Time has taken its toll on Leonardo da Vinci’s irreplaceable self-portrait. Sketched in red chalk, “foxing spots,” nasty reddish-brown spots, have erupted on the paper’s surface and threaten its survival.
A mystery to scientists for decades, new research has uncovered their cause. In recent years, researchers have sought a biological origin to the spots, using swabs and other media without success.
Guadalupe Pinar, a senior scientist at the University of Natural Resources and Life Sciences in Vienna, led a team of researchers, and recently published their evidence as to the cause. The scientists collected DNA from the drawing, amplified the fungus-specific sequence, and cloned recovered fragments to identify the responsible organisms. An electron microscope revealed a variety of fungal forms from smooth spheres wrapped in filaments, a congregation of spiky cells and flattened disks with cross-hatched scars.
Pinar and his team wrote, in a recently published paper,
The fungus, Eurotium halophilicum, was found in “foxing spots” using SEM analysis. Oxalates of fungal origin were also documented. Both findings are consistent with the hypothesis that tonophilic fungi germinates on paper metabolizing organic acids, oligo saccharides and proteic compounds, which react chemically with the material at low water activity, forming brown products and oxidative reactions resulting in “foxing spots.
Pinar thus suggests that the spots result from a two-step, chemically-induced process. First, dust-borne iron particles land randomly on the paper disrupting the cellulose structure. Second, with this opening, fungal organisms burrow into the paper and, when energy is available, spit out oxalic acid.
These findings may help conservationists treat the sketch. By understanding the precise composition of the spots, a game plan may be established. Without this knowledge, catastrophe could occur, as almost happened during conservation efforts in 1987. At the time, researchers wanted to soak the sketch in ethylene oxide. Had they done so, the sketch would have been irreversibly damaged.
This study shows that conservation treatments should be founded on sound scientific data. If Leonardo da Vinci were alive, I’m sure that he would have agreed.