Madder dyes

Krapp-farbe, /. madder color, madder dye. -filrbcn, n. madder dyeing, -farbstoff, -farbc-atoff, m. alizarin, -gelb, n. madder yellow, xanthin. -lack, m. madder lake. -rot n. madder red, alizarin, -stoff, m. alizarin, -wurzel, /. madder root. 

Red natural anthraquinone dyes on ancient textile materials can be readily identified by thin-layer chromatography (TLC) if they belong to the class of madder dyes. The method also shows which type of dye plant from the family Rubiaceae has been used for dyeing (Rubia tinctorum L., R. peregrina L.,... 

The separation of insect dyes and madder dyes by HPLC is discussed exhaustively by Wouters (7). Various other laboratories (8, 12-1A) also use this method for separating natural dyes. There are far more publications, however, on the use of TLC for separating natural dyes, including those belonging to the class of hydroxyan-thraquinones (A, 8, 9, 12-31). 

Pfister (41) assumes that Rubia cordifolia was used by antique Indian dyers. Another red madder dye that later was used in India is the Chay root of Oldenlandia umbellata L. (42). 

Madder dyes belong to the group of mordant dyes. They are dyed on wool or silk that has been previously mordanted with aluminum or iron salts. The madder dyes react with these salts to form on the fiber color lakes that are water-insoluble and do not bleed even when treated with dilute ammonia. For the identification of madder dyes with the aid of TLC, however, we require a dye solution that can be applied to the thin-layer plate. At acid pH (pH 3 or lower), the dyes are liberated from the lake, a process during which the color visibly changes, and the organic constituents can be extracted. 

In order to strip the dyes of madder dyeings completely from the fiber, they must be extracted in acid solution at temperatures up to roughly 100° C. Comparative solutions produced by extraction from dyer s plants such as madder root must also be prepared in the same manner at acid pH for the TLC. The anthraquinone dyes are present in the dyer s plants partly in the form of glycosides. These glycosides must be split hydrolytically with acids in order to obtain for the TLC a comparative solution that contains only free hydroxyanthraqui-nones and no hydroxyanthraquinone glycosides. 

In the extraction of powdered madder roots with 10% sulfuric acid, followed by shaking with ethyl acetate, we see that the ethyl acetate contains only a very small amount of pseudopurpurin or not at all. However, when we investigate madder dyeings on old carpets by the same method, the thin-layer chromatogram often shows that pseudopurpurin is present in roughly the same amount as purpurin. What is the explanation for this ... 

New improved extraction method for madder dyes. For these investigations, the following method of extraction is recommended ... 

For investigating dyeings with madder dyes, the extraction time of one minute is adequate, as described in the analysis of the "Rose madder" sample. 

When we strip a red dyeing by the improved extraction method with a mixture of 10% sulfuric acid and ethyl acetate (1 1) and.the upper layer has a yellow or orange-yellow shade and the lower layer is colorless, we can assume that this is a madder dyeing. 

Even with the new, improved extraction method, we often obtain this chromatographic picture in analyzing old madder dyeings, if... 

Ventilago mad(e)raspatana The dyes from this dyer s plant differ on the thin-layer chromatogram from all other madder dyes so markedly that the plant extract can be used for identification by TLC comparison. Although the dyes of Ventilago mad(e)raspatana are not quite unknown to us (see table III), it is not possible to classify the spots, because the necessary comparative material is missing. 

Figure 3. Comparison of the old and the new (improved) extraction methods for madder dyeings and madder pigments (see page 202) by TLC. 1, and 2, extractions of a sample of the artist s pigment Rose Madder Genuine 1, extraction by the new method 2, extraction by the old method 3, pseudopurpurin (lower spot) 4, purpurin 5, alizarin.

Figure 5 shows the comparison of the madder dyes taken from the Adler-dalmatika with samples of Rubia tinctorum, alizarin, and purpurin by TLC. The dye extracted from the sample of the Adlerdalmatika with dilute sulfuric acid is unmistakably madder. 

When the improved method of extraction is used for stripping madder dyes with 10% sulfuric acid and ethyl acetate (see page 202), and the ethyl acetate layer is not stained yellow, but orange, this could indicate the presence of insect dyes containing kermesic acid (XXVI). 

If the sulfuric acid layer is stained orange to red, this could indicate the presence of carminic acid (XXVII) or laccaic acids (XXVIII -XXXII). If the sulfuric acid layer is colorless and the ethyl acetate layer is orange, the ethyl acetate layer is separated off in a small separating funnel and a specimen for the TLC is prepared as described above for madder dyeings (page 202). 

The chromatographic separation and the identification with ura-nyl acetate are carried out as in the TLC of madder dyes. 

Figure 5. Identification of the madder dyes in the Aldlerdalmatika by TLC comparison. 1, dyes extracted from the Aldlerdalmatika by diluted sulfuric acid 2, madder dyes 3, alizarin 4, purpurin.

Chromatographic methods (1-25), in particular thin-layer chromatography (TLC) (5-17, 20-25) and more recently high-performance liquid chromatography (HPLC) (21, 22, 25), preferably are used. If dye mixtures are present, as is the case with madder dyeings, chromatographic comparisons are the best method of distinguishing the individual components. 

Red and violet madder dyeings on alum or iron sulfate mordant turn orange the dye bleeds a yellow shade, and after it has been shaken with ethyl acetate, it can be used for TLC comparisons. 

In dyeing with natural mordant dyes, widely varying shades are usually obtained, depending on the mordant that was used to pretreat the textile material to be dyed. Madder dyeings on an alum mordant are red, whereas those on an iron sulfate mordant are brown-violet to violet. 

Testing of Violet Natural Dyeings. Violet dyeings are first boiled in 102 sulfuric acid. If the dye dissolves with a yellow color, a madder dyeing on iron mordant is indicated. If the sulfuric acid turns orange and a blue remains on the fiber, the dyeing is a mixture of cochineal and indigo, which can be identified separately by the methods described earlier. 

---