Laser Cleaning of Bronzes at the Shanghai Museum in the Lab and in Practice: Lessons from a Decade of Successes and Setbacks

This study examines a decade of laboratory investigation and bronze conservation practice at the Shanghai Museum using a 1064 nm Nd:YAG laser (El.En. S.p.A., Calenzano, Italy). Preliminary irradiation tests on heterogeneous archaeological fragments explored effective-cleaning fluence ranges and adverse-alteration limits under dry and wet LQS and SFR irradiation; reflectance measurements assessed optical response at 1064 nm. Despite recurring tendencies, operational ranges overlapped substantially and remained case-specific, limiting the predictive value of nominal fluence and reflectance. Eight historical artefact cases highlighted situations with greater practical potential for laser treatment, including well-preserved gold-bearing surfaces and selected aged restoration residues, whereas sensitive patinas and degraded metallic decorative layers offered narrower treatment margins. For certain restoration residues, useful removal was achieved even when their major constituents were weak absorbers at 1064 nm. In selected chloride pits, high-frequency SFR was associated with delayed and reduced visible recurrence but was accompanied by surface modification and the continued need for conventional corrosion-control measures. These successes and setbacks inform a practice-oriented decision framework integrating pre-treatment assessment, preliminary testing, observed response and conservation judgement to determine whether laser treatment should serve as a primary method, make an auxiliary contribution or be avoided, with its role reassessed as treatment progresses.