This invention relates to a simple method for enhancing the weak colors displayed by certain iridescent pigments, so as to provide more vivid colors which are useful in printing, copying, and recording systems. This invention further relates to a novel method for recording indicia by means of iridescent colors. This invention still further relates to a novel method for forming intensely colored indicia from weakly colored recording fluids. This invention still further relates to a new series of novel printing inks.
In recent years there has appeared a new type of nacreous pigment which exhibits a weak iridescence when dispersed in liquid. These pigments are known commercially as "iridescent pigments." From an optics view point, they comprise a base substrate of essentially transparent (colorless) plate-like particles, with at least one overcoating of an essentially transparent material having a refractive index greater than that of the base substrate. They are prepared commercially by overcoating a micaceous substrate with an extremely thin film of a material such as titanium dioxide or zirconium dioxide, having a refractive index which is substantially higher than that of the base substrate and which is usually greater than about 2.0. Such pigments are disclosed in U.S. Pat. Nos. 3,071,482 and 3,087,828 and are available from Mearl Corp., 41 East 42nd St., New York, N.Y. 10017, under the name "Flamenco Nacreous Pigments."
In the course of another investigation, the present applicant examined these iridescent pigments for possible use in a recording paper. They displayed very weak colors which gave almost no contrast with normal backgrounds and, therefore, they had no practical value for that purpose. As graphite is strongly colored and has a plate-like structure similar to mica, an attempt was made to deposit an iridescent interference layer, the so-called Nobili rings, on graphite flakes, but the Nobili process did not work on graphite. (The Nobili rings and process are discussed in David Fishlock, Metal Colouring, Robert Draper, Ltd, Teddington, 1962.)