Disclosing the dyeing formulations of weld yellows from 18th-century recipe books with molecular fluorescence

Abstract

For millennia, organic dyes have been used in artworks such as textiles and have great artistic and historic value. They may provide clues to the understanding of the technology behind an artwork's production. The characterization of natural organic colourants in artworks is still a challenge to this day, and of the natural dyes used in cultural heritage, yellows are some of the most difficult to identify.

This paper explores the potential of combining molecular fluorescence in the visible range with high-performance liquid chromatography coupled with diode-array detection, to disclose historical dye formulations.

By analysing historically accurate reconstructions following recipes from two 18th c. French master dyers, Antoine Janot and Paul Gout, it was possible to demonstrate that molecular fluorescence is highly sensitive to the presence of specific dye ingredients and different use of recipes. Luteolin (Lut) and Luteolin 7-O-glucoside (Lut-7-O-glc) were identified as the main compounds, with the first being extracted in higher amounts, especially in Janot's variations. The analytical data demonstrated that molecular fluorescence is very sensitive to recipe differences: the presence of tartar correlated with a higher Lut presence, and the use of lime induced a slight shift in the emission maxima, even without affecting the spectral shape due to the exhaustion of molecules in the dye bath. Moreover, our findings revealed technological insights, showing that Paul Gout's optimized recipes achieved comparable yellow tonality to Janot's using a lower percentage of weld. This will provide key knowledge on the technological processes for dyeing with weld from these 18th c. French masters, while creating a molecular fluorescence database for the analysis of artworks.