Chapter 4 133 After the development time has been completed the product is rinsed off until the water is clear (~1-2 minutes) and then the hair is either shampooed or an after-color conditioner applied. Product Formulations: Product forms of these coloring products vary greatly but the essential requirements are similar. The product must be stable, both on shelf for several years and for at least 30 minutes after mixing the two components for use. In addition, ideally the product is easy to mix, easy to apply and is not drippy or messy. The majority of the products are lotions (thickened solutions or emulsions), creams (based on fatty alcohols) or gels (based on hydroxyethyl celluloses) depending on the components of the tint and the developer. A more recent innovation in the market has been the introduction of foams where the product before mixing is a liquid and the foam is generated as the liquid is delivered through a pump attachment to the bottle which aerates the product. The perceived advantage of the foam is that it is very easy to apply to the hair and does not drip easily off the head. As a consequence the consumer feels more comfortable to freely walk around the house as the color is developing. It is also claimed that the foam gives a more even coverage of the color. There is a limited market for powdered products, especially in the Far East, where water is added to activate the product. In these powder systems the oxidant used is typically either sodium perborate or sodium percarbonate—both of which dissolve in water, releasing hydrogen peroxide. These products have the advantage of reduced packaging because they do not contain water, which reduces shipping costs. For the lotions, gels and creams, there are two key types of thickening systems: passive thickeners and active thickeners. In a passive thickening system both the tint and developer are thickened by a system such as a fatty alcohol/surfactant mix (Formula 1). On mixing the final mixed product, thickness is just a combination of the two different thickening systems. In an active thickening system, thickeners are added that change the rheology of the product as a