If short cuttings of viscose fibers are exposed to solutions of certain dyes like Solophenyl Fast Blue Green BL and Sky Blue FF it is seen that the skin resists penetration of the dye for a considerable length of time, whereas the dye diffuses readily into the core from the fiber end. This phenomenon, designated as "anisotropic diffusion," gives a ready explanation of the selective staining of the core in fiber sections by a number of dyes.

The penetration x of the dye into the core with time t has been measured for one dye and a number of fiber samples; it conforms to the equation xt^-½ = constant. The speed of penetra tion varies over several orders of magnitude in the samples investigated, including ordinary tire cord and Lilienfeld rayons.

After a very long time at room temperature, and much faster if the temperature is raised sufficiently, the dye also penetrates into the skin. At equilibrium absorption both, skin and core become stained; often, not always, depending on the dye used, the core remains stained to a darker shade than the skin when either sections or fibers are dyed.

Attention is drawn to the striking fact that Bordeaux Extra is an example of a dye which readily penetrates the skin, although it is also a tetrazo dye with almost the same molecular size as Sky Blue FF, which does not.

In sections dyed to equilibrium, the dye readily bleeds off the core on washing with water, whereas the skin remains stained. This provides an easy means to obtain selective staining of the skin with a number of dyes.

The Victoria Blue technique for selective skin dyeing disclosed by Morehead and Sisson is discussed and a much simplified modification of it is recommended.

The differential staining of viscose and cuprammonium rayon with dye mixtures such as Neocarmin W is explained by the absence of a skin in cuprammonium rayon. It is shown that Lilienfeld-process rayon also has a skin exhibiting similar properties towards dyes as ordinary viscose rayon.

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Staining of Viscose Rayon Cross Sections