Abstract
Many objects of culture heritage, comprised of leather, need to receive the right treatment to be restored and to elongate their lifespan. Determination of the degradation degree and even better the type of the degradation is a crucial knowledge for the way of subsequent conservation, exhibition and long-term storage. Collagen based materials are very sensitive to the deterioration and undergo, mainly hydrolysis and oxidation. Namely acid hydrolysis and photooxidation are the most often causes of the disintegration of leather. Contrary to the leather investigation, a few studies dedicated to parchment described some typical features of hydrolysis, oxidation and gelatinization observed applying Attenuated Total Reflection Fourier-Transform Infrared spectroscopy (ATR-FTIR) which is widely used in collagen degradation type research. Except of the collagen secondary structure, followed by IR spectroscopy, we determined the shrinkage temperature of the collagen substrate by Micro Hot Table method (MHT) to reach the degradation level. In this paper, artificially degraded leather samples as a theoretical representative of cultural heritage objects were examined. We discuss the use of both techniques (IR and MHT) as potential methods for fast assessment of oxidation and hydrolysis of vegetable tanned leathers and degradation level. New samples of leather tanned by various vegetable tannins were artificially degraded under controlled conditions. We simulated the photooxidation by means of the Xenon arc lamp exposure, oxidation using the soaking in hydrogen peroxide, acid hydrolysis by the soaking in hydrochloric acid and alkaline hydrolysis using the soaking in natrium hydroxide. ATR-FTIR spectra of reference and tested samples were compared. Oxidation causes increase of the distance between amide I (AI) and amide II (AII) wavenumbers (Δν) above 100 cm−1 and the intensity ratio between AI and AII bands (AI/AII) above 1.6. The AI/AII ratio depends on the type of hydrolysis. The increase above 1.8 proves acid hydrolysis while the decrease under 1 demonstrate alkaline hydrolysis. MHT results are not so obvious. Generally, mainly the hydrolysis causes the decrease of the temperatures. We have found out that knowledge of the whole shrinkage interval is important and provides more appropriate information about the leather disintegration.
Highlights
Leather, as all other materials, is disintegrated over time
Photooxidation Even though the photooxidation of leather samples was performed only for 1 week significant changes were observed in IR spectra in comparison with spectra of reference samples (Fig. 4)
Our results indicate that the leathers, tanned by hydrolysable tannins hydrolysable tanned leather samples (HL), are more sensitive to alkaline hydrolysis than leathers tanned by condensed tannins condensed tanned samples (CL)
Summary
As all other materials, is disintegrated over time. It is hard to decide which treatment method is the best if the reason of the disintegration is unknown. The main component of leather is protein called collagen. More than 28 different types of collagen exist [1]. They differ themselves by their chemical composition, molecule weight, and by content of glycosides compounds. Collagen type I is the main component of the leather. The fundamental structural units of collagen are amino acids linked through the peptide bonds to polypeptide chains. Each of peptide chain contains about 1000 amino acids residues which are divided to tripeptide segments of the repeating structure Gly–X–Y, where glycine (Gly) forms about 30%, X and Y are generally
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