HAIR COLORING--MODERN FORMULATION CONSIDERATIONS 299 unfortunately also to the skin. Scalp staining can be reduced somewhat by pH control e.g., DC Red 19 will stain skin most at pH 8. It dyes hair quite well at any pH though best in alkaline solutions above pH 8. CHOOSING A DYE COMBINATION When choosing a combination of dyes, the shade they give to the hair is often the last of many qualities to be considered. It is elementary but often forgotten that a hair dye shade mixture must react uniformly when subjected to the following common treatmen rs: 1. Each of the components must come off at the same rate when the hair is washed. Otherwise, a brown shade made from primary colors may turn green if the red component should wash out first. This problem is fm ther magnified by the di•culty of choosing a combination which will wash off damaged hair at the same rate as flora normal hair. Figure 1 shows our adaptation of the A.A.T.C.C. Launder-ometer, used to standardize dyeing and washing of small samples of hair and wool. Rotating •t 45 r.p.m. to allow tmnbling, it is simply a block of wood to which standard jar caps have been attached upsidedown, allowing various size jars to be mounted and dismounted quickly for timed experiments. It allows multiple experiments to be carried out reproducibly and easily. 2. Similarly, the components of a dye mixture must fade uniformly in daylight so that the shade does not change character. Light-fastness ratings to be found in the literature are good guides for preliminary work, but permanent dye formulations must be checked in a Fadeometer or pre-

300 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ferably in actual use before being sold. The inclusion of substa•/ive UV absorbers can help such formulations. 3. Shades must not change character when applied to bleached or per- manent waved hair or to hair treated with cationic creme rinses, or when followed by such treatments. It is currently accepted that they may ap- pear stronger or weaker due to these common processes, but they should not "turn" green or red. Preferably, they should show no change at all of course. 4. The hair dye should not sublime off when subjected to the dry heat of electric hair driers. Anthraquinone dyes occasionally do this. The salon operator will be quite angry if some of your product sublimes into her machine and a freshly bleached blonde then sits under it, emerging with polka dots all over her coiffure! 5. If vou use solvents in your p•oduct, it is wise to use only dyes with similar solubility characteristics. The shade given by solvent formulations can be altered quite radically if the ultimate user changes the partition ratio by diluting with water. 6. Finally, the idealproduct should appear in the bottle as it will on the hair, or at least as you want the customer to think it will appear on the hair! A "Golden Brown" should preferably not appear black in the bottle, nor a "Steel Grey" purple. SOLVENTS In this day of polyfunctional cosmetics, hair dyes must not only contain coloring matter, but also softeners or hardeners, antistatic agents, sheen producers, curling or straightening agents, and if possible prevent dandruff and baldness as well! We chemists keep each of these agents in separate bottles in our supply room, but when management requests a particular combination, we sometimes need cosolvents to prevent the final product from resembling that famous seven-layered pousse car& Solvents are used in hair dyes for several other reasons. They make possible greater dye concentrations, thereby increasing the hair color depth which depends in part on the law of mass action. One of the more ingenious uses of solvents is to take advantage of the solubility partition ratio exhibited by some dyes between the solvent and water. To make full use of this trick, you must have a solvent with the following properties: 1. It must not be too soluble in water--about 5 per cent seems ideal. 2. It must be a much better solvent for the dye than water is. 3. It must sorb onto the hair out of water solutions or emulsions. When this is the case [as shown by Karrholm and Lindberg (14) for amyl and butyl alcohols, and for high molecular weight aldehydes by Widner, Buehler and Roesti (15) ], dyeing rates can be increased up to ten times or more. The