Transformation of collagen into gelatine in historical leather and parchment caused by natural deterioration and moist treatment

The main constituent in leather and parchment is the fibrous protein collagen type I. Experiments and studies have shown that the chemical mechanisms causing deterioration of the collagen structure in these materials are acid hydrolysis and oxidation and that these can be correlated to the hydrothermal stability of the material. For parchment and leather tanned with condensed types of vegetable tannins stored under acid conditions, the chemical deterioration may lead to a transformation of the collagen into a gelatine-like substance that may dissolve in water even at ambient temperature. This process follows certain typical denaturising patterns that can be observed as characteristic morphological transformations of the collagen fibre structure at the microscopic level. Moreover, this transformation process has shown to be the same as that taking place during measurement of the hydrothermal stability of fibres by heating in water. Eight major characteristic morphological breakdown features have been identified as typical and representative for historical parchments. In most cases, one or more of these characteristics of damage are present to various degrees in a fibre sample and often in the single fibre of the sample, reflecting the complexity of breakdown. Normally damage occurs as a process of unfolding into flat bands, splitting, forming of pearls on string-like structures and/or fraying of the fibres. This may develop 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 and a relative humidity normal for storage of cultural heritage objects. The morphological transformation of the fibres can be observed partly by the naked eye and not least by means of simple microscopic methods of characterisation of the fibres in dry state or in water at room temperature as well as by measurement of the shrinkage activity. Furthermore, experiments and observations of historical objects, especially parchment documents which have been exposed to active conservation and restoration treatments, have revealed that water and ethanol based treatments accelerate the morphologic transformation of the fibres in vegetable tanned leather and parchment.