Decay and durability assessment of sculptural siltstone: applied petrography of polychromous artworks from Baroque Livonia

Five Latvian artworks, linked to the 16th—18th c. Livonia were examined for their petrography, selected properties, and state of decay. Their natural stone, composed of prevalent silt size clastic components, bound by clay-rich matrix and calcitic cement, was found to correspond petrographically to Burgsvik (Gotland) sandstone, a historically widespread sculptural stone in the Baltic region. Petrophysical properties, specifically effective porosity and porosimetric characteristics quantified by mercury intrusion porosimetry (MIP), indicate distribution patterns similar to the assumed source locality. Because the current study relies on small, irregularly shaped specimens obtained from artworks showing various degrees of decay or conservation treatment, their durability was assessed by indirect indices based on MIP data. Permeability and open porosity allowed for a more subtle indication of the degree of decay than computed durability indices, and also proved their potential in provenancing estimates. Following the results of the current study, a combination of a detailed applied petrography study, including the usage of quantitative methods such as petrographic image analysis, complemented with petrophysical characterisation, proved its usefulness in the complex characterisation of stone material from precious heritage artworks.