Study of biodeterioration of diterpenic varnishes used in art painting: Colophony and Venetian turpentine

Natural diterpenic varnishes have been widely used in works of art, traditionally fulfilling an important aesthetic and protective function. However, deterioration processes cause major changes in the mechanical, optical, and chemical properties of varnishes, seriously affecting the conservation and visual appearance of paintings and polychromed sculptures. This paper studied fungal and bacterial colonization of Venetian turpentine and colophony varnish by examining the chemical changes brought about by microbiological growth. Test samples were prepared on standard laboratory glass slides coated with a solidified varnish layer. Samples were inoculated with fungi and bacteria, acquired from collections as well as from oil paintings showing evidence of biodeterioration. Oxidative processes were detected by GC–MS in the inoculated samples, caused by Chrysonilia sitophila, Streptomyces celluloflavus, Bacillus amyloliquefaciens, and Arthrobacter oxydans.