Thousands of Bibles were made during the 13th century as more Christians carried the holy text around with them.

But the mysterious material on which the tissue-thin pages of these pocket books were made from has baffled archaeologists for centuries - until now.

Using a simple eraser to reveal the parchment beneath, the experts have discovered they are not made from abortive or newborn animals as previously thought but were instead the result of a skilled production process.

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A team of bioarchaeologists have used a simple eraser to reveal that the parchment used in medieval Bibles was not made from abortive or newborn animals as thought, but was the result of a skilled production process

The majority of the Bibles were made in France but also in England, Italy and Spain.

They featured the use of parchment, often called 'uterine vellum', which some scholars believe was made from the skin of foetal calves.

Others have discounted the theory, though arguing that it would not have been possible to sustain livestock herds if so much vellum was produced from foetal skins.

Some claim unexpected animals such as rabbits or squirrels may have been used instead, while other medieval sources suggested the hides must have been split by hand through the use of a long-lost technology.

Thousands of the Bibles were made in the 13th century, principally in France but also in England, Italy and Spain. They featured the use of parchment, often called 'uterine vellum' which some scholars believed was made from the skin of foetal calves. A stock image of a newborn calf is shown

Scientists from France, Belgium, Denmark, Ireland, the USA and the UK analysed 72 medieval pocket Bibles and 293 parchment samples from the 13th century. Two examples are shown above

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In a bid to solve the mystery, a team of international experts led by Dr Sarah Fiddyment and Professor Matthew Collins from the University of York, developed a simple technique to identify the animal origin of parchment, without damaging the artefacts.

The non-invasive method, called ZooMS extracts protein from the parchment using the electrostatic charge generated by the gentle rubbing of a PVC eraser on the membrane surface.

The protein can then be analysed and matched to an animal.

Scientists from France, Belgium, Denmark, Ireland, the USA and the UK analysed 72 medieval pocket Bibles and 293 parchment samples from the 13th century.

The parchment samples ranged in thickness from 0.03 to 0.28mm.

Dr Fiddyment said: 'We found no evidence for the use of unexpected animals; however, we did identify the use of more than one mammal species in a single manuscript, consistent with the local availability of hides.

'Our results suggest that ultrafine vellum does not necessarily derive from the use of abortive or newborn animals with ultra-thin skin, but could equally reflect a production process that allowed the skins of maturing animals of several species to be rendered into vellum of equal quality and fineness.'

Professor Collins told MailOnline that in order to make parchment, the hair and fat is removed from the skin by placing it in a lime bath.

'In order to make older animal skins into so called "uterine vellum" it requires more hard work!

'In the case of goat or young calf this requires that you "pounce" (rub the skin with pumice) to grind away the surface and attempt to remove all the hair follicles.'

He added: 'With no evidence of unexpected species, such as rabbit or squirrel, we believe that "uterine vellum" was often an achievement of technological production using available resources.'

This is the first time the new method has been used to extract protein from parchment.

The experts developed a simple technique to identify the animal origin of parchment without damaging the artefacts. The non-invasive method (illustrated), called ZooMS extracts protein from the parchment using the electrostatic charge generated by the gentle rubbing of a PVC eraser on the membrane surface

Alexander Devine, of the Schoenberg Institute for Manuscript Studies at the University of Pennsylvania, added: 'The Bibles produced on a vast scale throughout the 13th century established the contents and appearance of the Christian Bible familiar to us today.

'Their importance and influence stem directly from their format as portable one-volume books, made possible by the innovative combination of strategies of miniaturisation and compression achieved through the use of extremely thin parchment.

'The discoveries of this innovative research therefore enhance our understanding of how these Bibles were produced enormously, and by extension, illuminate our knowledge of one of the most significant text technologies in the histories of the Bible and of Western Christianity.'

The parchment conservator involved in the study has put the findings to the test by recreating a parchment similar to 'uterine vellum' from old skins.

Jimí Vnouček said: 'It is more a question of using the right parchment making technology than using uterine skin.