Are patinas of the Mediterranean monuments really related to the rock substrate?

Abstract

Patinas of different mineralogies and colours occur on most monuments of the Mediterranean area. The origin(s) of the latter have often been related to previous painting and/or protective treatment(s). Microbial and biogeochemical pathways of formation, however, have as well been proposed and discussed. Paintings and photographs make it clear that coatings of changing colours have covered famous monuments in the past 200 years. Also the presence of microbiota has been recorded already 150 years ago. The work recorded here was done on several buildings in the Mediterranean area. A wide variety of localities and rock substrates and the patinas on them have been studied. Among the rocks were marbles, limestones calcarenites, granites and sandstones. In all cases studied patinas, their thickness and colour as well as their mineralogical composition were related to the exposure type. The patinas usually are multi-layered, the individual layers characterized by different crystal size, texture, mineralogy, and colour. Orange to grey layers are characterized by calcium carbonates and oxalates with some phosphates admixed, while the dark grey to black layers are rather characterized by gypsum with some trapped air-borne particles in cases. The petrology and mineralogy of the patinas is practically identical in all cases. Many of the patinas are inhabited by a variated microflora causing pitting and exfoliation in cases of the patina. Sometimes the pitting is only seen in the patina, and it reaches also the bedrock. A relationship between patina formation, preservation, destruction and climatic changes over the past 200 years is derived from these findings.