Covering Layers on Granite Buildings of Northwestern Iberian Peninsula: When Observable Characteristics and Lab Characterization Do Not Match

Abstract

Illustrated glossaries on stone pathologies help to describe deterioration forms in built heritage without resorting to any laboratory analyses. In this way, terms such as crust, deposit, and soiling which according to ICOMOS-ISCS: Illustrated Glossary on Stone Deterioration Patterns may include exogenic material, a patina which results from ageing of the material in an endogenous process, and a film included under the broad term of a coating layer in the glossary, can be macroscopically identified on site. However, a definition on the basis of characteristics only observable with the naked eye (without further analysis in the laboratory) is certainly complicated, and if in addition, the case studies are on granitic rock (a major building stone used across Europe), the picture becomes even more complicated. The intention of this brief report is to engender an open, constructive debate about the casuistry of the covering layers on granite (a poorly reactive and less porous rock) and the difficulty of using the ICOMOS nomenclature on them.

Rare Biogeochemical Phenomenon Associated to Manganese Patinas on Mural Painting and Granite Ashlars

“…The source of Ca in historic buildings can be found in joint mortars [21] and plagioclases when the building stone is a silicate rock (e.g., granites) [22] or from calcite/dolomite, when raw stone is a limestone or marble. Also, iron oxy-hydroxides and Fe-rich clay minerals can be found on heritage stones causing colour changes on its surface [23].…”

“…The EPS matrix is a result of complex metabolic activities underlying the production of (in)organic acids, anionic exopolymers, amino acids and proteins [30]. These metabolites are even responsible for the diagenetic changes of minerals, causing detrimental effects, and influencing the occurrence of some chemical reactions such as acidolysis, alkaline hydrolysis, complexation and oxidoreductive corrosion processes, triggering biocorrosion of building materials and changes in the pigmentation of stone artefacts and painted artworks [23,31].…”