The works of art and artifacts that constitute our cultural heritage are subject to deterioration, both from internal and from external factors. Surfaces that interact with the environment are the most prone to aging and decay; accordingly, soiling is a prime factor in the degradation of surfaces and the attendant disfigurement of a piece. Coatings that were originally intended to protect or contribute aesthetically to an artwork should be removed if they begin to have a destructive impact on its appearance or surface chemistry. Since the mid-19th century, organic solvents have been the method of choice for cleaning painted surfaces and removing degraded coatings. Care must be taken to choose a solvent mixture that minimizes swelling of or leaching from the original paint films, which would damage and compromise the physical integrity of all the layers of paint. The use of gels and poultices, first advocated in the 1980s, helps by localizing the solvent and, in some cases, by reducing solvent permeation into underlying paint layers. Unfortunately, it is not always easy to remove gels and their residues from a paint surface. In this Account, we address the removal problem by examining the properties of three classes of innovative gels for use on artwork--rheoreversible gels, magnetic gels, and "peelable" gels. Their rheological properties and efficacies for treating the surfaces of works have been studied, demonstrating uniquely useful characteristics in each class: (1) Rheoreversible gels become free-flowing on application of a chemical or thermal "switch". For art conservation, a chemical trigger is preferred. Stable gels formed by bubbling CO(2) through solutions of polyallylamine or polyethylenimines (thereby producing ammonium carbamates, which act as chain cross-links) can be prepared with a wide range of solvent mixtures. After solubilization of varnish and dirt, addition of a weak acid (mineral or organic) displaces the CO(2), and the resulting free-flowing liquid can be removed gently. (2) Incorporation of magnetic, coated-ferrite nanoparticles into polyacrylamide gels adds functionality to a versatile system comprising oil-in-water microemulsions, aqueous micellar solutions, or xerogels that act as sponges. The ferrite particles allow the use of magnets both to place the gels precisely on a surface and to lift them from it after cleaning. (3) Novel formulations of poly(vinyl alcohol)-borate gels, which accept a range of organic cosolvents, show promise for swelling and dissolving organic coatings. This family of gels can be quite stiff but can be spread. They are non-sticky and have sufficient strength to be removed by peeling or lifting them from a sensitive surface. These three classes of gels are potentially very important soft materials to augment and improve the range of options available for conserving cultural heritage, and their interesting chemical-physical properties open a rich area for future scientific investigation.

Tackling the degradation of cultural heritage requires a global effort. We call on all material scientists to develop new nanomaterials and methods for the preservation of artwork.

This study reports the physicochemical investigation of oil-in-water (o/w) microemulsions and micellar solutions formulated to solubilize acrylic and vinyl polymers. Three different four-component systems formed from (i) water, p-xylene, sodium dodecylsulfate, and 1-pentanol; (ii) water, propylene carbonate, sodium dodecylsulfate, and 1-pentanol; and (iii) water, p-xylene, Tween-20, and 1,2-ethandiol, and one five-component system (water, p-xylene, commercial nitro-diluent, sodium dodecylsulfate, and 1-pentanol) have been studied. The o/w microemulsions or micellar solutions (system ii) have been characterized by quasi-elastic light scattering experiments to obtain the hydrodynamic radius and the polydispersity of the microemulsion or micelle droplets. The application of these microemulsion or micellar systems as solubilizing agents for acrylic and vinyl polymers from works of art (mainly wall paintings) and monuments (stones) has been investigated. Acrylic and vinyl polymers have been extensively used in the past decades, and are still used, for art conservation. The aging produced both yellowing and serious degradation of the painted layers or of the stone surfaces, imposing their removal from the artistic and architectonic surfaces. Contact angle measurements, FTIR, and SEM/EDX results showed that the microemulsions and micellar solutions investigated were very effective in removing acrylic or vinyl polymeric resins from several solid surfaces. Hydrophobic acrylic copolymers have been completely removed from a Renaissance fresco by Spinello Aretino (used during a restoration performed in the sixties) in the Cappella Guasconi in San Francesco Cathedral, Arezzo, Italy, and poly(vinyl acetate) resins (used in a restoration performed during the fifties) from Renaissance frescoes decorating the external walls of the Cathedral of Conegliano, Northern-East Italy. The nanocontainers route represents a new, safe, and very efficient method for removing aged polymer from surfaces of works of art, otherwise condemned to complete loss.

The synthesis and characterization of a novel series of polymer gels are reported. They are formed at ambient temperatures by bubbling CO(2) through solutions of polyallylamine (PAA; a latent gellant) in several organic liquids, including aliphatic alcohols and 1-methyl-2-pyrrolidone. The stabilities of the alcohol gels, as indicated by the (irreversible) degelation temperature, T(g)(dt), were strongly dependent on the number of carbon atoms (C(n)) in the alkyl chains of the alcohol liquids. The mechanism of formation and the microenvironmental properties of PAA-based gels containing a small amount of a pH-sensitive probe, 2,6-naphthalenedicarboxylic acid (2,6-NDCA), have been probed using static and dynamic fluorescence measurements. A measurable pH change and significant alterations to the fluorescence spectra were coincident with gelation of PAA solutions in 1-butanol as CO(2) was bubbled through, and the fluorescence spectra were monitored over several hours until no further changes were detected. Analyses of dynamic fluorescence decay histograms indicate the presence of three decay times due to different microenvironments where the 2,6-NDCA molecules are located.