Advanced methodologies for the cleaning of works of art

Abstract

Cultural heritage assets constitute a fundamental socioeconomic resource, but the actual works of art need to be maintained, counteracting degradation processes, to transfer these benefits to future generations. In particular, the removal of soil, aged coatings, and vandalism/overpaints is one of the most needed interventions in art restoration. Traditional cleaning methodologies, based on classical solution and polymer chemistry, only grant limited control of the cleaning interventions, with the risk of affecting the original components of the artifacts, and often involving the use of toxic or non-environmentally friendly compounds. Alternatively, materials science, colloids, and soft matter have provided valuable and safe solutions in the last decades. This review provides a selection of the most recent and advanced methodologies for the wet cleaning of works of art, spanning from nanostructured cleaning fluids (microemulsions, surfactants swollen micelles) to physical and chemical gels. The new methodologies work on different physico-chemical mechanisms, such as processes for detaching/dewetting, to selectively remove the unwanted layers in sustainable and cost-effective interventions. The best performing systems, like microemulsions confined in “twin-chain” polyvinyl alcohol gels, have been assessed in the cleaning of masterpieces such as works by Pablo Picasso, Jackson Pollock and Roy Lichtenstein. Particular attention is dedicated to “green” chemistry systems, using low-toxicity solvents or bio-based/waste materials to build gel networks. Finally, current trends and future perspectives are given, showing that advanced systems for art cleaning link with transversal fields of crucial importance even beyond Cultural heritage conservation, e.g., detergency, tissue engineering, drug-delivery, food industry and cosmetics.

From Nanoparticles to Gels: A Breakthrough in Art Conservation Science

“…Successively, different nanostructured cleaning fluids (NCFs) were also developed, using nonionic surfactants and partially water-soluble solvents, which are mostly found in the fluid’s continuous aqueous phase and only partially in the surfactant micelles. ,− These NCFs were designed to remove aged polymeric coatings (e.g., acrylate, vinyl acetate, epoxy), which they do following nonclassic mechanisms. Essentially, these NCFs promote the dewetting of polymer layers (see Figure ), driven by two main factors: (1) good solvents and the surfactant swell and mobilize the polymer chains, and (2) surfactant molecules favor the formation of interfaces and polymer detachment areas, lowering the activation energy to initiate dewetting and speeding up its kinetics.…”

“…Even in cases where the presence of amphiphilic additives in the coating film discourage its dewetting from the artistic substrate, the fast dynamic exchange of solvent, surfactant, and cosurfactant from the NCFs continuous and dispersed phases to the polymer layer causes its swelling, softening, and feasible detachment. Overall, these features are key to achieve the high versatility and efficacy that the NCFs have exhibited in the past decades over a range of case studies spanning from classic frescos and the Renaissance to modern/contemporary masterpieces by Pablo Picasso. ,,,− Figure shows two examples where the NCFs were used to remove aged varnish from a painted wood panel and a complex superimposition of synthetic polymer coatings accumulated on wall paintings in the Annunciation Basilica in Nazareth (Israel) owing to past restorations in the past decades.…”

“…Current challenges involve the formulation of nanostructured cleaning fluids using “green” solvents (e.g., alkyl carbonates and other esters, food grade oils) and cleavable or biodegradable surfactants. , The goal is to keep the same versatility and efficacy of state-of-the-art formulations while adopting sustainable components, and the outcomes of this research are expected to impact also on detergency, cosmetics, and other fundamental industrial fields …”

“…As fundamental as they are in many sectors of material science and colloids, gels have also gained growing attention in cultural heritage conservation through the past decades, in particular polymer gels designed and synthesized with different approaches. ,,− ,− The first main rationale relies on the time and spatial control achievable in art cleaning operations by confinement of cleaning fluids into gel matrices. This allows safe cleaning interventions without the need for lengthy steps checking for possible damages to water- or solvent-sensitive artistic surfaces, overall making the operations time-effective and allowing safe cleanings, in some specific cases ones not achievable with conventional conservation methodologies.…”

“…In response to these issues, interfacial and colloidal chemistry has taken the lead over the past decades in devising and establishing novel advanced solutions for the remedial and preventive conservation of cultural heritage. ,,− Systems like nanoparticles, composite nanomaterials, and soft condensed matter have proven to be valuable tools in the consolidation, cleaning, and protection of works of art belonging to different artistic and historical productions. While this retrospectively comes as a natural result considering that degradation processes often start at the nano- and mesoscale of the artifacts’ surface and interfaces, it must be noticed that the formation of a scientific framework fully dedicated to the design of colloidal systems for art conservation took some time to emerge, initially starting as germinal ideas not yet validated by common views in science.…”