The origin of calcium oxalates on historical buildings, monuments and natural outcrops

Abstract

Calcium oxalates (whewellite and weddellite) occur widely in nature. They usually form a patina on the marbles and limestones of monuments, historical buildings and artifacts, as well as natural outcrops. Whewellite and weddellite are produced by a pervasive transformation of the calcium carbonate, occurring in the presence of lichens (or more rarely of other microorganisms such as blue algae and fungi) which provide the oxalic acid required to precipitate the calcium in the form of oxalates. In the past, when the quality of the atmosphere was more suitable for lichen support, the formation of oxalate patinae was a widespread weathering phenomenon in urban areas. Today, in highly polluted sites, weddellite and whewellite are found only as a testimony to such a past transformation. The limited thickness of the oxalate patina must be related to the shallow penetration of the fungi hypae into the rock. Because of this, the phenomenon of oxalate formation: (1) has been first observed on monuments, where superficial weathering processes are better studied; and (ii) has been found on the vertical surfaces of monuments where this phenomenon is not masked by other weathering processes, as usually occurs on natural outcrops. Finally, regarding the oxalates present on monuments, such as the so-called “scialbatura” of Roman imperial marble: their origin has been widely reported in the literature as being due to protective treatments. Our experimental results firmly exclude this.