IDENTIFICATION OF DYES ON OLD TEXTILES
8 MIXTURES OF NATURAL DYES AND INDIGO
IF THE FABRIC has a green or violet shade obtained by the use of natural dyes,and in the preliminary test a blue color is extracted by boiling glacial acetic acid, one should test for indigo by the method described by Hofenk-De Graaff (“A Simple Method for the Identification of Indigo,” Studies in Conservation, v. 19, : 54–5). If the result of the test is positive the indigo should be removed from the fibre by repeated boiling with dimethylformamide until the solvent remains colorless. Once the indigo has been removed the yellow component (in green dyeings) or the red component (in violet dyings) is left behind and can be identified by forming the various colored lakes as described above.
An interesting example of the method just described was the examination of a fragment of Coptic fabric dating from the 6th centruy that had been dyed with indigo and madder, a mixture that has been called Egyptian purple. The color of the original sample was dark violet. Treatment with ammonia did not change the color but extraction with dimethylformamide took out the indigo, leaving the red on the fibre. The colored lakes formed by the red dye matched those of the red obtained from wild madder (Rubia peregrina). It was confirmed by thin-layer chromatography that ordinary madder from Rubia tinctorum was not present, since only purpurin (1, 2, 4-trihydroxy-anthraquinone) was found: alizarin (1, 2-dihydroxyanthraquinone) was absent.