Mild extraction of dyes by hydrofluoric acid in routine analysis of historical paint micro-samples

The paper demonstrates the broad applicability of a mild extraction method, based on the use of HF and organic solvents, for dyestuffs in lakes. Originally developed for acid-labile components of madder lakes, such as pseudopurpurin and glycosides of alizarin and purpurin, this method has shown its efficiency for virtually all mordant-type dyestuffs, which includes anthraquinones from kermes, lac, or madder, homoisoflavonoids from brazilwood, flavonoids from dyer's broom or other plants, and some synthetic dyes. It is also suitable for lakes based on complexes of cations other than aluminium, such as barium or tin, and for non-mordant dyestuffs such as indigoids. All this is illustrated through HPLC analyses of commercial and home-made lakes, paint samples and of historical micro-samples. In many of these samples the mild extraction method has allowed to show the presence of glycosidic precursors of the dyestuffs.