22 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS measurement has its own advantages and disadvantages. Even a skilled colourist has difficulty in reproducing his results, although he can dis- criminate well. The tristimulus colorimeter produces X, Y, Z (or some similar set of coordinates). Its accuracy relies on the three filters provided and colours can only be described as if viewed under one light source. The spectrophotometer is probably more accurate than the other methods for pale colours, but the curve obtained is difficult to interpret and manipulate. In the textile field, the current practice is to use all three methods, the spectrophotometer in the laboratory to establish reference data, and the colourist and the colorimeter in the dyehouse. Applications of colour measurement The major developments have taken place in three areas: (a) Colour atlases. (b) Colour matching. (c) Colour tolerances. Colour atlases have been established over many years based on two main principles. Either the amount of colourant or the amount of per- ceived colour is regularly changed. A full discussion is given in Judd (8). Examples of the former are the Ostwald, NuHue and Colour Harmony Manual, whereas the Munsell chart is based on visual differences. This has been extended recently, .for example, by Bell et al (12), who have set up an atlas of sewing cottons that are approximately equally spaced in the planes of colour space. With regard to colour matching, Park and Sterns (13) in 1941 pointed out that if a transform of the reflectance could be found that was additive, the amounts of dyestuff necessary to match a colour could be found by calculation. Apart from one or two exceptions, notably the work of Duncan (14) and further work by Sterns (15), this method received little attention until a few years ago. Then Alderson and Atherton (1t3) showed that general purpose digital computers could be used to predict colour matches. Also, Davidson and Hemmendinger (17) announced the develop- ment of the Comic computer to calculate matches. Now most major dye manufacturers, and a number of dyers, have developed their own methods of match prediction. Much work has been done on setting up equations for measuring colour tolerance between similar colours. These are used for quality control and establishing the goodness of a match.

HAIR COLOURANTS -- A PRACTICAL APPROACH 23 MECHANISM OF DYEING The introduction of a dye molecule into hair (or any substrate) involves three distinct processes. These are :-- (1) Diffusion of the dye from the bulk solution to the hair/solution interface, (2) adsorption of the dye on the hair surface, and (3) penetration/diffusion of the dye into the swollen hair where the dye molecules may or may not subsequently participate in reactions with the hair or with themselves. Diffusion in water has been extensively studied. It is known that the higher the concentration of dye in solution, the more dye will reach the hair in a given time. The rate of movement is determined by the size of the dye molecule. The larger the cross-sectional area of the dyestuff, the slower it diffuses. Viscosity is also important the more viscous the dye solution, the slower is the movement of the dye molecules. There is still much to be learned about the processes of dye uptake on human hair. More is known, however, about the dyeing of wool and so we find ourselves in the not unusual position of working by analogy. However, there are factors such as dyeing time and temperature which make comparison difficult. The textile dyer often dyes near boiling point, a temperature which would, to say the least, be uncomfortable on the human scalp. Temperature is important since it is one of the major factors controlling the movement of the particles in solution. The higher the temperature, the more rapid is the frequency with which the dye molecules strike the hair surface. The number of dye molecules adsorbed on the hair surface depends on the collision number, the zeta potential of the hair and the charge carried by the dye molecule. From the hair surface/solution interface the dye diffuses into the hair. It has been assumed that the hair consisted of a sieve which prevented access of molecules greater than 6 A. This idea arose from the observations of Speakman on the swelling of wool in various alcohols (18). He observed that when wool was immersed in water, methanol or ethanol, the strength was decreased by almost the same amount, whereas in alcohols of molecular weight greater than propanol, no decrease was observed. He concluded, therefore, that molecules of a radius greater than propanol were incapable of penetrating the fibre rapidly. It has therefore been postulated that in a dry, unswollen fibre, there are pores of approximately 6 A. By observing the elastic properties in mixtures of methanol and octanol, it was calculated