A famous red chalk on paper drawing, widely accepted as a self-portrait of Leonardo da Vinci, is rapidly deteriorating from the effects of years of exposure to pollution, light, and heat. Worst of all, many centuries in unregulated and humid storage has led to extensive yellowing and browning of the paper. Recently, however, researchers from Italy and Poland have developed a new non-destructive, nano-level method to identify the root causes of the degradation and assist in planning appropriate conservation strategies.

In an extraordinary concession by Turin's Royal Library, the da Vinci picture was made available for diagnostic investigations at the Central Institute for the Restoration of Archival and Library Heritage in Rome, Italy. Concentrating on areas across a range of degradation levels, the team conducted a spectral analysis on the paper to offer a deeper insight into the chemical changes seen at the molecular level and to ascertain the concentration of light-absorbing molecules known as chromophores.

Ordinarily, in a living plant, chromophores act as part of the energy transfer chain in photosynthesis. But when plant material is turned into paper, the chromophores become responsible for the yellowing effect on the cellulose contained in that paper. That is, chromophores in cellulose absorb visible light in the higher energy range of violet and blue, and so largely scatter the yellow and red portion, thereby generating the yellow-brown hue characteristic of old paper.

The results definitively showed that humidity and oxygen have produced volatile chemicals responsible for the paper’s degradation, as well as the promotion of the effects of the chromophores in discoloring the portrait. It is envisaged that a continued repetition of this analysis will not only offer an ongoing, measurable assessment of the rate the degradation is occurring, but also increase the understanding of the degradation processes themselves. Such information should prove invaluable to restorers and conservators in their on-going efforts to maintain and preserve such rare works of art.

The results were published in the journal Applied Physics Letters.