Collagen: a key to preserving historical parchments

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.

“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.”

“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.”

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 13^th Cen­tu­ry, at a time before paper, when parch­ments were com­mon­ly used. But they have analysed doc­u­ments that go as far back at the 8^th 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.

“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 13^th 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

• https://​por​tail​.poly​tech​nique​.edu/​l​o​b​/​f​r​/​r​e​c​h​e​r​c​h​e​/​m​i​c​r​o​s​c​o​p​i​e​s​-​a​v​a​n​c​e​e​s​/​m​u​l​t​i​p​h​o​t​o​n​-​c​h​a​r​a​c​t​e​r​i​z​a​t​i​o​n​-​c​u​l​t​u​r​a​l​-​h​e​r​i​t​a​g​e​-​a​r​t​e​facts
• https://​hal​.archives​-ouvertes​.fr/​h​a​l​-​0​3​0​2​8​0​9​1​/​d​o​c​ument

Interview by James Bowers