Crystalline modification of a water-insoluble monoazo dye

A process to produce a water insoluble monoazo dye of the formula I

• having a crystal modification characterized by an X-ray diffraction pattern (CuKα) showing one strong peak at a diffraction angle (2θ) of 11.1±0.1° and also four intermediate peaks at 8.1±0.1°, 21.8±0.1°, 23.1±0.1° and 25.2±0.1°, which comprises synthesizing by diazotizing 6-chloro-2,4-dinitroaniline, and then carrying out a coupling reaction with 3-(N,N-diallyl)amino-4-methoxyacetanilide coupler in an aqueous medium at a temperature of 0 to 15° C. from 0.5 to 15 hours
• thereby forming the β-form and subjecting said β-form to further treatment which comprises
• 1)dispersing the β-form in an aqueous medium optionally in the presence of a dispersing agent then stirring at a temperature of 60 to 130° C. for 0.5 to 30 hours or
• 2)dispersing the β-form in an organic solvent and then stirring at a temperature of 15 to 100° C. for 0.5 to 10 hours.