Development of hydrogels for the conservation of copper

Abstract

Abstract: In archaeological sites, there are many metals found in different conditions and different levels of damage. The metal artifacts are generally covered by oxide-based corrosion layers. Copper-based artifacts, including bronze products, can be found in a major part of cultural heritage sites, being extremely important to be conserved, as well as in need of careful cleaning procedures and effective means of oxidation protection. Accordingly, the aim of this work is to develop new chemical methods for the corrosion removal by using a green and environmentally friendly gel that enables both area and material selective cleaning, depending on the additives. Hydrogels are known to be very promising materials for corrosion removal, as they enable cleaning of the parts that are not easily reached or that cannot sustain liquids and can also keep cleaning solvents in the desired area and depth, i.e., high area selectivity. In addition, hydrogels are affordable, friendly and safe for the environment and for the conservators. The development of the hydrogels has been carried out in the following steps: 1) analysis of the copper surface in terms of hydrophilicity by means of contact angle measurement; 2) selection of the suitable polymer structure that has high wettability and compatibility with copper, such as poly(vinyl alcohol) (PVA) and sodium carboxymethyl cellulose (CMC), and preparation of their aqueous gel having a polymer concentration of 15% (w/w); 3) formation of hydrogel containing 7wt% citric acid as cleaning agent on electrochemically corroded copper surfaces. The cleaning efficiency and mechanism were evaluated using X-ray Diffraction (XRD) analysis and Infrared Spectroscopy (IR). Throughout these analyses, the desired gel formulation was obtained, which was then tested using chemically corroded coins to confirm the applicability of the gel for the cleaning of different objects.