing a † Part of the Molecular and Polymer Gels; Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding author. Phone: +39 0554573033. E-mail: piero.baglioni@ unifi.it.
(1) Ferroni, E.; Malaguzzi, V.; Rovida, G. Proceedings of the ICOM Conference, Amsterdam, September 1969. (2) Ferroni, E.; Baglioni, P. Proceedings of the Symposium ‘‘Scientific Methodologies Applied to Works of Art’’, Florence, Italy, 1984. (3) Wheeler, G.; Mendez-Vivar, J.; Fleming, S. J. Sol-Gel Sci. Technol. 2003, 26, 1233–1237. (4) Wheeler, G. Alkoxysilanes and the Consolidation of Stones; Getty Conservation Institute: Los Angeles, 2005. (5) Mahltig, B.; Swaboda, C.; Roesslerc, A.; Bottcher, H. J. Mater. Chem. 2008, 18, 3180–3192. (6) Michalski, S. In Preprints of the Contributions to the Congress Cleaning, Retouching and Coatings; Mills, J. S., Smith, P., Eds; IIC: London, 1990; pp 85-92. (7) Stolow, N. Stud. Conserv. 1957, 3, 40–44. (8) Stolow, N. Nature 1957, 179, 179–182.
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decrease in the stability of the paint. In the last two decades, gels have been employed to decrease these deleterious effects. A major advance was proposed by Wolbers in the late 1980s.9,10 His systems basically act as modifiers of pure solvents, enzymes solutions, and so forth that are traditionally used in the cleaning procedures. Gels offer advantages over neat solvents related to the evaporation control of the solvent entrapped in the gel, the control of the penetration inside the work of art by limiting capillary action, and the control over the area to be treated. The gels proposed by Wolbers were characterized by the presence of a polymer (mainly poly(acrylic acid)) as a gellant.9-12 Other components such as cosolvents, detergents, and enzymes can be added to the gel to improve the cleaning capacities. Responsive gels13-23 offer several advantages over nonresponsive physical solvent gels for conservation applications. These gels are both effective as cleaning tools and are easily removable from the painted surface once they have carried out their function because they are converted rapidly to free-flowing liquids. Thus, by modulating the chemical and/or conformational properties of the gelator, it is possible to apply a gel and, after the (9) Wolbers, R. C. In Preprints to the Conference Restoration ’92: Conservation, Training, Materials and Techniques, Latest Developments; Todd, V., Marsden, J., Talley, M. K., Lodewijks, J., Sluyterman, V., Koeno, W., Eds; IIC: London, 1992; pp 74-75. (10) Wolbers, R. C. Cleaning Painted Surface. Aqueous Method; Archetype Publications: London 2000. (11) Stulik, D.; Miller, D.; Khanjian,N.; Wolbers, R.; Carlson, J.; Petersen, W. C. In Solvent Gels for the Cleaning of Works of Art. The Residue Question; Dorge, V. Ed.; Getty Conservation Institute: Los Angeles, 2004. (12) Cremonesi, P.; Curti, A.; Fallarini, L.; Raio, S. Progetto Restauro 2000, 7, 25–33. (13) Aggeli, A.; Bell, M.; Boden, N.; Keen, J. N.; Knowles, P. F.; Mcleish, T. C.; Pitkeathly, M.; Radford, S. E. Nature 1997, 386, 259–262. (14) Yoshida, M.; Asano, M.; Suwa, T.; Katakai, R. Radiat. Phys. Chem. 1999, 55, 677–680. (15) Matsumoto, S.; Yamaguchi, S.; Ueno, S.; Komatsu, H.; Ikeda, M.; Ishizuka, K.; Iko, Y.; Tabata, K.; Aoki, H.; Ito, S.; Noji, H.; Hamachi, I. Chem.;Eur. J. 2008, 14, 3977–3986. (16) Saunders, J. M.; Tong, T.; Le Maitre, C. L.; Freemont, T. J.; Saunders, B. R. Soft Matter 2007, 3, 486–494. (17) Suzuki, H. J. Intell. Mater. Syst. Struct. 2006, 17, 1091–1097. (18) Bonini, M.; Berti, D.; Di Meglio, J. M.; Almgren, M.; Teixeira, J.; Baglioni, P. Soft Matter 2005, 1, 444–454. (19) Carretti, E.; Dei, L.; Baglioni, P.; Weiss, R. G. J. Am. Chem. Soc. 2003, 125, 5121–5129. (20) Carretti, E.; Dei, L.; Weiss, R. G.; Baglioni, P. J. Cultural Heritage 2008, 9, 386–393. (21) Carretti, E.; Dei, L.; Weiss, R. G. Soft Matter 2005, 1, 17–22. (22) Carretti, E.; Macherelli, A.; Dei, L.; Weiss, R. G. Langmuir 2004, 20, 8414–8418. (23) Juntanon, K.; Niamlang, S.; Rujiravanit, R.; Sirivat, A. Int. J. Pharm. 2008, 356, 1–11.
Published on Web 05/19/2009
DOI: 10.1021/la900961k
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Figure 2. Relined canvas where a polymer used for the relining process has been removed with a “hard gel”.
Figure 1. Eighteenth century painted wood sculpture with a surface layer of a degraded natural varnish before (right side) and after (left side) cleaning by a gel of poly(ethylene imine) as the gellant and 1-pentanol as the continuous phase.19,20
activation of a chemical or physical switch, induce a physical gel/ sol transition.21,22 Removing the sol from the surface of a work of art minimizes mechanical action over the surface of the work of art and diminishes the possibility of surface damage. An example of cleaning action by a rheo-reversible gel is shown in Figure 1. A new highly elastic viscous poly(vinyl alcohol) hydrogel for conservation applications has been generated by a cross-linking agent (usually borate).24 Adhesion of the gel to the art object can be modulated by controlling the polymer structure and the degree of cross-linking. After the cleaning action, the gel can be easily peeled from the painted surface without the addition of a second liquid component, as is necessary with the Wolbers solvent gels. Recently, the functionalization of a chemical gel by adding magnetic nanoparticles to the 3D polymeric network has been developed to make it responsive to an external magnetic field.25,26 This allows the complete removal of the cleaning gel from the painted surface without leaving residues and avoiding additional contact between a conservator and the work of art while maintaining good cleaning action. These systems can be shaped as desired and applied to a specific area with fine spatial control of the area to be treated. Control of polymer cross-linking and therefore of the mesh size of the gel network allows additional control of the compartmentalization of the cleaning system (24) Dei, L.; Carretti, E.; Grassi, S.; Cossalter, M.; Natali, I.; Caminati, G.; Weiss, R.; Baglioni, P. Langmuir 2009, http://dx.doi.org/10.1021/la804306w. (25) Bonini, M.; Lenz, S.; Giorgi, R.; Baglioni, P. Langmuir 2007, 23, 8681–8685. (26) Bonini, M.; Lenz, S.; Falletta, E.; Ridi, F.; Carretti, E.; Fratini, E.; Wiedenmann, A.; Baglioni, P. Langmuir 2008, 24, 12644–12650.
8374 DOI: 10.1021/la900961k
(i.e., microemulsions or micellar solutions26) that can be embedded in the gel.25-29 For example, as shown in Figure 2, microemulsion release can be efficiently controlled by increasing the cross-linking degree (leading to harder gels), and polymer removal from the canvas can be done to avoid the unwanted spreading of the microemulsion into the canvas.30 The nanocompartimentalized gel systems are effective in cleaning wall paintings, stone objects, and painted surfaces without leaving undesired residues. Because they are easily prepared and used, they are expected to have a dramatic impact on the methods used in the conservation field, especially where fine-tuning of the release or uptake of a confined material is required. Other gels already described in the literature may be useful for art conservation as well. Promising systems include (i) redoxresponsive gel-sol/sol-gel systems of aqueous poly(acrylic acid) containing Fe(III) ions that can be switched reversibly by light;31 (ii) chemically responsive supramolecular gels;32 and (iii) temperature-responsive ionic gels that allow the control of specific ion transport by changing the charge density in response to temperature. These latter systems consist of an interpenetrating network of two modified poly(vinyl alcohol) polymer chains: the first chain with sulfonic acid groups and the other with poly(N-isopropylacrylamide) and [poly(nipaam)] grafted chains (nipaam corresponds to N-isopropylacrylamide).33,34 Since the time of Florence’s flood, conservation science has matured and evolved into a branch of nanoscience where soft matter, surface science, and polymer and organic chemistries merge. Many new avenues for additional improvements and approaches abound for art conservation using gels. Acknowledgment. We thank Aurelia Chevalier and Michel Menu (C2MRF-Louvre Museum) for helpful discussions on the use of gels on canvases. Financial support from CSGI and MiUR is acknowledged. (27) Carretti, E.; Dei, L.; Baglioni, P. Langmuir 2003, 19, 7867–7872. (28) Carretti, E.; Giorgi, R.; Berti, D.; Baglioni, P. Langmuir 2007, 23, 6396–6403. (29) Carretti, E.; Berti, D.; Fratini, E.; Dei, L.; Baglioni, P. To be submitted for publication. (30) Chevalier, A.; Chelazzi, D.; Baglioni, P.; Giorgi, R.; Carretti, E.; Stuke, M.; Menu, M.; Duchamp. R. In Proceeding of 15th Triennal Conference ICOM-CC (International Council Of Museum - Committee for Conservation); New Delhi, India, Sept 22-26, 2008; Vol. 2, pp 581-589. (31) Peng, F.; Li, G.; Liu, X.; Wu, S.; Tong, Z. J. Am. Chem. Soc. 2008, 130, 16166–16167. (32) Qingtao Liu, Q.; Wang, Y.; Li, W.; Wu, L. Langmuir 2007, 23, 8217–8223. (33) Higa, M.; Yamakawa, T. J. Phys. Chem. B 2004, 108, 16703–16707. (34) Higa, M.; Yamakawa, T. J. Phys. Chem. B 2005, 109, 11373–11378.
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