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

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.

Fig. 1

Historic silk costumes and sampling procedure. a King Gustav II Adolf (GIIA) doublet showing where sample was taken from the lining; b detail of GIIA doublet lining; c GIIA breeches showing where sample was taken; d detail of the seam allowance. e King Karl X Gustav (KXG) cloak showing where samples were taken; f detail of seam allowance. Photo: a–d, f courtesy of Erik Lernestål; e courtesy of Göran Schmidt

Fig. 2

Extract acidity of the historic and artificially aged silk textiles

Fig. 3

Effect of the different artificial degradation environments on the brightness (expressed as % of transmittance at 457 nm) of silk textiles. a UV exposure; b thermo-oxidation (black, 125 °C; grey 60 °C); c RH exposure (black, 60 °C); d pH immersion (black square 60 °C; grey rhombus, 25 °C). The area in grey corresponds with the reference silk

Fig. 4

FTIR spectra of selected artificially aged and historic silk textiles: a overview of the mid-infrared region (600–2000 cm⁻¹); b free carbonyl region (1700–1775 cm⁻¹); c amide I region (1600–1700 cm⁻¹); d amide III region (1200–1280 cm⁻¹); tyrosine doublet region (800–875 cm⁻¹)

Fig. 5

FTIR crystalline and carbonyl indexes: a UV exposure; b thermo-oxidation (black, 125 °C; grey, 60 °C); c RH exposure (black, 60 °C); d pH immersion (black squares: 60 °C; grey rhombus, 25 °C); e historic silk samples. The area in grey corresponds with the reference silk

Fig. 6

SEC chromatograms for the artificially aged and historic silk samples: a UV exposure; b thermo-oxidation; c RH exposure at 25 °C for 28 days; d immersion into pH solutions at 25 °C for 28 days; and e historic samples

Fig. 7

a Exploratory principal component analysis (PCA) of the analytical markers for the degradation of historic and artificially aged silk textiles. Loading plot for the analytical markers (variables) under the first two principal components. These two principal components explain 66.8 % of the total variance. Nomenclature: amino acid composition (as in Table 1), carbonyl index (CarbI), crystalline index (CrI), molar mass from SEC (Mw), acidity (pH), Young’s modulus from tensile tests (Mod), elongation at break (EaB), tenacity at break (TaB); b PCA score plot for the silk textile samples (observations) under the first two principal components. Nomenclature: B reference silk, UV4d UV irradiation (4 days), UV10d UV irradiation (10 days), T28d thermo-oxidation (125 °C, 28 days); King Gustav II Adolf doublet from 1617, King Gustav II Adolf breeches from 1617 and King Karl X Gustav cloak from 1654