The Magnetic Resonance Laboratory (SCIB/LRM), in collaboration with Arc Nucléart, iRTSV and INRAP, has succeeded in explaining the origin of the excellent preservation of old leather, after having spent several centuries buried in sediments.
During works for the construction of Saint-Georges car park in Lyon, a true archaeological gold mine was brought to light: a dump of used leather shoe soles. The richness of this deposit comes from their age, varying from 200 to 800 years. The shoes are dated precisely by how they are sewn.
Leather is composed mainly of collagen, a natural polymer from the skin of the animal. In order to maintain the original flexibility, chemicals (tanning) that play the role of plasticizer, are added. For our old leather soles, these were natural tannins extracted from plants, oil and fat. Despite their having aged, the leather specimens were found only slightly damaged. We have shown by solid nuclear magnetic resonance (NMR) that no de-compositional changes occurred, regardless of the age of the sample.
Strangely, all of the tanning agents (tannins, oil and fat) had disappeared. However, electron paramagnetic resonance (EPR) revealed a significant amount of iron oxides that came from the silt in which the soles were buried. In this oxygen-poor environment, iron oxides replaced the natural tannins that were gradually leached out. Throughout this process, the leather retained some of its flexibility and its degradation in the landfill was avoided. Hence, the secret of good preservation of old skins: no cosmetic chemicals, long live mud baths!
Further reading: M. Bardet, et al., Analytical Chemistry 81 (2009) 1505
Last update : 02/20 2014 (992)