Comparative analysis of coatings on granitic substrates from urban and natural settings (NW Spain)

Abstract

Rock coatings are frequent on natural rock outcrops and even more on stone surfaces of historical buildings and monuments. They are interesting as some of them have a protective effect against erosion. In the last 30 years, research on rock coatings has been performed separately on either buildings or natural outcrops. This paper studies and compares thin coatings found on granitic rock surfaces of historical buildings with coatings found on natural granite outcrops in a same climatic area (NW Spain) by analysing their surface and cross-sections. As rock surfaces of buildings have known age they can be used to assess rates of weath-ering and coating growth. This is the fi rst comparative study of formation of coatings on natural and built sur-faces in the same region. Both endogenous (rock characteri stics) and exogenous factors (climatic conditions, air pollution) that could affect the coatings formation are considered to assess the origin and formation of coatings. X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF) analyses of the underlying rocks, XRD ana-lyses of the coatings and Scanning Electro n Microscopy (SEM) observations of surface and cross-sections of the coatings, performing Energy Dispersive Spectromet ry (EDS) and X-ray m aps, were carried out to estab-lish their composition. Fieldwork and results indicate that natural outcrops are widely covered by biological coatings or coatings formed due to rock weathering, while coatings found on rock ashlars are highly variable with air pollution , other buildin g materials, organic droppings and micro-environmental factors contributing to coatings on rock ashlars in a more signi fi cant way than substrate characteri stics and exposure time.