 The ink identifies the presence of the photocatalyst coating by changing colour upon irradiation of the latter at a rate (usually < 10 min) which provides a measure of the film's activity.For example, it has been established that the rate of change in colour of an "Paii" on commercial self-cleaning glass is directly related to the rate at which the glass is also able to photo-oxidatively mineralise, via reaction (1), the wax-like, natural fatty acid, stearic acid, found in finger prints.By making the dyes in the ink increasing difficult to reduce chemically, for example by using