A Leonardo da Vinci portrait—thought to be a self portrait of the artist in his 50s—has been fading ever since it was first drawn with red chalk on paper in the early 1500s. Now, thanks to a new technique, scientists say they've been able to quantify the damage, which could ultimately help them preserve the work. They did so by measuring the levels of light-absorbing molecules called chromophores in the cellulose that makes up the paper, reports LiveScience. Because chromophores tend to absorb blue and violet light and reflect reddish and yellow light, old documents often develop a yellowish hue. And because the portrait was done in red chalk, the yellowing reduced contrast with the paper, making the lines appear to vanish.
Almost since it was first drawn, the portrait's history has been shrouded in mystery. It disappeared for centuries until a Sardinian king handed it over to the Royal Library of Turin in 1839, and was displayed publicly for decades in the 1900s. The scientists who were offered rare access to the now fragile art, which has been locked away in a temperature- and humidity-controlled vault at the library since 1998, determined that the work is in such poor condition because it was kept for centuries in "extremely humid conditions or within a closed environment," a researcher tells Heritage Daily. Now that the damage has been quantified, preservationists can map out what restoration technique to use to save this piece and other ancient documents as well. They also want to study the work again later to see if its degradation rate has stabilized.