Paper PS2-WeA9
Advanced Plasma Treatments for Cleaning and Protection of Metal Artefacts

Wednesday, October 17, 2007, 4:20 pm, Room 607

In the framework of conservation of cultural heritage, protection of metal artefacts from corrosion and degradation phenomena which take place during burial and /or during storage and exhibition, is of outstanding importance. Restoration procedures must be non-destructive, reversible, they have to respect the integrity of the objects and to satisfy specific requirements for preserving their aesthetic appearance. In this contribution we present the development of a non-destructive and reversible procedure for cleaning and protecting precious archaeological objects, mainly Ag-based alloys, in low pressure plasma. Reference Ag alloy with micro-chemical and micro-structural features similar to ancient artefacts were used for the plasma treatments. To mimic the real artefacts, the specimens have been buried in the archaeological site of Tharros (Sardina, Italy) for 6 months. To accelerate the corrosion degradation 5% of NaCl has been added to the soil, since chlorides play a key role in the degradation of silver alloys during burial. Furthermore, to better mimic the patina present on real ancient artefacts (mainly consisting of AgCl and Ag₂S), the Ag-based alloys were also immersed in a solution of Na₂S. Cleaning of the tarnished specimens was carried out in RF hydrogen plasma. As detected by XRD and EDS analyses, the treatment effectively decreases the thickness of the patina to an extent which depends on the plasma treatment time. The hydrogen plasma removes the chlorides by forming HCl and this is accompanied by the reduction of oxidized silver (Ag⁺) to metal silver (Ag⁰). The conversion to Ag⁰ can represent an advantage of the plasma cleaning with respect to conventional methods, generally leading to removal of silver from the artefact. Furthermore it is important to mention another benefit: existing cleaning methods are based on chemicals with environmental concerns. For protection of the Ag-based alloy from tarnishing, SiO₂-like coatings were deposited from a RF plasma fed with tetraetoxysilane/oxygen/argon mixture and the protective effectiveness was tested by electrochemical Impedance Spectroscopy (EIS) analyses. Results show that serious tarnishing of the coated samples only occurs after 72 h of immersion in 0.1 M Na₂S solution. Chemical (FTIR, XPS) and morphological (AFM, SEM) characterization of the films will be also presented.