Study supporting restoration of rapakivi granite in St. Petersburg

Abstract

The focus of this ongoing study has been on two rapakivi granites, wiborgite and pyterlite, that have been utilized in St Petersburg since the 1760s but originate from south-eastern Finland. The aims of this study have been to describe the two types of rapakivi granites in their geological settings, to assess the needs for restoration of stones utilized in constructions and to establish databases of stones utilized in historical buildings and of the original and alternative quarries. The focus of this study has been on defining the original quarries for the old building stones or finding other similar substitute granite deposits. Chemical analysis with a handheld XRF instrument was found to be the most promising approach when combined with multivariate mathematical tools. In addition, the applicability of chemical treatment of the surface of rapakivi granite for changing the appearance of stones was investigated. The visual appearance and texture of natural stones used in restoration are the key factors, in addition to the stone type and quality. The chemical treatment of stone surfaces using acidic or metal salt treatments altered the colour tone and visual pattern. Thermal treatment was also found to be effective, but it might be harmful to the surface structure. With further optimisation, chemical treatment could be a feasible method to change the visual appearance of fresh stone in the case of substitution of weathered stone. In addition, this project has initiated cross-border activities that aim to create opportunities for establishing first contacts between the authorities, construction companies, stone producers and the manufacturer who produces the needed high-quality stone pieces.