Evaluating degradation of silk’s fibroin by attenuated total reflectance infrared spectroscopy: Case study of ancient banners from Polish collections

•ATR-FTIR estimators to describe crystallinity and oxidation of fibroin were chosen.•A new, proposed estimator ECOOH monitors the hydrolysis of fibroin.•ATR-FTIR estimators were classified as primary and secondary structure estimators.•Primary and secondary structure estimators help in museum objects’ risk assessment.•Estimators can discriminate surfaces directly exposed to ex humidity, pollution.

In this study a part of research where artificially aged model samples were used as a guideline to the mechanism of degradation is presented. In previous work Bombyx Mori silk samples were exposed to various environments such as different oxygen, water vapour and volatile organic products content, all at the temperature of 150 °C [11]. Based on those results gathered with by Attenuated Total Reflectance/Fourier Transform Infrared Spectroscopy (ATR-FTIR) the degradation estimators were proposed and classified as follows:(1)Primary functional groups estimatorsEAmideI/II – intensity ratios of Amide I CO stretching vibration to Amide II NH in-plane bending and CN stretching vibrations A1620/A1514.ECOOH – band 1318 cm−1 integral to band integral of CH3 bending vibration band located at 1442 cm−1P1318/P1442.(2)Secondary conformational estimatorsEcCO2 – intensity ratios within Amide I CO stretching vibration of parallel β-sheet to antiparallel β-sheet A1620/A1699.In this work estimators were verified against estimators calculated from spectra of silk samples from 8 museum objects: 3 from 19th, 2 from 18th, 1 from 17th and 2 from 16th century including 3 banners from the storage resources of the Wawel Royal Castle in Cracow, Poland.

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