Patent ID: 8313576

Claim:
A method of preparing a pigment comprising the steps of: providing a mixture including: a pigment, or a mixed crystal of a pigment comprising at least 50 wt % of a total weight of pigment in the mixture, selected from the group of pigments consisting of C.I. Pigment Violet 23, C.I. Pigment Red 146, C.I. Pigment Red 176, C.I. Pigment Red 177, C.I. Pigment Violet 19, and C.I. Pigment Orange 13; an alkyl nitrile; and at least one acid selected from the group consisting of sulfuric acid and chlorosulfuric acid, wherein the at least one acid is present in the mixture in an amount more than 2 wt % based on the total weight of the pigment in the mixture; heating the mixture for more than 2 hours at a temperature of at least 70° C.; and filtering the mixture and washing a filtrand obtained by the step of filtering with a washing liquid to remove residual acid.