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The aim of this study was to advance the current understanding on the mechanism of deterioration of historical vegetable tanned leathers and establish new criteria for quantifying their deterioration using micro differential scanning calorimetry (micro DSC) and micro hot table (MHT) method. Ten historical leather objects were investigated to this purpose. The calorimetric indices of macromolecular change identified for fibrous collagen provided quantitative results on the bulk material and deconvolution of DSC denaturation peaks revealed the dynamics of deterioration in historical leather. The results brought clear evidence that long-term natural ageing of leather induces the destabilization of chemically modified collagen thereby promoting its partial de-tanning and allowing micro-unfolding of chemically unmodified collagen. As deterioration continues toward the extreme, collagen molecules become highly unstable allowing for gelatinisation and irreversible denaturation. It is thus explained the coexistence of collagen populations with distinct thermal stability in historical leather. They are grouped in three main structural domains, namely "leather-like", "parchment-like" and "gelatine-like", whose mass percentages determines leather stability against further natural ageing and deterioration. As a result of the MHT and micro DSC parameters correlation, a more comprehensive set of criteria, including T f and T l values, as well ∆C and ∆T intervals' lengths, was introduced for better interpreting the shrinking activity of collagen in historical vegetable-tanned leathers and open thus the way for the highly sought in situ evaluation of leather artefacts.

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