IDENTIFICATION OF DYES ON OLD TEXTILES
9 RAPID TEST FOR DISTINGUISHING AMONG MADDER, COCHINEAL, AND KERMES
ONE TAKES A SMALL PORTION of the red dyeing, adds a few drops of concentrated sulfuric acid, and waits for a few minutes until the dye has colored the acid. Madder gives a dull red solution that fluoresces orange in ultraviolet light; cochineal (and lac dye) gives a magenta solution; kermes gives a dull red-violet solution. If a few milligrams of boric acid are then added to the sulfuric acid, color changes may be observed. If cochineal (or lac dye) is present the color changes to blue; if kermes is present the color changes to brownish-violet; if madder is present there is no change in color. If the solution is diluted with about ten times its volume of water and shaken with a little ether, madder and kermes go into the ether phase, coloring it orange; cochineal (and lac dye) remains in the aqueous phase but can be extracted with pentanol.
The ether and pentanol extracts can be washed throughly with water to remove acid and used for identification of the dyes by means of thin-layer chromatography. The latter is the only simple way by which one can distinguish between cochineal and lac dye.