Exploring elucidation of red dye mixtures on woolen historical textiles via non-destructive spectroscopic analysis and multivariate cluster analysis

Abstract

Abstract One of the foremost challenges facing analysis of historical textiles is that the gold standard technique—high performance liquid chromatography (HPLC)—is inherently destructive. This is especially problematic considering many historical textiles are exceptionally fragile, be it from age, poor care over time, etc. One proposed solution to this is the implementation of non-destructive, namely spectroscopic, techniques, such as diffuse reflectance (Fiber Optic Reflectance Spectroscopy, FORS). In this work, 204 well-provenanced red Norwich textiles were measured with FORS and analyzed to attempt to determine chromophore combinations used to dye the original textiles. To these ends, cluster analysis algorithms and spectroscopic domain knowledge were coupled with selective HPLC validation to assess overall ability of FORS to discern changes in chromophore combinations. It was found that the near infrared (NIR) region of the spectrum contained little meaningful information in multivariate space, while the VIS region, particularly 380–469 nm, showed a narrow visible region that was primarily responsible for clustering behavior that correlates with HPLC-validated samples. This indicates that FORS shows promise for discerning chromophores in textile swatches. Additionally, X-ray fluorescence (XRF) analysis was used to confirm that the observed FORS spectral inflection point shift in the 600 nm region did not correlate with the presence or type of mordant used when dying these textiles. From this work, three main conclusions can be drawn: (1) FORS adequately identifies visual infon, which shows reasonable correlation to HPLC-validated dye recipes, warranting further investigation, and indicating utility for cois or use for those with visual impairments; (2) XRF analysis confirms that the ~ 600 nm inflection point shift and mordant are not correlated when measuring dyed textiles, unless mordant is present below analytical detection limits or not present at all; (3) many documented structural-to-spectral relationships established in the conservation literature are too weak in dyed textiles for statistical analysis and, by extension, expert spectral identification.

Keywords

• Textiles
• Red
• Cochineal
• Madder
• Norwich
• Chemometrics