Stone science and its importance for the adoption of conservation actions: an example of interdisciplinary work in Brazil

Due to the action of intrinsic and extrinsic factors, such as mineralogical compositions, textures, weathering, climatic conditions and pollution in urban centers, constructions using stone that are part of a country's cultural heritage require permanent care for their preservation, which must involve collaboration between professionals from different areas of knowledge. The case presented here serves as an example of a conservation action developed after unprecedented work involving the participation of geologists and a biologist and confirms that an interdisciplinary approach is indeed essential to guarantee a holistic understanding of stone in buildings. The cultural asset treated consists of a sculptural set made up of twelve life-size statues, representing prophets. Produced between 1800 and 1805, the set was built using inhomogeneous steatite blocks, ranging from isotropic to anisotropic types and is part of a monument located in the city of Congonhas, Minas Gerais, Brazil. Recognized by UNESCO as a Cultural Heritage of Humanity, the complex suffers degradation of different types, with emphasis on the natural and recurring action of biological colonization, which motivated this interdisciplinary work. At the beginning of 2023, after twelve years without any intervention, the elements of the complex appeared almost completely taken over by biological colonization, which caused visible aesthetic degradation, compromising the religious function for which they were produced. Initially, a detailed macroscopic petrography of all the statues was carried out, seeking to identify the different textures present in the steatite blocks used, but also all types of stone degradation. In addition to biological colonization represented by the presence of cyanobacteria, algae and lichens, losses of pieces, black crusts, dissolutions, patinas and cracks were observed. Of the mentioned microorganisms, mainly whitish gray or greenish lichens were found, described as foliaceous and other whitish lichens forming crusts on the substrate and described as crustose. Algae without stems or leaves and showing colors varying between yellow, green and reddish brown were also observed, occurring in a subordinate manner in relation to lichens. Other forms, such as mosses, were observed filling cavities and developing along fissures and fractures in the stone, as well as in spaces between blocks and between the bases of statues. Following our assessments on the extent of degradation of the stone material and the respective degree of deterioration of the set of statues, procedures were adopted by the conservators of the company Grupo Oficina de Restauro aiming to combat microorganisms with applications of a biocide and 70% alcohol. Geological monitoring during the microorganism removal process, evaluating possible losses to the stone substrate, and a new macroscopic petrography carried out sixty days later confirmed the absence of new damage to the stone. In summary, the presence, especially of geologists, who know the properties of the stone materials present in buildings, particularly historic ones, was fundamental to the success of the conservation action, which must always have a multidisciplinary character.