Abstract
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 Tf and Tl 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.
Highlights
Historical leathers, in a huge variety of items as footwear and garments, bookbinding, wall tapestry, upholstery, harnesses, armours, storage vessels, household tools, cases, musical instruments, toys, ritual objects, are important parts of our cultural heritage, from remote time to modernity
The micro Differential scanning calorimetry (DSC) analysis provided quantitative results on the bulk material and unambiguously pictured the deterioration state of historical leather through several calorimetric indices of macromolecular change identified for fibrous collagen, including (1) the temperature at the endothermic peak maximum (Tmax), (2) the specific enthalpy of denaturation (∆H), (3) heterogeneity in the structural domains with distinct thermal stabilities (∆T1/2)
Separation through peak deconvolution of the endothermic peaks associated with these structural domains confirms that historical leathers are complex blends of chemically modified, chemically unmodified and gelatinised collagen having distinct hydrothermal stabilities
Summary
Historical leathers, in a huge variety of items as footwear and garments, bookbinding, wall tapestry, upholstery, harnesses, armours, storage vessels, household tools, cases, musical instruments, toys, ritual objects, are important parts of our cultural heritage, from remote time to modernity. The more versatile vegetable-tannage has continuously progressed to become the most used method to produce leather in Western and Mediterranean Europe until the end of the XIXth century [5, 6], when chromium mineral tanning came into use and gradually replaced vegetable tannins. Tannins derive their name from the Latin word tannum, which means “crushed oak bark,” since in early times oak trees served as a major source of tannin for the leather production. The terms tanning and tannage are applied for all different processes of leather making which include vegetable tanning with plant polyphenols, mineral tanning with metal salts, oil and aldehyde tannages, synthetic tanning agents and organic tannages based on natural polyphenols or synthetic organic oligomers [9]
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