(Digital Presentation) Application of Plasma Cleaning in the Preservation and Restoration of Cultural Heritage

Abstract

The chemical reactivity of metals at inappropriate climate conditions can be a threat to metallic cultural heritage. It is known that oxygen plays an important role in the corrosion and decay of metallic objects. Oxide formation is among the most prevalent destructive phenomena in this field. The metallic archaeological objects lose their original shape due to the oxide layer that develops on their surface. Among physical-mechanical and chemical techniques used in the past to remove the oxide layer from these antiques, Cold plasma technology has been proposed as a novel safe method to successfully achieve this goal. In a plasma medium, which can be formed by an electrical discharge, chemically reactive species exhibit a unique behavior in response to the injected gas type, electrical and chemical parameters, etc. This special behavior aids in achieving the user's goal through physicochemical phenomena. In this study, Hydrogen gas, with its reductive character, in the plasma phase provides free radicals of Hydrogen reacting to the copper oxide layer on ancient coins. As a result of this plasma reduction reaction, separation of the oxide layer from the copper surface is achievable. This phenomenon which is called Plasma cleaning, is a type of etching. Plasma cleaning does not destroy surface structure and information contained within metallic heritage artifacts. The feasibility of this non-destructive cleaning method has been investigated for the preservation and restoration of ancient copper coins dating back roughly 300 years ago. These ancient copper coins were exposed to a cold hydrogen plasma for 120 minutes. Results show that Tenorite and Spertiniite formed on the surface of coins have been successfully removed. The black color of Tenorite, a type of copper oxide, gives a dark appearance to copper coins' surface, and Spertitnite, as a copper hydroxide, exhibits a blue-green color on copper coins. According to the results, after 2 hours of hydrogen plasma treatment, not only these green spots have been removed, but also the dark appearance of the entire surface of the coin has been reduced. This amount of clearance is a good result considering the untreated area. Moreover, Figure 1 illustrates the coin's surface using SEM (A) before and (B) after Hydrogen plasma treatment. These images were obtained at a 200x magnification from the surface. In contrast to Figure 1(A), which had heavily polluted surfaces, case B's dark pollution has been removed, reducing the amount of darkness on the coin's surface. This study shows the efficacy of Plasma cleaning for the preservation and restoration of ancient copper coins. Figure 1