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