Using FTIR spectroscopy to detect sericin on historic silk

Silks represent some of the most precious ancient and historic textile artefacts in collections worldwide. Their optimum preservation demands an appreciation of their characteristics. One important concern, especially with regard to ancient Chinese silks, is whether the fabrics have been degummed. Silks with remnant sericin gum coating the fibroin fibres would require different conservation protocol. In previous research on aged silks, the presence of sericin has been inferred from amino acid analysis of hydrolysates. In the study reported here, the potential of FTIR spectroscopy to provide a simpler and rapid method of detecting sericin on silk has been investigated. Both fibroin and sericin exhibit singular IR absorptions. Attenuated total reflectance spectroscopy was found to highlight the sericin coating more effectively than transmission and reflectance spectroscopy. Three particular peak intensity ratios were identified which might provide a quantitative estimate of the sericin content of new silk, to a sensitivity of 1%-2%. These were also shown to be valid indicators for the presence of sericin on artificially aged and archaeological silks, although quantitation was now not possible. Besides the peak intensity ratios, two signature peaks were also seen to be useful markers for silk fibroin, and their presence in a spectrum could be used to infer a degummed silk.