Microscopic, chemical, and molecular-biological investigation of the decayed medieval stained window glasses of two Catalonian churches

► Na-rich and K-rich (or K-Ca-rich) glasses were distinguished and investigated. ► Biological activity evidenced orange patinas, bio-pitting and mineral precipitation. ► Bacteria belonged to Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria. ► Fungi were less diverse and belonged to species of the genus Cladosporium and Phoma. ► The microorganisms detected were related to the observed biodeterioration phenomena.