Lake Pigments and Thin-Layer Chromatography of Natural Dyes: Paired Experiments for Nonmajors

Lakes are artists' pigments made from colorants bound to an inert substrate, usually hydrated aluminum oxide. Before the late 19th century, lakes were made from natural pigments extracted from plants and insects, extracted either directly from the dyestuff or indirectly from waste materials generated in the manufacture of dyed textiles. The first of two experiments described here recreates the second indirect method using silk yarn dyed with madder roots and cochineal insects. Heating the yarn in potassium carbonate solution extracts the hydroxyanthraquinone colorants into solution; adding potash alum precipitates the lakes. A reflectance spectrometer is used to characterize the colors of the lakes through their CIELAB values; the pigments can also be used to make paint swatches. In the second experiment, thin-layer chromatography is used to identify the principal colorants in wool yarn dyed with madder. Alizarin and purpurin are readily distinguished by their retention factors and colors and purpurin's fluorescence. Although this pair of experiments is situated in a nonmajors' course about paint pigments, it can also find a place in a more general nonmajors course. Besides giving students first-hand experience preparing an important class of artists' pigments by a historic method and with thin-layer chromatography, these experiments reinforce fundamental chemistry topics such as line structures, polar bonds, complex ion formation, and solute–solvent interactions.