JPS618854B

A process for the manufacture of a red pigment from perylene-3,4,9,10-tetracarboxylic acid N,N'-bis-methylimide which has been obtained by methylating the corresponding diimide or by fusing naphthalic acid N-methylimide with an alkali metal hydroxide which comprises vatting a suspension of N,N'-bis-methylimide dye with 0.5-1.5 times by weight of an alkali metal dithionite in the presence of up to 4 times by weight of an alkali metal hydroxide relative to the quantity of the dithionite, at a temperature of from room temperature to 100 DEG C., to obtain the corresponding leuco compound, cooling the resulting solution to 20 DEG -40 DEG C., isolating and washing the leuco compound with fresh vatting solution until the solution is virtually colorless and clean and then oxidizing the leuco compound.