Gel-supported liquid extraction of natural dyes: testing the performance and the invasiveness of an innovative protocol

Abstract

Colorants in cultural heritage can provide information on provenance, technique, and use, as well as inform on contemporary conservation and preservation. Naturally, the development of minimally invasive techniques is critical for their analysis. Consequently, this thesis seeks to test the performance and invasiveness of an innovative protocol applied to madder and indigo dyes. Gel-supported liquid extraction was performed on mock-ups of madder and indigo, applied by egg tempera and dyed onto wool textiles. The invasiveness was evaluated by visual analysis, microscope photography, and FORS colorimetry. Quantification of the dye extracts was calculated using HPLC-MS/MS analysis, allowing for an evaluation and comparison of the efficiency of each extraction solution tested (acidic, alkaline, organic). Visual examination of the mock-ups yielded virtually indistinguishable differences before and after extraction, but the ΔE00 values for some of the acidic and alkaline extraction solutions applied to the paint and textiles were above the upper limit of perceptible color change. However, this is likely due to the heterogenous nature of the mock-ups that can impact colorimetric measurements. The HPLC-MS/MS quantification resulted in good extraction performance from the acidic and alkaline solutions when compared to the organic solution, while the tempera paint mock-ups generally yielded more dye extracts than the textile mock-ups. Finally, a preliminary Raman analysis of the agar-gels and the extraction solutions suggested potential molecular modifications due to the interaction of the extraction solution with the agar-gel matrix. Overall, this thesis demonstrates the effectiveness of gel-supported liquid extraction, while remaining almost completely non-invasive and allowing multi-analytical study of organic dyes from cultural heritage.