Characterization of corrosion mechanism and traditional soldering treatment of a composite bronze lamp from the Greco-Roman period of Egypt

Abstract

This paper presents a study of treatment by soldering for a unique bronze oil lamp combined with Eros and dog figurines. In this object, there are two types of soldering; the first is ancient, used to join the three pieces together, and the other is used in conservation treatments to stop the degradation of the object. Soft solders have been a modern method of repair, with its historical use as the original joining technique demonstrating its general durability and effectiveness. Optical microscopy, portable X-ray fluorescence, and micro-Raman spectroscopy were used to identify the soldering treatment, the chemical composition of the alloy, and the corrosion products. Investigation techniques showed the use of a modern tin-lead alloy as soft soldering in past interventions to treat cracks and fill losses. Cracks propagated due to soil pressures and ongoing corrosion processes, as well as the combined action of tensile forces and the surface-active molten solder. The ancient solder used to attach the dog and Eros to the lamp was a hard solder. The three pieces were made of high-leaded tin bronze, with Pb contents ranging from 10.5 to 13 wt%. The chloride and sulfate corrosion products detected by µ-RS were more aggressive in the object structure. The corrosion products of the soft solder consisted mostly of lead and tin oxides, basic carbonates, and sulfates.