A process is provided in which a pigment is oxidated with ozone in an aqueous environment in which the mixture of ozone, water and pigment is subjected to a dispersive mixing operation; andor the pH of the mixture is maintained in a range of 6-8, whereby the pigment is self-dispersing and preferably has an acid value of less than 3 \u03bcmolesM2. The reading voltage has such magnitude that the displacement of the polarization of the ferroelectric film is restored to that obtained before reading the multi-valued data by removing the reading voltage. , and a WHSV over the catalyst of from 0.