One of Leonardo da Vinci's masterpieces, drawn in red chalk on paper during the early 1500s probably in Turin, Italy, and widely believed to be a self-portrait, is in extremely poor condition.
Centuries of exposure to humid storage conditions has led to widespread yellowing and browning of the paper.
Now, researchers from Italy and Poland have developed an approach to non-destructively identify the concentration of light-absorbing molecules known as chromophores in ancient paper.
Chromophores are the culprit behind the "yellowing" of the cellulose within ancient documents and works of art.
"During the centuries, the combined actions of light, heat, moisture, metallic and acidic impurities and pollutant gases modify the white colour of ancient paper's main component: cellulose," explained Joanna Lojewska, a professor in department of chemistry at Jagiellonian University in Krakow, Poland.
Using a new approach, "we were able to evaluate the state of degradation of Leonardo da Vinci's self-portrait and other paper specimens from ancient books dating from the 15th century," informed Adriano Mosca Conte, a researcher at University of Rome Tor Vergata in Italy.
Yellowing occurs when "chromophores within cellulose absorb the violet and blue range of visible light and largely scatter the yellow and red portions - resulting in the characteristic yellow-brown hue," said Olivia Pulci, a professor in at University of Rome Tor Vergata.
The findings revealed that the type of chromophores present in Leonardo's self portrait are "similar to those found in ancient and modern paper samples aged in extremely humid conditions or within a closed environment, which agrees with its documented history," said Mauro Missori, a researcher at the Institute for Complex Systems, CNR, in Rome, Italy.
The results were published in the journal Applied Physics Letters.