Gradual hair dyes, also known as metallic or progressive hair dyes, require repeated application to result in gradual darkening of the hair shaft. These products will change the hair color from gray to yellow-brown to black over a period of weeks. There is no control over the fi nal color of the hair, only the depth of color. The hair can only be darkened and not lightened with this technique. The most commonly used gradual hair dyes employ water-soluble lead salts, which are deposited on the hair shaft in the form of oxides, suboxides, and sulfides. The lead is in an inert form, thus gradual hair dyes pose no threat of lead poisoning.
This type of hair coloring is most popular among men who wish to blend their gray hair gradually over time with the surrounding darker hairs. Continued use is necessary to maintain the hair color. Gradual dyes cannot be combined with permanent waving or other hair-coloring techniques. The presence of the lead salts on the hair shaft creates unpredictable results if further chemical processing is undertaken. After prolonged use, gradual hair colorings may weaken the hair shaft and precipitate hair breakage.
