Summary:
The advent of the Green Chemistry principles is nowadays an unconditional driving force for the research carried out in the field of Conservation Science. Great attention is currently addressed in response to the potential risks derived from cleaning methods conventionally employed in art conservation, which are frequently relying on the use of petroleum-based and toxic substances, such as solvents and complexing agents. Within this scenario, the doctoral research here discussed was addressed to explore greener alternatives for the cleaning of altered historical metal collections. The core issue for the preservation of metals lies in the spontaneous and irreversible process of corrosion towards which they naturally tend over time. Therefore, conservator-restorers (CRs) typically rely on the application of organic coatings to protect the metallic surface from atmospheric agents (e.g., water, gaseous sulphide species). However, also these materials tend to deteriorate through time due to several environmental factors (e.g., temperature, UV light), leading to a failure of these protective systems.
Consequently, it is common that the appearance, functionality, or conservation conditions of metal artworks are jeopardised by the presence of altered organic coatings associated to corroded underlying metal substrates, leading to the necessary removal of both these degradation features by CRs. Therefore, innovative greener gelled solutions were designed in order to tackle individually (i.e., selective action) or simultaneously (i.e., controlled double-action) corrosion and altered organic coatings possibly present on historical metal collections. Great attention was addressed to the selection of possibly renewable and bio-degradable gel “building-blocks”. The research led to a first investigation of several bio-polymers, among which poly-3-hydroxybutyrate (PHB) was selected as suitable thickening agent to design doubleaction cleaning systems. Namely, two organogels were developed using a poly-3-hydroxybutyrate matrix, loaded with ethyl lactate and complexing agents for the removal of organic coatings and corrosion, respectively. Specifically, the potential of biodegradable complexing agents, such as deferoxamine B (DFO)
and ethylenediamine-N,N′-disuccinic acid (EDDS), were explored. The cleaning targets were selected being the most representative of alloys and organic coatings present in indoor historical metal collections.
Therefore, steel, brass, and sterling silver were chosen as metal substrates, whereas an acrylic resin (i.e., Paraloid® B72) and a nitrocellulose lacquer (i.e., Zaponlack) were the organic coatings of interest. A multi-modal analytical protocol was performed on gelled formulations and metal samples before and after cleaning intervention in order to evaluate the efficiency and reliability of the innovative cleaning methods. In general terms, both cleaning systems provided an effective removal of the organic materials while yielding moderate cleaning on the corrosion layers. This outcome would ensure a versatile intervention, fine-tuning gel application time and reiteration, which is a sought feature for the controlled cleaning of historical metal collections.
From an audacious perspective, the results obtained within the doctoral research were shared with conservator-restorers and the scientific community aiming to promote the exploitation of the innovative methods and materials, while fostering the interest in researching for more sustainable approaches of production (e.g., EDDS manufactured without the use of ethylene bromide) at the industrial scale.
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