Abstract
In this research, we propose an advanced system for the cleaning of wax-based coatings applied on indoor bronzes. To this aim we developed a new kind of eco-friendly gel based on PHB (poly-3-hydroxybutyrate) used as thickening agent, biodiesel (BD) and dimethyl carbonate (DMC). BD is a mixture of methyl esters obtained from palm oil, which acts as cleaning agent while DMC was added as additional solvent to partially solubilize PHB and forming a gelly phase. For the first time a PHB-based gel obtained by mixing two solvents with different proprieties was proposed, expanding the range of possible formulations, that can be used according to the specific restoration purpose. After the preliminary characterization of chemical and physical properties of the gel, an ad hoc analytical protocol was implemented to evaluate both the cleaning efficiency and the release of residues on the treated surfaces. Standard samples were prepared following ancient recipes and submitted to spectroscopic and chromatographic analysis before and after the cleaning procedures. Finally, the performances of PHB-DMC/BD gel were assessed on a real case of study presenting a wax-based coating: the Pulpito della passione attributed to Donatello and dated back to 1460. In situ analysis demonstrated the high cleaning efficiency of the proposed systems also for the removal of aged coatings.
Highlights
The surface of indoor metal objects is a complex system characterized by the presence of inorganic products and organic coatings
Gel formulation and characterization In order to assess the ability of a mixed solvent system such as dimethyl carbonate (DMC)/BD to produce a gel in the presence of PHB, two gel formulations, obtained by varying the ratio between solvents (DMC: BD, Table 1), were produced
To date, scientific research has mainly focused on the proposal of new cleaning systems for painted surfaces, while little attention has been paid to the evaluation of new solutions for the restoration of indoor and outdoor bronzes
Summary
The surface of indoor metal objects is a complex system characterized by the presence of inorganic products and organic coatings. Lacquer, or varnish were applied to saturate the surface color and protect the patina and the metal surface from corrosion [4,5,6] Among these different materials, natural wax - such as beeswax - is one of the mostly used coating for indoor bronzes, thanks to its properties such as low water. Some alkaline compounds could be used to remove the waxes from the surfaces, subsequently treated with weakly acidic solutions to neutralize their action. This application has been limited due to its aggressiveness for the artwork [10]. This cleaning approach may induce drawbacks mainly owing to the unrestricted action of the solvent, which leads to its penetration into porous matrices, producing undesired phenomena [11,12,13,14]
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