Transformation of Collagen into Gelatine in Historical Leather and Parchment Caused by Natural Deterioration and Moist Treatment

It is known that, for parchment and leather tanned with condensed types of vegetable tannins stored under acid conditions, chemical deterioration may lead to a transformation of the collagen into a gelatine-like substance that may dissolve in water even at ambient temperature. In a previous study, eight major morphological features of the breakdown process have been characterized as typical and representative for degraded historical parchments. In most cases, one or more of these characteristics of damage are present to various degrees in a sample, and often even in a single fibre sample, reflecting the complexity of breakdown. Damage was most frequently observed as a process of unfolding into flat bands, splitting, forming of pearls on string-like structures and/or fraying of the fibres. In some cases, this developed further to a fragmentation of the fibres and finally to a gelatine-like substance that may dissolve on contact with water or even at room temperature at relative humidity levels normal used for storage in archives, libraries and museums. Morphological transformation of fibres upon wetting at room temperature can be observed partly by the naked eye, and characterized more fully by microscopic examinations, as well as by hydrothermal shrinkage activity measurements. Examinations of degraded historical parchment documents and leather objects show that the gelatinisation may take place as part of the natural deterioration. However, especially parchment documents which have undergone conservation and restoration water-based treatments have been co-related with the frequent presence of some level of gelatinisation damage, especially in the case of degraded parchment, a finding which is consistent with the fact that direct exposure to water can cause gelatinisation of such degraded fibres. Caution is needed with water or alcohol based-treatments since they pose a risk of accelerating the morphologic transformation of degraded collagen fibres, especially in the case of parchment.

The present paper is a slightly revised version of that given with kind permission from the 5th Freiberg Collagen Symposium, where it was part of the program and proceedings (ISBN 978-00-039421-8). The Symposium was held on September 4-5, 2012, in Freiberg, Germany and was organized by Forschungsinstitut für Leder und Kunststofbahnen, FILK, Freiberg, Germany. More information can be found at http://www.filkfreiberg.de