Calcium oxalate films on works of art: A review

This work presents a review of the findings of calcium oxalate films, a widespread decay phenomenon recovered on stone and other substrates (mortars, wall and easel paintings, written materials, glass). The specific attention given to the issue in the 1990s has decreased, although articles have documented films until the present day. The review provides critical insights into the literature, focusing on the general properties of films, numerous case studies, the instrumental techniques used to characterise and date films, insights into the possible origin of the decay phenomenon, and an evaluation of the protective role of calcium oxalate layers. The practice of creating artificial films on stone surfaces for protective purposes is also considered. An evaluation of the literature over the last few decades shows various open issues. The origin is still up for debate, and the issue is still of major concern to conservation scientists, conservators, and restorers. The scientific community tends to attribute a biological origin to these films, however further studies are needed to study exactly how they form, focusing for example on simulation tests of the chemical and atmospheric pathway. How these films protect the artwork in terms of the mechanical properties of the surface underneath deserves more study. This would also help restorers to reproduce the calcium oxalate. The bibliography highlights the prevalence of calcium oxalate findings in the Mediterranean Basin and the formation of the least stable form, i.e. weddellite, which has yet to be explained.