Using Microscopic Techniques to Characterise Rock Coatings on Granitic Heritage Buildings: An Overview

Different terms of variable meanings have been used to refer rock coatings. Such coatings are frequent on natural outcrops and façades of rock of Heritage buildings. Their formation and growth have been linked to factors related to the rock composition or the surface environment. Research on rock coatings is based on microscopic techniques, mainly scanning electron microscopy (SEM). SEM and electron dispersive spectrometry (EDS) provide qualitative and quantitative data on the coatings by using secondary electrons to observe surface features, and backscattered electrons and X-ray maps to study polished cross sections of the coatings. Data obtained on their composition, texture and structure is very useful to elucidate their origin. A suitable protocol to collect and prepare samples to SEM observations is very important, as sampling on heritage buildings is very restricted. Carefully sampling and preparation of samples is suggested. The existing different types of coatings have been identified and classified in the last years based on SEM studies. Depending on the coating composition, lithobiontic coatings, artificial coatings, different salt crusts, Si-rich patinas, Fe and metal-rich films, and dust crusts have been identified among others on most granite rock surfaces and mainly on Heritage buildings constructed with granitic rocks. This paper reviews the research on rock coatings on granitic heritage buildings in the last 30 years.