Low-impact cleaning systems for the removal of low-polar materials in cultural heritage conservation

This dissertation aims to present the main research topics I have engaged in over the past three years, in collaboration with several national and international researchers from Sapienza University of Rome and other universities. The primary focus is the investigation of potential alternative cleaning systems for the removal of low-polar substances, centering around solutions with a low impact on health and the environment. The study explores the use of greener solvents with effectiveness comparable to that of more traditional solvents, such as petroleum derivatives. Recent advancements in sustainable and safe cleaning technologies for cultural heritage have increasingly centered on the development and application of composite systems, including microemulsions, nanostructured fluids, and both chemical and physical gels. While these solutions present effective, low-impact alternatives, literature remains largely deficient in research on pure substances that could directly replace traditional, hazardous solvents. This dissertation aims at addressing this gap by thoroughly investigating the properties of selected greener solvents. The primary focus is on fatty acid methyl esters (FAMEs) with varying chain lengths (C6-C18), which are widely used in other industries as more environmentally friendly substitutes for petroleum derivatives. The research evaluates two essential aspects of FAMEs for their application in the conservation of cultural heritage: (i) their efficacy in removing reference low-polar coatings, through an analysis of their solubility parameters, physicochemical properties, and laboratory tests on solubility and cleaning performance; and (ii) their interactions with the original materials of the object to be preserved through retention studies and swelling tests. Considering the wide range of substances present on cultural heritage objects and the diverse substrates on which they can be found, this dissertation does not attempt to cover the full spectrum of low-polar materials encountered in the field. For this reason, in relation to point (i), the research focused on the solubility of beeswax and microcrystalline wax – used as reference low-polar materials – in FAMEs through X-ray fluorescence (XRF) scanning, digital microscopy, spectrophotometry, Fourier Transform Infrared spectroscopy in the ATR mode (FT-IR ATR), and Scanning Electron Microscopy (SEM). Further analyses, including contact angle measurements, solubility tests, and quantification of the cleaning agent used, provided data on the economic and health-related sustainability of the solvents. Cleaning tests were performed on stone and bronze samples, as these substrates are frequently coated with protective wax layers that often require removal, especially in outdoor works exposed to pollutants and weathering effects. As an additional case study, cleaning tests were conducted on lime-based surfaces coated with soot deposits from oil lamps traditionally used in Kerala temples in India. This research, part of Moupi Mukhopadhyay's doctoral project at UCLA, aims to develop accessible and sustainable restoration methods for preserving mural paintings in local communities. Regarding point (ii), the interaction between the solvents and artwork materials was examined on marble and naturally aged zinc white oil paint. Retention and evaporation analyses of the solvent were performed on marble samples, exploring the potential use of nuclear magnetic resonance relaxometry (NMR) for this purpose. Swelling tests, along with a preliminary assessment of potential leaching in the oil paint samples, were conducted following the method described by Phenix (2002) [1]. The second focus of the dissertation is the result of work conducted within the organization Youth in Conservation of Cultural Heritage (YOCOCU APS), in collaboration with the University of Calabria. The work represents an exploratory study on hydrophobic deep eutectic solvents (HDESs) for the solubilization of low-polar substances, again with a focus on beeswax and microcrystalline wax. The study primarily focused on the affinity between the solvent and solute and the removal capacity of the solutes from inert surfaces, to eliminate substrate interference from the solubilization process assessment. The third focus of this dissertation involves identifying a delivery system capable of encapsulating low-polar solvents, including both petroleum-based solvents and FAMEs. This research began with a comprehensive literature review, drawing from drug delivery systems, to identify potential natural gelling agents [2]. Based on this review, preliminary gelling experiments were conducted using metallic stearate, which was then tested on bronze and stone substrates to evaluate its solvent release capacity, cleaning effectiveness, and residual levels. Further research on a delivery system for encapsulating FAMEs was conducted in collaboration with PhD candidate Laura Giuliani (Earth Sciences Department, Sapienza University of Rome), focusing on Poly(vinylalcohol)-borate hydrogels (HVPDs). Preliminary results indicated promising inclusion capabilities and provided insights into the rheological and compositional characteristics of the gel.