Agar and Chitosan Hydrogels' Design for Metal-Uptaking Treatments.

In the field of cultural heritage, the use of natural gels is rising for the application of active agents. Here, two natural polymers are assessed: agar, a pioneer hydrogel for conservation treatments, and chitosan, a rather novel and metal-binding gel. For chitosan, a state-of-the-art based formulation (CS-ItA-LCys) is evaluated as it was reported for silver-complexing properties. It is evaluated whether these polymers can withstand the addition of the chelating compound deferoxamine, which is a bacterial siderophore. This allows for the obtainment of completely bio-sourced gel systems. A Fourier-transformed (FT) infrared spectroscopy characterization is performed, completed with rheological measurements and Cryo-Scanning Electron Microscopy (cryo-SEM) to investigate the physico-chemical properties of the gels, as well as their interaction with deferoxamine. Both polymers are also tested for their inherent complexing ability on silver ions using FT-Raman spectroscopy. A multi-analytical comparison shows different microstructures, in particular, the presence of a thick membrane for chitosan and different mechanical behaviors, with agar being more brittle. Neither hydrogel seems affected by the addition of deferoxamine; this is shown by similar rheological behavior and molecular structures in the presence or absence of the chelator. The intrinsic abilities of the chitosan formulation to make silver complex are demonstrated with the observation of two peaks characteristic of Ag-S and Ag-O bonds. Agar and chitosan are both proven to be reliable gels to act as carriers for bio-based active agents. This paper confirms the potential asset of the chitosan formulation CS-ItA-LCys as a promising gel for the complexation of soluble silver.

Testing of the siderophore deferoxamine amended in hydrogels for the cleaning of iron corrosion.

Bioderived alternatives to commonly used complexing agents for the cleaning of iron artworks are sought for their natural origin and better biodegradability. Indeed, complexing agents currently used for the removal of undesired corrosion products from iron artworks can be difficult to control and their environmental impact is often overlooked. This paper studies the use of siderophores, focusing on the ability of one of them, deferoxamine, to be employed as an active agent loaded in polysaccharides hydrogels, on corrosion phases. Preliminary tests were conducted on artificially aged steel samples and further studies were performed on naturally corroded steel to assess the most performing application parameters. Long-term behavior of cleaned surface was assessed. Cleaning outcomes were compared with those obtainable with disodium ethylenediaminetetraacetic acid using optical microscopy, colorimetry and atomic absorption spectroscopy as well as Infrared and Raman micro-spectroscopies. Among the different gelling agents evaluated, agar applied when hot and gellan gum prepared at room temperature were the most effective gel formulations and agar left few residues over the treated surfaces. The protocol was then tested on altered steel artifacts belonging to heritage institutions in France. Encouraging outcomes in the removal of iron corrosion phases with green approaches are here presented.

The Use of Hydrogels in the Treatment of Metal Cultural Heritage Objects.

Currently gels are widely used in the restoration of paintings, graphic arts, stuccowork and stonework, but their use in metal restoration is less widespread. In this study, several polysaccharide-based hydrogels (agar, gellan and xanthan gum) were selected for use in metal treatments. The use of hydrogels allows to localize a chemical or electrochemical treatment. This paper presents several examples of treatment of metal objects of cultural heritage, i.e., historical or archaeological objects. The advantages, disadvantages and limits of hydrogel treatments are discussed. The best results are obtained for the cleaning of copper alloys via associating an agar gel with a chelating agent (EDTA (ethylenediaminetetraacetic acid) or TAC (tri-ammonium citrate)). The hot application allows to obtain a peelable gel, particularly adapted for historical objects. Electrochemical treatments using hydrogels have been successful for the cleaning of silver and for the dechlorination of ferrous or copper alloys. The use of hydrogels for the cleaning of painted aluminum alloys is possible but it has to be coupled with mechanical cleaning. However, for the cleaning of archaeological lead, the cleaning using hydrogels was not very effective. This paper shows the new possibilities of using hydrogels for the treatment of metal cultural heritage objects: agar is the most promising hydrogel.