Using a simple PVC eraser, an international team of researchers has solved the long-standing mystery surrounding the origin of the tissue-thin parchment used to produce the first pocket Bibles in the Middle Ages.
The secret of the ultra-thin material derives from a highly specialized craft technique rather than the supply of particular animal skins, said the researchers.
More than 20,000 copies of pocket Bibles were produced by scribes in the 13th century, mainly in France but also in England, Italy and Spain.
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The books were written on an ultra-fine, almost translucent parchment known as "uterine vellum." It was believed to have come from the thin skins of unborn calves or sheep.
But most paleographers have discounted the theory, viewing the Medieval uterine vellum simply as a myth.
"Its production on the scale implied by extant manuscripts would have entailed an untenably high number of aborted fetuses," researchers led by Sarah Fiddyment and Matthew Collins of the BioArCh research facility in the Department of Archaeology at York, wrote in Proceedings of the National Academy of Sciences (PNAS).
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Older research suggested smaller, more thin-skinned mammals such as rabbit, squirrel or even rats as the source of the unique writing material.
To identify the animal origin of parchment, the researchers used a novel, non- invasive technique – a triboelectric extraction of skin collagen.
A variant on ZooMS (ZooArchaeology by Mass Spectrometry) peptide mass fingerprinting, the simple method extracts protein from the parchment surface simply by rubbing a PVC eraser on the membrane surface.
"By passing the eraser gently across the surface, you generate an electrostatic charge - in the same way you can rub a silk cloth on amber or a balloon on your hair," Matthew Collins told Discovery News. "The eraser exfoliates into charged sheets, and the protein and dirt are lifted off onto the resulting crumbs," he added.
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Using this method, Collins and colleagues analyzed 72 pocket Bibles originating in France, England, and Italy and 293 additional parchment samples from the 13th century.
The thickness in the parchment samples ranged from 0.03 to 0.28 mm.
"We found no evidence of unexpected species, such as rabbit or squirrel. However, we did identify the use of more than one mammal species in a single manuscript, consistent with the local availability of hides," the researchers wrote.
According to Collins and colleagues, parchment makers simply had the skills to make the finest parchments from calf, goat and sheep skins.
"It is more a question of using the right parchment-making technology than using uterine skin," said co-author and parchment conservator Jiří Vnouček.
Based on the new findings, Vnouček has recreated parchment similar to uterine vellum from old skins.
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"Skins from younger animal are of course optimal for production of thin parchment, but I can imagine that every skin was collected, nothing wasted," he said.
It is the first time the triboelectric effect was used to extract protein from parchment.
"The method is non-invasive and requires no specialist equipment or storage," Collins said.
"In order for us to analyze parchment, all we need to do is ask conservators who are dry cleaning parchment to save their waste crumbs," he added.