Studies in Wool Yellowing

Abstract

Changes in the yellowness of wool during exposure to sunlight or to an artificial source resembling sunlight (the Xenotest) depend not only on the relative proportions of blue light and uv radiation, which cause partial bleaching and yellowing, respectively, but also on the exposure period and on the initial natural yellowness of the sample. The bleaching reaction is rapid and short-lived and is most pronounced with deeply yellow wools, whereas uv yellowing is more apparent with paler specimens and is sustained throughout irradiation. Even with the deeply yellow samples, the uv yellow ing reaction eventually supervenes and proceeds at a similar rate for all wools, although the level of yellowness at any particular stage of irradiation varies from one wool to another. A linear relationship exists between the initial natural yellowness of scoured wool and the change in yellowness during Xenotest irradiation for any particular period. Action spectra for changes in shade of naturally yellow wool and of wool after yellow ing in the laboratory by either uv radiation, dry heat, or hot alkaline buffer are similar. yellowness increasing to a maximum with decrease in wavelength down to 300-320 mμ (the lowest waveband examined) and decreasing to a minimum at 440-460 mμ in the blue region. However, the absorption spectra determined by the o-dichlorobenzene immersion method differ and so, in general, do the changes in yellowness during irradiation in the Xenotest. Natural yellowness is more likely to be attributable to the deposition of a thin pigment layer at the surface of the wool fiber than to the effects of uv radiation, heat, or alkaline conditions while on the sheep. This important difference in the distribution of pigment between natural and artificially yellowed wools may suggest new methods of improving the color of wool.