Analytical markers for silk degradation: comparing historic silk and silk artificially aged in different environments.

Francisco Vilaplana,Sigbritt Karlsson,Dorte V P Sommer,Johanna Nilsson

doi:10.1007/s00216-014-8361-z

Abstract

Suitable analytical markers to assess the degree of degradation of historic silk textiles at molecular and macroscopic levels have been identified and compared with silk textiles aged artificially in different environments, namely (i) ultraviolet (UV) exposure, (ii) thermo-oxidation, (iii) controlled humidity and (iv) pH. The changes at the molecular level in the amino acid composition, the formation of oxidative moieties, crystallinity and molecular weight correlate well with the changes in the macroscopic properties such as brightness, pH and mechanical properties. These analytical markers are useful to understand the degradation mechanisms that silk textiles undergo under different degradation environments, involving oxidation processes, hydrolysis, chain scission and physical arrangements. Thermo-oxidation at high temperatures proves to be the accelerated ageing procedure producing silk samples that most resembled the degree of degradation of early seventeenth-century silk. These analytical markers will be valuable to support the textile conservation tasks currently being performed in museums to preserve our heritage.Electronic supplementary materialThe online version of this article (doi:10.1007/s00216-014-8361-z) contains supplementary material, which is available to authorized users.

Highlights

Silk from the cultivated Bombyx mori larva is an appreciated material in historic costumes due to its valuable properties such as high lustre, smoothness, strength and lightness
Low pH values during textile washing are usually correlated with specimen deterioration and this needs to be taken into account during their conservation to avoid further catalytic degradation action
The decrease in brightness is thought to be caused by the formation of chromophoric groups during the degradation environments to which silk textiles are exposed; this will be correlated with the chemical changes by FTIR and with the amino acid content

Summary

Introduction

Silk from the cultivated Bombyx mori larva is an appreciated material in historic costumes due to its valuable properties such as high lustre, smoothness, strength and lightness. The H-fibroin primary structure consists of 20 amino acids, mainly glycine (Gly) 45.9 %, alanine (Ala) 30.3 %, serine (Ser) 12.1 % and tyrosine (Tyr) 5.3 %. The Gly-X repeats are distributed in 12 “crystalline” domains with varying length between 39 and 612 amino residues, separated by almost identical copies of boundary “amorphous” sequences These amorphous spacers in B. mori silk are tyrosine rich and contain most of the other amino residues that are absent in the Gly-X domains, basically amino acids with bulky and polar side chains. These amorphous domains break the Gly-X alternate and terminate the crystalline regions. In the amorphous regions distorted β-sheets are present [7]

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Conclusion