Traditional lithographic ink is oil based but differs from oil paint in that the ink must be ground to the very finest sort of grain and the most complete dispersion possible, by being repeatedly put through powerful and accurately adjusted roller mills. The consistency requirements differ from those of oil colors in that print making inks must be capable of depositing on the paper a smooth, level, uniform stain rather than a thick painty layer. In order to meet these requirements the ink makers produce a very stiff paste, capable of being spread or rolled out to a level flat film on the inking roller, and having just the necessary degree of snap or sticky tackiness, combined with certain oiliness. Recently, water-based lithographic inks have been developed specifically for use in the classroom and unventilated areas. Relief printing inks can also be either oil or water based.
Water based inks are generally easier to clean up and emit less toxic fumes.