Low treatment temperatures, selectivity, efficiency, no waste products, are all peculiar characteristics of plasma at atmospheric pressure, characteristics that benefit its specific use in conservation and restoration.
Since the advent of vacuum devices, plasma has been used for patina reduction/reconversion studies on metal artifacts. Thanks to atmospheric pressure plasma, the disadvantages of the vacuum plasma are avoided (non-selective process, not controllable by the operator, expensive), becoming a non-contact cleaning method, which makes use of reactive species in the ionized gas to initiate oxidation-reduction reactions.
In fact, the Plasma Jet was born and was applied for the first time in the field of cultural heritage conservation. Many studies have been performed to validate the Plasma Jet in the cleaning of metal artifacts. The use of an argon plasma, enriched with hydrogen, favors processes of reduction of oxides and sulphides patinas, reconverting the surface of the object to metal. Thanks to plasma, the historical reading of the metallic artefact is preserved as there is no mechanical removal of material from the surface but only chemical reduction of the patinas in a gaseous reaction environment. The reading of the surface roughness, inexorably attacked over time, is preserved. Furthermore, the gradualness and selectivity of the method confirm that atmospheric plasma is a controllable cleaning method.
An emblematic case of the conversion of a metal work was conducted by the Cabinet of Scientific Research applied to Cultural Heritage and the Metal and Ceramics Restoration Laboratory at the Vatican Museums. Here, thanks to the Nadir Plasma Jet, the complete cleaning of the Capsella Vaticana was completed (reliquary casket in engraved and embossed silver from the Treasury of the Sancta Sanctorum of the Chapel of San Lorenzo in Palatio al Laterano, Rome): an innovative cleaning method for conservation workers.
At the basis of the cleaning of the Vatican Capsella there is a process of reconversion in three phases:
– the Plasma Jet in reducing mode (argon-hydrogen) reconverts the patinas of oxides and sulphides to metallic silver in a selective and gradual cleaning process alternating passages of plasma and chemical cleaning in the narrowest points such as undercuts and mechanical removal of excess tin from past interventions
– work degreased in solvent baths, dehydrated and protected with nitrocellulose varnish
The level of cleaning achieved is satisfactory and leaves a good reading of the decoration.