This was caused by the oil gilded surfaces having similar solubility parameters with the bronze paint. Conservators at the studio treated several gilded picture frames from Knole’s collection using an energy attenuated Fotona (Dualis XS) Erbium:YAG laser.
An English frame, probably dating from the 1630s, and reused for the slightly earlier portrait of Thomas Sackville was cleaned with the laser as part of the project. A range of approaches were taken for pre-wetting the surface, laser parameters, clearing with solvents and mechanically thinning layers disrupted by the laser. Using Er:YAG facilitated the partial or full removal of the bronze paint, while reducing the damage to underlying oil gilding associated with using solvents alone.