Identification of an unusual pale green material on the surface of an ancient Chinese bronze vessel and application of laser cleaning to its removal

Abstract

AbstractAn ancient Chinese bronze vessel represented a typical difficult situation for conservation: a large quantity of an unwanted pale green material of unknown nature adhered to a surface fully decorated with delicate relief. The diagnostic itinerary, beginning with in-situ Raman spectroscopy analysis and followed by scanning electron microscopy-energy dispersive spectroscopy and X-ray diffraction, was found to be advantageous in revealing the presence of both inorganic and organic compounds in the pale green material. Irradiation with pulsed Nd:YAG 1064 nm laser in LQS regime (100 ns) followed by chemical cleaning using a low-toxicity solvent mixture proved to be respectful towards the original patina of the bronze as well as of high efficiency. Tentative analyses with the Raman spectroscopy and hyperspectral imaging were conducted to provide further indications on quality and visual effect during the cleaning result assessment. In parallel with the analyses and interventions on the bronze vessel, experiments were also carried out on mock-up samples in order to further explore the Raman scatter property and reactivity with laser ablation of mixed materials. The study provided reference for the optimization of diagnostic and conservation for the bronze artwork with similar situation.