Metal soap efflorescent hazes and crusts are one of the most visually disturbing of all metal soap related alterations in paintings. The phenomenon presents particular challenges for conservators as the surface deposits that form are insoluble and intimately bound with the paint film. The superficial deposits accumulate following migration of metal carboxylates, a product formed within the paint film from reaction of free fatty acids with metal ions from pigments and/or driers. Upon deposition at the surface, the metal carboxylates undergo a process of remineralisation, the products of which are determined by the atmospheric conditions that the painting is exposed to. The material can appear optically similar to the whitish haze of organic efflorescence though manifestations also range from crystalline clusters to thick crusts, and in some cases the painting may consequently be rendered partially unreadable. This study aimed to investigate the removal of visually disturbing lead soap efflorescence with Ethylenediaminetetraacetic acid (EDTA), which sequesters the lead ions from the remineralised crust. Through a careful balance of variables such as pH, concentration and application, it is feasible to thin the superficial material significantly. By this means it is possible to saturate the surface of the paint film and render the previously disfiguring efflorescence transparent. Paintings previously treated with this method show no signs of recurrence and thus it appears to be a viable, long-term treatment option.
