No one is surprised that old parchments are discolored and hard to read, but have you ever pondered why? The cause of the purple spots found on many ancient parchments has resisted investigations until now, but a study of a 13th-century Vatican roll has found a two-stage bacterial infection to be responsible. The work may help archivists prevent the decay of other ancient manuscripts.
In around 1205 the soldier Laurentius Loricatus accidentally killed a man. Despite living in an era where homicide was common, Loricatus was distressed enough to move to a cave for 34 years, where he flagellated himself and wore instruments of penance. In 1244 his story was written onto a 5-meter (17 foot) roll of goat parchment as consideration for canonization, stored since the late 18th-century in the Vatican’s Secret Archives.
Although goat parchment is valued for its longevity, it is far from eternal. Collagen, its main ingredient, develops purple spots that make it hard to read. According to a paper in Scientific Reports, the damage to Loricatus’ scroll is particularly severe on the first and last pages, and on some of the margins of the pages between. Since the Vatican Secret Archives are kept in carefully temperature controlled conditions, the damage probably occurred in the five and a half centuries before the scroll was moved there.
First author Dr Luciana Migliore of Tor Vergata University and colleagues were aware that several techniques have been used to identify the microorganisms defacing similar parchments, including traditional culturing techniques, culturing on new parchments and DNA sequencing. All failed, or at best produced doubtful results.
However, by applying a process called Next Generation Sequencing, Migliore identified a pattern of differences between microbe populations where the scroll was intact and damaged. Apparently, many different bacteria have made their home on the Loricatus parchment over the centuries.
“In the undamaged, uncolored samples Pseudonocardiales was the dominant order (68.2 percent of the identified sequences),” the paper notes. Yet this type of bacterium represented only 41.4 percent of the purple colonies. None of the many bacterial types common in the discolored areas appeared capable of doing the damage alone.
Instead, it seems that several types of bacteria combined, with Halobacteria initially growing on a layer beneath the parchment surface and breaking down the collagen matrix. This, in turn, made space for Gammaproteobacteria, which leave behind purple stains, but could not grow on the original parchment, particularly after it was protected with salt. Fungi probably joined the Gammaproteobacteria party when the already-damaged parchment got wet.
The findings may help in future preservation, and even restoration of important historical documents. Conditions can be adjusted to prevent either the first or second stage of colonization, as appropriate.