A Preliminary Investigation into the Use of Laser Cleaning to Stabilize Bronze Disease

Emily B. Frank, Michaela Paulson, Pablo Londero, and Carol E. Snow

Abstract

A preliminary study of the use of lasers to stabilize active corrosion on ancient copper alloys was undertaken through collaborations at the Yale University Art Gallery and the Institute for Preservation of Cultural Heritage. This paper will describe the processes and results of the initial findings. The chosen samples contained isolated pits of active corrosion, which are time consuming to treat, difficult to remove completely, and often require tight environmental controls to stabilize. The use of lasers to clean metals has been documented for removing coatings, unwanted patinas, and surface accretions, but to our knowledge, has not been tested as a tool for spot treating bronze disease (Sansonetti et al. 2015, Siano et al. 2012, Drakakki et al. 2010, etc.). This project explores the viability of a 1064nm Nd:Yag laser to remove ongoing chloride corrosion from archaeological copper alloy samples from two sites—Sardis, Turkey and Dura Europos, Syria. Samples were treated with the laser to determine the feasibility of removing chlorides more efficiently than by mechanical cleaning with a scalpel. We investigate how laser cleaning might enhance existing treatment methods, allowing for increased efficiency and improved long-term preservation. The conservation of archaeological copper alloys is complex due to immeasurable variability in composition and environment. In determining treatment approach, conservators must consider variables including, but not limited to, alloy composition, manufacturing technique, pre-burial wear/ use and associated materials, burial environment, excavation method, post-burial stabilization, on-site conservation, storage method/environment, and subsequent treatment/retreatment. Over the years, conservators have approached the long-term stabilization of such objects with a wide-range of chemical and mechanical treatments, which have resulted in inconsistent success rates with regards to long-term preservation. Visible light green, powdery copper trihydroxychlorides (atacamite and paratacamite) and underlying waxy cuprous chloride (nantokite) are part of an autocatalytic cycle of corrosion which results in complete powdering of metallic copper alloy objects, termed bronze disease. Moisture and oxygen activate the corrosion resulting in pitted surfaces with light green powdery spots of varying size or layered structures with inaccessible active corrosion. To be a flexible tool for the treatment of bronze disease, the laser spot size must be independent from the energy output, targeting only the afflicted area at the optimal fluence. For this study, we manipulated the fixed energy output of a Compact Phoenix Laser without interfering with the handheld unit. Through a series of lenses and a polarizer, one can reduce the power of the laser while maintaining a small spot size, thus reducing fluence values to those less likely to negatively affect metallic surfaces (Yandrisevits et al. 2017, Abdel-Kareem et al. 2016, Siatou et al. 2006). A modified 1064nm laser successfully micro-excavated pits of bronze disease with diameters smaller than 2mm. Preliminary visual examination of the treated samples with microscopy is promising, in many cases, the laser treatment appears to have exposed a layer at the base of the pit that is visually similar to tenorite. Further microscopy and scanning electron microscopy with energy dispersive x-ray spectroscopy will be used to evaluate the treatment.

2019 | Uncasville | Volume 26