Scientists have developed a new non-destructive technology to help preserve one of Leonardo da Vinci’s masterpieces — a self-portrait drawn in red chalk on paper during the early 1500s that is fading fast.

Centuries of exposure to humid storage conditions or a closed environment has led to widespread and localised 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 with expertise in paper degradation mechanisms was tasked with determining whether the degradation process has now slowed with appropriate conservation conditions — or if the ageing process is continuing at an unacceptable rate.

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 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,” said 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 negatively affects the aesthetic enjoyment of ancient art works on paper,” said Lojewska.

Chromophores are the key to understanding the visual degradation process because they are among the chemical products developed by oxidation during ageing and are, ultimately, behind the “yellowing” within cellulose.

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 the Physics Department at the University of Rome Tor Vergata.

To determine the degradation rate of da Vinci’s self-portrait, the team created a non-destructive approach that centres on identifying and quantifying the concentration of chromophores within paper.

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 fibres.

The research was published in the journal Applied Physics Letters.

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