Volume 35, Issue 2 (March 1990)
An Investigation of Known Blue, Red, and Black Dyes Used in the Coloration of Cotton Fibers
Previous work on blue, red, and black cotton samples dyed with unknown dyes showed that, within a color class, the use of microspectrophotometry can give a significantly higher degree of discrimination than is possible using microscopy alone. The present study was undertaken (1) to assess the frequency of matching spectra being produced from dyes known to be different; (2) to show what extra level of discrimination, if any, is obtained when thin-layer chromatography (TLC) can be carried out on the extracted dyes; and (3) to examine the extent of intrasample spectral variation.
Spectra were recorded from 77 blue, 32 red, and 26 black cotton samples dyed with known examples of sulfur, leucosulfur, direct, reactive, and vat dyes. TLC was attempted on all spectrally matching samples. Spectral variations (shifts of peak maxima and peak reversals) were noted for each sample.
The occurrence of matching spectra from different dyes in each color class was very small (0.2% for blue dyes, 1.5% for red dyes, and 1.5% for black dyes). TLC was only effective in separating 5 out of 21 spectrally matching sample pairs. All color classes showed occasional examples of peak reversal, especially in pale blue and pale black samples. Shifts of absorption maxima were sometimes considerable. The casework implications of these results are discussed.