The Color Purple: Mystery of What Has Been Killing Ancient Parchments Solved

Hoping to help save ancient documents, including at the Vatican, scientists discover what has been eating parchments going back millennia.

Ruth Schuster
Ruth Schuster
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Purple spots damaging the scroll of ex-soldier Laurentius Loricatus the Blessed, dating to 1244 C.E. Photo shows how the calligraphic writing, presumably in Latin, disappears as the mysterious spots "ate" the parchment.
Purple spots damaging the scroll of ex-soldier Laurentius Loricatus the Blessed, dating to 1244 C.E.Credit: G. Vendittozzi
Ruth Schuster
Ruth Schuster

Ancient parchments are rare, precious and delicate, and it turns out they require protection not only from antiquities looters but from turning purple, then falling apart.

Any ancient document found by archaeologists was preserved because it had been somehow protected from the elements, or because the environment was conducive to preservation, such as deep desert. The Dead Sea Scrolls found in the Judean Desert are a good example.

Once unearthed and exposed, parchments tend to deteriorate fast – and some begin to develop mysterious light purple spots. One result of the empurplement is that the parchment layers separate, often destroying the inked text.

Using a 13th-century empurpled scroll, genetic analysis and microbiological detective work, scientists at Tor Vergata University, Rome have now figured out how these blemishes are caused, as described in Nature, Scientific Reports on Thursday.

In a discovery that may help preserve or restore fragile ancient documents, scientists found the spots are caused by not one, but a succession of bacteria that eat at the collagen fibers that are crucial to the parchment's integrity.

From homicide to hermit cave

Parchment is made of untanned skin, usually of sheep or goats. biologist Prof. Luciana Migliore and her team worked on a five-meter long scroll written in 1244 C.E.

The document tells the story of a soldier, Laurentius Loricatus, who accidentally killed a man. Horrified by his deed, he laid down his arms and made a pilgrimage of penance to Santiago de Compostela, Spain. He then sequestered himself in the the ruins of a monastery near Subiaco, Italy, for the next 34 years.

Though falling short of sainthood despite tormenting himself by wearing chain mail on his bare skin - which is actually what his surname means in Italian, he is today known as Laurentius Loricatus the Blessed. According to the record of Catholic Saints, he died in 1243. The scroll was written the following year, for a Vatican canonization inquiry, they explain.

The 5-meter long parchment, A.A. Arm. I-XVIII 3328, from the Vatican Secret Archives, describing the travails of the accidental killer Laurentius Loricatus.Credit: G. Vendittozzi

Canonization can take time. Laurentius was beatified in over five centuries later, in 1778.

Meanwhile, the purple spots that the text describing Loricatus' travails and humility had developed are a peculiarity of parchment. Though, as Migliore points out to Haaretz, bacteria can destroy plant-based documents as well. "Usually paper documents are more heavily damaged by fungi," she observes.

She suspects the Loricatus parchment was damaged before the scroll was stored at the Vatican Secret Archive, in the late 18th century.

By sampling the spots, and parchment bits that didn't have spots, and performing genetic analysis of the microbial communities colonizing the scroll, Migliore and the team discovered a type of gammaproteobacteria: they were present in the purple spots but absent in the undamaged areas of the parchment, the article explains.

The rub is that the gammas were not responsible for the purple stains.

What our eyes have in common with bacteria

The scientists concluded that such stains on ancient parchments are caused by a microbial succession, the gammas replaced a different germ, a halobacteria — which is the one that caused the purple spots, by producing chemicals called rhodopsins.

Rhodopsins may sound familiar: also known as visual purple, it's a pink-colored protein that we have in our eyes that is involved in converting the light that hits our retinas into electrical signals. In bacteria, they are involved in light-driven metabolism. Halobacteria produce pinkish-colored proteins called rhodopsins, gammaproteobacteria do not.

In any case, first the ancient parchments are colonized by halobacteria that produce rhodopsins and stain the parchment. Later these are supplanted by the gammaproteobacteria that the scientists found. "Ecological succession is a common feature of all natural communities," Migliore says.

How the purple spots cause detachment of the superficial layer of the damaged areas and loss of readability. Shown on the Laurentius scroll from 1244 C.E.Credit: G. Vendittozzi

Naturally there could be more germs in this sequence, as there are millions of species of bacteria and some produce rhodopsins: broader research is needed. In any case, there is no lack of ancient parchments affected by the malady. "A lot of ancient documents all over the world show this kind of damage," Migliore says.

"The microbial succession we hypothesized on the base of metagenomic, chemical and physical analyses support the idea that halobacteria are the responsible [party]. Research work is ongoing, to demonstrate the model on different documents of different historical periods," Migliore elaborates. That said: "The type of damage on this document is the same as in the great majority of the damaged ancient parchments. Hence we suppose that the dynamics of the attack are always the same."

Having identified the responsible bacteria, how can parchments be better protected against them?

"We hope that the model of microbial colonization we hypothesized will open new perspectives to the conservation and restoration of ancient parchments, because we could work not only on microbes but also on the pigments and on collagen deterioration," Migliore says.



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