Delamination of sheepskin parchment: an interdisciplinary discovery published in Heritage Science

For several years now, heritage sciences have been experiencing a particularly significant boom. This deeply interdisciplinary field of research aims to foster dialogue between the humanities and natural sciences with a view to improving our knowledge of heritage objects, whether they be parchments, works of art, or artifacts discovered during excavations.

Manuscripts bear witness to ancestral practices and know-how, which unfortunately are poorly documented. It is still unclear why legal documents were preferably written on sheepskin parchment in England from the 13th century until 1925. Among the hypotheses put forward is the fact that sheepskin is whiter, and therefore more attractive, but above all that documents written on it were considered unforgeable due to the tendency of sheepskin to delaminate (any malicious attempt to erase the text would thus be revealed). This delamination property was exploited because it allowed the production of high-quality writing surfaces. It was also used to prepare strong repair pieces used to fill any tears that appeared during the parchment manufacturing process. Understanding why sheepskin delaminates is of interest in the context of traditional parchment preparation techniques, offering valuable insights into the interaction between animal biology, craftsmanship, and historical needs.

Delamination is the phenomenon whereby the inner layers of the skin separate along their interface as a result of mechanical stress. The diagram (a) below shows the structure of the skin, which consists mainly of the epidermis, dermis, and hypodermis. The dermis is divided into two layers, the papillary dermis and the reticular dermis, which contain hair, hair follicles, and sebaceous glands. 

During the parchment manufacturing process, a step following liming involves scraping the skin to remove the hair. This step crushes the sebaceous glands, releasing fats and creating a void where the hair was located (diagram b). 

The study showed that delamination occurs within the papillary dermis itself, in this structurally weakened area, rather than at the papillary-reticular junction as previously assumed. 

The unique nature of the delamination process in sheepskin is highlighted by the skin structure, which differs from that of other animals (calves, goats) used to make parchment, as it has a high fat content associated with a large number of primary and secondary hair follicles. In the study, the presence of fats was confirmed using Raman spectroscopy.

This study combines experimental archaeology and advanced analytical techniques, including scanning electron microscopy (SEM) and micro-Raman spectroscopy, to characterize the delamination process and the adhesion of repair pieces on experimentally produced sheepskin parchment. It benefits from the expertise in archaeometry, biology, chemistry, and physics of the researchers involved.

Beyond its visual and structural implications, delamination has contributed to promoting the use of sheepskin for prestigious documents, improving the surface properties of parchment. The study of the interaction between metal-gallic ink and delaminated sheepskin (wetting experiments) showed that ink diffusion and writing quality are improved, a key finding that provides insight into how surface morphology and composition influence writing performance.

At UNamur, Marine Appart, a PhD student in physics, is conducting this multidisciplinary research on the archaeometry of delamination and repairs on a sheepskin parchment under the supervision of Professor Olivier Deparis (Department of Physics, NISM Institute). 

Also part of the UNamur team are:

• Professor Francesca Cecchet (expert in Raman spectroscopy), Department of Physics, NARILIS and NISM Institutes
• Professor Yves Poumay (skin specialist), Department of Medicine, NARILIS Institute
• Dr. Caroline Canon (histology specialist), Department of Medicine
• Nicolas Gros (PhD student in heritage sciences), Department of Physics, NARILIS and NISM Institutes

Other international experts

• Professor Matthew Collins (world expert in biomolecular archaeology, Department of Archaeology, The McDonald Institute, University of Cambridge, Cambridge, UK)
• Jiří Vnouček (curator and expert in parchment production, Preservation Department, Royal Danish Library, Copenhagen, Denmark)
• Marc Fourneau (biologist) 

This study and the resulting article were inspired by the delamination experiments conducted in 2023 by Jiří Vnouček during a symposium in Klosterneuburg, Austria, in which Prof. Olivier Deparis participated. The symposium was organized by Professor Matthew Collins as part of the ABC and ERC Beast2Craft (B2C) projects.

But it all began in 2014, when the Pergamenum21 project, dedicated to the transdisciplinary study of parchments, was launched.  Pergamenum21 is a project of the Namur Transdisciplinary Research Impulse (NaTRIP) program at the University of Namur. The project received an additional grant in 2016 from the Jean-Jacques Comhaire Fund of the King Baudouin Foundation (FRB).

The projects and events followed one after another, including: 

• May 2014: a transdisciplinary seminar on parchment, the scientific techniques used to characterize this material, and historical questions at the Mauretus Plantin Library (BUMP)
• May 2017: "Autopsy of a scriptorium: the Orval parchments put to the test of bioarchaeology," a transdisciplinary research project co-financed by the University of Namur and the Jean-Jacques Comhaire Fund of the King Baudouin Foundation
• April 2019: a publication in Scientific Reports, Nature group - Jean-Jacques Comhaire Prize: discovery of an innovative technique based on measuring the light scattered by ancient parchments. This technique makes it possible to characterize, in a non-invasive way, the nature of the skins used in the Middle Ages to make parchments
• September 2020: a residential workshop on making parchment from animal skins at the Domaine d'Haugimont – a first in Belgium
• July 2022: a new project on parchment bindings for the restoration workshop at the Moretus Plantin University Library (BUMP) thanks to the Jean-Jacques Comhaire Fund of the King Baudouin Foundation.
• September 2024: a residential symposium-workshop at the Domaine d'Haugimont on the theme of the physicochemistry of parchment and inks using experimental and historical approaches 

Overall, the work of Marine Appart and her colleagues clarifies the structural and material factors that make sheepskin parchment susceptible to delamination and offers new insights into the surface properties of this ancient writing material. UNamur is now establishing itself as a major player in parchment research.

Professor Olivier Deparis, along with several of the researchers involved in this research, are also working on the ARC PHOENIX project.  This project aims to renew our understanding of medieval parchments and ancient coins. Artificial intelligence is used to analyze the data generated by the characterization of materials. This joint study will address issues related to the production chain and the use of these objects and materials in past societies.