Moisture Sorption as a Potential Condition Marker for Historic Silks: Noninvasive Determination by Near-Infrared Spectroscopy

Abstract

Given their ephemeral nature, the preservation of historic silks can be problematic. Rapid, on-site condition monitoring would offer significant benefits to conservators and museum curators concerned with continued access to collections. In this paper, near-infrared spectroscopy (NIR) is investigated as a noninvasive approach to the characterization of silk fabrics and particularly for determining the moisture content of silks as a potential age-related marker. Bands within the NIR spectrum of silk are assigned to contributions from water and the silk fibroin polymer. The water bands may be deconvolved to show separate contributions from bound and structural water. When silk is exposed to deuterium oxide, the water OH NIR bands are rapidly lost. The accompanying changes in the amide-related NIR absorptions reflect differential accessibility of regions within the semi-crystalline fibroin aggregate. NIR spectra were recorded while silk was maintained at a range of relative humidity; complementary gravimetry provided absolute reference data for moisture sorption. A single spectral parameter, the intensity of the water combination band, is sufficient to indicate the relative moisture content of silk and allows distinction of unaged and heat, light, and humidity aged silks. The results confirm that NIR has significant potential for on-site studies at collections in support of the preservation and access of our silk heritage.