BLUE DYE IDENTIFICATION ON CELLULOSIC FIBERS: INDIGO, LOGWOOD, AND PRUSSIAN BLUE
Ann Cordy, & Kwan-nan Yeh
3 RESULTS AND DISCUSSION
3.1 Non-Aged Thread
3.1.1 Acid Digestion Colors.
The colors of the dyed thread solutions were noted after acid digestion and then after extraction with methylene chloride (see Table I). All indigo samples yielded a blue color on extraction. The organic indigo dye molecule was soluble in the organic layer, while some water-soluble sulfonated indigo appeared to remain in the acid layer. Logwood-dyed thread produced a pink acid solution. Prussian blue-dyed samples yielded light blue in the acid layer. Undyed flax thread produced a yellow color in the methylene chloride layer.
Table I Acid Digestion Colors for Dyed and Undyed Flax Thread
3.1.2 UV-VIS Spectroscopy.
Only indigo colored the methylene chloride layer, producing a visible peak at 602 nm. This was close to the peak found by Whiting of 600 nm in methylene chloride.6Table II lists VIS spectroscopic maximum peaks for indigo. Prussian blue gave poor readings with this technique so no results are presented. Logwood produced only one peak as discussed below. UV spectrum for all samples produced weak peaks.
Table II Observed Absorption Maxima (NM) for Indigo in the Visible Region
Vis spectroscopic analysis of the acid layer yielded a 645 nm peak for indigo (possibly sulfonated indigo) and a 518 nm peak for logwood. Whiting had obtained a peak of 520 nm for logwood in dilute acid.7
The acid layer was neutralized and evaporated to a residue which was dissolved in water. It was hoped that a useful UV-VIS spectrum could be produced. However, the residues were not readily soluble in water and the resulting suspension gave ambiguous results. The methanol extraction of the residues produced clear peaks only for indigo at 607 nm.
3.1.3 IR Spectroscopy.
Characteristic organic functional groups were searched for from the methylene chloride layers. Due to water interference, attempts to use potassium bromide pellets of evaporated neutralized acid layers were not successful.
As expected from the color of the solutions (possible indication of dye presence), only indigo provided identifiable peaks from the methylene chloride layer, and those were for the sulphonated indigo that had been formed during the acid digestion step (sulphonated aromatic rings).
3.1.4 Wet Chemical Analysis.
Wet chemical identification gave a positive response for ferrocyanide ions only with Prussian blue. No positive responses were detected for ferricyanide ions.
Ferrous ions were found with all three dyes. The ferric ion test produced weak positive tests for all dyed samples. Ferrous sulphate was used as a reducing agent for indigo dyeing on cellulosics in the mid-nineteenth century and was also used in this study. An iron mordant was used to produce logwood blue. Iron was also present in Prussian blue. Testing for ferric or ferrous ions was, therefore, not a discriminating test between these dyes but does point to the presence of a dye since the undyed flax gave negative results for all the above ion tests.
3.2 Aged Thread
3.2.1 Color on Aging.
The Prussian blue and indigo-dyed thread remained blue throughout the light treatment. After only ten hours the logwood sample had lost its original color and had become tan.
All control thread was brittle when removed from the oven after 254 hours and broke easily when bent or gently pulled. Again, the indigo and Prussian blue dyed threads retained their blue color. The Prussian blue lost its intensity and dulled to a grey-blue similar to that of the indigo samples (a point to be aware of as some dark blue textiles of an “indigo” color may actually be dyed with Prussian blue that has faded). All logwood dyed samples became dark brown with less then ten hours of heat. The undyed thread became dark brown by the end of 86 hours.
3.2.2 Acid Digestion Colors.
Colors for aged samples in the extraction solvents are given in Table I. Undyed aged flax produced an amber color in the acid layer rather than in the organic methylene chloride layer, an indication of some fiber degradation. The indigo and Prussian blue samples still produced blue in the methylene chloride and acid solutions as had the non-aged thread. Logwood samples did not retain their blue color with aging.
3.2.3 UV-VIS Spectroscopy.
Aged dyed thread responded similarly to non-aged dyed samples in UV-VIS spectroscopy analysis except for indigo in acid and the logwood-dyed thread. The UV spectrum of aged samples did not provide positive identification of a dye.
Aged indigo samples produced VIS maxima peaks in both methylene chloride and methanol solutions that were extremely close to those of the non-aged thread (see Table II). Indigo-dyed thread produced a differing maxima peak in acid after heat treatment (670 versus 645 nm non-aged). The peak shift in acid with aging could cause unclear identification and therefore the acid layer VIS analysis might not be a good choice for aged samples even though the textile may still be blue.
Aged logwood thread lost its color within ten hours of both heat and light treatment and produced no peaks in the visible range. Prussian blue samples yielded no identifiable peaks whether aged or not.
A change in color of the dyed textile will not give the same VIS spectroscopic results due to the color shift and therefore is not a suitable test if a color change is suspected to have occurred.
3.2.4 IR Spectroscopy.
Artificially aged indigo-dyed samples no longer showed absorption peaks of indigo or sulphonated indigo despite the blue color of the methylene chloride layer. This was perhaps due to the decreased amount of dye in this layer with aging (lighter blue solution than non-aged). More sensitive IR detection would be needed to identify the indigo. Heat and light treated indigo, logwood, and Prussian blue samples showed only peaks for undyed flax.
3.2.5 Wet Chemical Analysis.
Even with severe artificial aging the ferrocyanide ion test still yielded positive results for Prussian blue. The ferricyanide test still produced negative results for all samples as it did for the non-aged thread. Positive tests were found for ferric and ferrous ions on all aged samples except for ferric ion with heat-treated logwood. Undyed flax yielded negative results for all tested ions.