One of Leonardo da Vinci’s masterpieces, drawn in red chalk on paper during the early 1500s and widely believed to be a self-portrait, is slowing fading.
Centuries of exposure to humidity has led to widespread yellowing and browning of the paper which is reducing the contrast between the colours of chalk and paper and substantially diminishing the visibility of the drawing.
A group of researchers from Italy and Poland has been tasked with determining whether the degradation process has now slowed with appropriate conservation conditions or if the aging process is continuing.
To do this, as they describe in the journal Applied Physics Letters, the team developed an approach to non-destructively identify and quantify the concentration of light-absorbing molecules known as chromophores in ancient paper, the culprit behind the yellowing of the cellulose.
“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,” says Joanna Lojewska, a professor in the Department of Chemistry at Jagiellonian University in Krakow, Poland.
“This phenomenon is known as yellowing, which causes severe damage and C the aesthetic enjoyment of ancient art works on paper.”
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.
The team used a spectroscopy setup to study paper samples in the near-infrared, visible and near-ultraviolet wavelength ranges.
Once reflectance data is gathered, the optical absorption spectrum of cellulose fibers which form the sheet of paper can be calculated using special spectroscopic data analysis.
“Using our 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,” says Adriano Mosca Conte, a researcher at the University of Rome Tor Vergata.
“By comparing the results of ancient papers with those of artificially aged samples, we gained significant insights into the environmental conditions in which Leonardo da Vinci’s self-portrait was stored during its lifetime.”
Their work 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.
One of the most significant implications of their work is that the state of degradation of ancient paper can be measured and quantified by evaluation of the concentrations of chromophores in cellulose fibers.
This approach can serve as a precious tool to preserve and save not only Da Vinci’s work but others as well.
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