Silver colloidal pastes for dye analysis of reference and historical textile fibers using direct, extractionless, non-hydrolysis surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is an ideal tool for analyzing dyes on historical textiles because it requires very little sample compared to other available analytical methods and analysis can be done directly on the fiber. This paper reports on the first systematic study of the use of citrate-reduced silver colloidal pastes for the direct, extractionless, non-hydrolysis detection of dyes directly on wool, silk, cotton, and flax fibers. This type of study provides greater insight into the optimal conditions required for accurate analysis of dyes in historical samples. In this work, Ag colloidal pastes were characterized using localized surface plasmon resonance and scanning electron microscopy. The pastes were then employed for SERS analysis of twelve reference samples of different vegetal and animal fibers dyed with cochineal and eleven dyed with brazilwood. Furthermore, six historical textiles from an important collection of Mariano Fortuny (1871-1949) textiles at the Art Institute of Chicago were also examined, to test the efficacy of the paste on aged samples, and to shed light on Fortuny's fascinating production techniques. A mixture of cochineal and brazilwood was detected in some of the historical samples demonstrating, for the first time, simultaneous identification of these colorants used in combination. In addition, the findings give substance to the claim that Fortuny kept using natural dyes at a time when many new and attractive synthetic products became available.