Hair dyeing
The dyeing of hair fibres offers a singular challenge to the colourist, and several factors set this technology quite apart from other dyeing procedures. The option, indeed the necessity, of periodic reapplication is notable. This is less to restore faded colour than to apply colour for the first time to new growth hair. This constant development of new substrate is perhaps the most peculiar feature of this dyeing category.
Repeated application does, however, allow the industry to use dyes with lower fastness than in other categories. Hair fibres as a dyeing substrate are rather complex. The fibres themselves are not uniform cross-sectionally and even, to some extent, from root to tip. The outer layer, or cuticle, consists of overlapping cells, generally five to seven layers, while the inner cortex has longitudinally arranged keratinous fibrils within elongated corticular cells. It is the cortex which contains the natural melanin colorant. New growth hair near the root has a tight corticular structure while the weathered and treated tip ends are far more porous, thus making uniform dyeing quite challenging under the mild conditions of product use.
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