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Collagen: a key to preserving historical parchments

Marie-Claire Schanne-Klein
Marie-Claire Schanne-Klein
CNRS research director at the Optics and Biosciences Laboratory (LOB*), IP Paris
Gael Latour
Gaël Latour
Senior lecturer, University of Paris-Saclay, Researcher at the Optics and Biosciences Laboratory (LOB*), IP Paris

Whilst we may asso­ciate it more with aes­thet­ics than antiq­ui­ty, col­la­gen is cen­tral in some cul­tur­al her­itage objects. Indeed, this vital pro­tein is a key struc­tur­al mol­e­cule hold­ing bod­i­ly tis­sues togeth­er to sup­port cells. And since ani­mal skin was used in cen­turies gone by to make parch­ments, the pages of many his­tor­i­cal doc­u­ments are full of col­la­gen, too.

Collagen, collagen everywhere…

“There are around 26 dif­fer­ent types of col­la­gen, that are found in many organs – ten­dons, skin, cornea, arter­ies, lungs and so on,” says Marie-Claire Schanne-Klein a physi­cist at École poly­tech­nique. She spe­cialis­es in bio­pho­ton­ics, which essen­tial­ly means that she uses tech­niques from physics to study liv­ing tis­sues. In her case, she works with advanced opti­cal imag­ing, known as mul­ti-pho­ton microscopy.

She explains, “we use flu­o­res­cent mark­ers in bio­log­i­cal sam­ples to label dif­fer­ent cel­lu­lar com­po­nents so we can see them under a micro­scope. But col­la­gen doesn’t need a mark­er as it nat­u­ral­ly gen­er­ates har­mon­ics detect­ed in mul­ti-pho­ton microscopy which makes it glow with­out fur­ther inter­ven­tion on our behalf.” Hence, this pro­tein, which cre­ates a ‘fib­ril­lary matrix’ sup­port­ing cells, can be observed thanks to its har­mon­ic sig­nal. “These intrin­sic imag­ing modal­i­ties, with­out any labelling are a spe­cial­i­ty of our lab.”

Analy­sis using the mul­ti­pho­ton micro­scope from the Lab­o­ra­toire d’Optique et Bio­sciences with a medieval man­u­script on parch­ment from the Chartres’ library (Pho­to cred­it: M. Schmeltz, LOB).

“Col­la­gen forms fib­rils that are arranged dif­fer­ent­ly depend­ing on the tis­sue we are study­ing,” she clar­i­fies. “In skin, for exam­ple, we would expect to see tan­gled bun­dles of large fib­rils, which are respon­si­ble for its sup­ple tex­ture. Where­as in the cornea col­la­gen fib­rils are very thin and high­ly ordered, arranged in lay­ers (or lamel­lae) that make it rigid, giv­ing it the prop­er­ty of focus­ing light cor­rect­ly onto the reti­na.” How­ev­er, col­la­gen may degrade, los­ing its fib­ril­lar struc­ture ulti­mate­ly, form­ing a gela­tine that no longer gen­er­ates a har­mon­ic sig­na­ture, but still fluorescence. 

Marie-Claire Schanne-Klein and her col­leagues study the col­la­gen prin­ci­pal­ly for bio­med­ical pur­pos­es. “Col­la­gen struc­ture plays an impor­tant role in many dis­eases. It can change in cer­tain extreme sit­u­a­tions like skin burns or scar­ring, which leave vis­i­ble traces. But also, in cer­tain can­cers because tumours seem to form around struc­tures of col­la­gen as a scaf­fold. We tend to study them for that rea­son, to bet­ter under­stand the patholo­gies asso­ci­at­ed to collagen.”

Collagen in cultural heritage

Inter­est­ing­ly, col­la­gen is found in oth­er places too, includ­ing many his­tor­i­cal man­u­scripts writ­ten on parch­ments that are made from ani­mal skin. Gaël Latour from Uni­ver­sité Paris-Saclay stud­ies these mate­ri­als. “Parch­ments can degrade over time due to stor­age con­di­tions, and as they do so they become increas­ing­ly trans­par­ent and rigid, lead­ing to the loss of the read­abil­i­ty of the writ­ing,” he out­lines. This trans­par­ent mate­r­i­al is, in fact, degrad­ed col­la­gen, often referred to as gela­tine.

“It is com­mon knowl­edge in the world of cul­tur­al her­itage that objects or doc­u­ments made from skin will ‘gela­tinise’ in this way. And now we know that this hap­pens because the col­la­gen fibres in the parch­ment unwind as it degrades, pro­gres­sive­ly trans­form­ing the mate­r­i­al into gela­tine. In doing so, the parch­ment grad­u­al­ly becomes more homo­ge­neous, let­ting more light through.” More­over, the process is irre­versible: once the col­la­gen-to-gela­tine process has occurred the doc­u­ment is lost forever.

With his col­leagues they have been study­ing doc­u­ments used in West­ern Europe in the 13th Cen­tu­ry, at a time before paper, when parch­ments were com­mon­ly used. But they have analysed doc­u­ments that go as far back at the 8th Cen­tu­ry. “Remark­ably, some of the doc­u­ments are still very well pre­served, with just dam­age at the bor­ders of the pages where they have been han­dled over the hun­dreds of years since they were made.”

Due to the col­la­gen con­tent of parch­ment, the team realised that they could use mul­ti-pho­ton microscopy to study it. “Cur­rent­ly, the main method for test­ing degra­da­tion of parch­ments is called dif­fer­en­tial scan­ning calorime­try; it’s a method that requires tak­ing a sam­ple from the page that is crushed into a pulp to be test­ed destroy­ing a part – how­ev­er small – of the doc­u­ment,” he explains. “But if we use mul­ti-pho­ton microscopy to study the col­la­gen, we can do this in a non-inva­sive way.” In 20161, Latour, Schanne-Klein and col­leagues, pub­lished a report show­ing that it was pos­si­ble to see whether a parch­ment was degrad­ed using their technique.

The researchers show that this method can be used to analyse the lev­el of degra­da­tion – or “gela­tin­i­sa­tion” – of parchments.

(Left) Pic­ture of a medieval man­u­script on parch­ment from the Chartres’ library (pho­to cred­it: CNRS–IRHT). (Right) Mul­ti­pho­ton microscopy images with well-pre­served col­la­gen in the cen­ter (Sec­ond har­mon­ic sig­nals in green) and degrad­ed col­la­gen on the periph­ery (two-pho­ton excit­ed flu­o­res­cence in red).

Preserving history

“Ini­tial­ly the idea was to show that we could see degrad­ed col­la­gen in the parch­ments. So, in the begin­ning the idea was a sim­ple ‘yes’ or ‘no’,” he says. “But now, we are seek­ing to look at how we can quan­ti­fy the amount of degra­da­tion. It could help us keep an eye on which doc­u­ments need to be tak­en care of more effec­tive­ly or help towards restora­tion efforts.”

Going fur­ther, they recent­ly pub­lished anoth­er study2 in which they show that the tech­nique can be used to analyse the amount of degra­da­tion – or gela­tin­i­sa­tion – of parch­ments. They used these tech­niques to analyse cus­tom doc­u­ments, then his­tor­i­cal ones pre­served since the 13th Cen­tu­ry from the archives of the Library of Chartres, France. In this par­tic­u­lar case, over 200 of the doc­u­ments had been exposed to heat from a fire dur­ing a bomb raid dur­ing World War II, result­ing in exten­sive dam­age and gela­tin­i­sa­tion. Gaël Latour and his col­leagues used these invalu­able pages to pro­vide evi­dence that it is pos­si­ble to quan­ti­fy the amount degra­da­tion using mul­ti-pho­ton microscopy, whilst caus­ing them no fur­ther harm.

“Now we want to also under­stand how the degra­da­tion hap­pens,” Gaël Latour adds. We have been start­ing from mod­ern parch­ments that we have arti­fi­cial­ly degrad­ed. The team expose them to dry con­di­tions and tem­per­a­tures above 100°C as if they were ‘aging’ and then analyse them using microscopy to quan­ti­fy the degra­da­tion. Marie-Claire Schanne-Klein adds, “nor­mal­ly, gela­tine is formed from expos­ing col­lage­nous ani­mal tis­sue to high tem­per­a­tures – that’s how gela­tine used for can­dy is made, for exam­ple, and in the case of these doc­u­ments. But we know that in most cas­es, the parch­ments haven’t been exposed to such heat. So, in oth­er cas­es, it is like­ly the result of acid­i­fi­ca­tion due to bac­te­r­i­al activ­i­ty on the doc­u­ments, which can pro­duce an acidic flu­id due to humid con­di­tions dur­ing stor­age.” The team are set­ting about study­ing how this hap­pens in more detail and to chal­lenge this hypothesis.

And they don’t plan on stop­ping at parch­ments. “There are many oth­er his­tor­i­cal objects that con­tain col­la­gen, too.” Indeed, in muse­ums skin can be found in range of mate­ri­als includ­ing rawhide or leather that was used to make clothes or in nat­ur­al his­to­ry spec­i­mens. There are oth­er bio­mol­e­cules that exhib­it har­mon­ics, notably cel­lu­lo­sis in plants, so that we can analyse old fab­rics and musi­cal instru­ments made of wood and more gen­er­al­ly a range of oth­er objects – each with their own sto­ry to tell for years to come.

For fur­ther reading

Interview by James Bowers
1https://​www​.nature​.com/​a​r​t​i​c​l​e​s​/​s​r​e​p​26344
2https://​advances​.sci​encemag​.org/​c​o​n​t​e​n​t​/​7​/​2​9​/​e​a​b​g​1​0​9​0​.​a​b​s​tract

Contributors

Marie-Claire Schanne-Klein

Marie-Claire Schanne-Klein

CNRS research director at the Optics and Biosciences Laboratory (LOB*), IP Paris

After studying at the Ecole Polytechnique and completing a thesis in physics, Marie-Claire Schanne-Klein teaches in the physics department of the Ecole Polytechnique and is responsible for physics at the Doctoral School of IP Paris. She was awarded the CNRS silver medal in 2019. *LOB: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris, Inserm

Gael Latour

Gaël Latour

Senior lecturer, University of Paris-Saclay, Researcher at the Optics and Biosciences Laboratory (LOB*), IP Paris

Gaël Latour is a physicist specialising in optics, and a lecturer at the science department of the University of Paris-Saclay. He teaches physics, and more particularly optics, from bachelors to masters level. His research activities at the Optics and Biosciences Laboratory are focused on the development of new imaging modalities, in particular non-linear optical microscopy, and on their applications in the biomedical field (cornea, skin) and for the study of heritage objects. *LOB: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris, Inserm