An .alpha.-indanthrone pigment having improved tinctorial and technological properties is produced by swelling .gamma.-indanthrone in sulfuric acid of from 73 to 80 percent strength by weight at from 20.degree. to 50.degree. C. in the presence of from 0.1 to 20% by weight, based on indanthrone, of a hydroxylammonium salt, a hydrazinium salt, amidosulfonic acid, urea, a salt of urea, formaldehyde or paraformaldehyde or a mixture of these. After precipitation in water which may or may not contain a surfactant, at below 50.degree. C., an .alpha.-indanthrone pigment of high tinctorial strength is obtained, which gives clear and pure colorations and is easily dispersible.
A .delta.-indanthrone blue pigment having an average primary particle diameter in the range of 0.5 .mu.m or less and having a chroma C* value of 20 or greater; a hue angle in the range of 274-283 when colorimetry is performed on a paint film made from a melamine alkyd paint in which the proportion of pigment in the solid portion is 9.6%; and containing .delta.-indanthrone blue pigment and titanium dioxide pigment in a ratio of 1 to 20 by weight.
A composition comprising (a) an indanthrone blue pigment of formula I ##STR1## and (b) a sulfonated indanthrone blue derivative of formula II ##STR2## wherein X.sup..sym. is H.sup..sym. or a group of the formula M.sup.n.sym. /n or N.sup..sym. (R)(R.sub.1)(R.sub.2)(R.sub.3), M is a monovalent, divalent or trivalent metal cation, n is 1, 2 or 3, each of R, R.sub.1, R.sub.2 and R.sub.3 is independently hydrogen, C.sub.1 -C.sub.18 -alkyl, C.sub.5 -C.sub.6 cycloalkyl, phenyl or phenyl which is substituted by C.sub.1 -C.sub.18 alkyl, or R.sub.2 and R.sub.3, together with the linking nitrogen atom, are a pyrrolidine, imidazoline, piperidine, piperazine or morpholine radical, or R.sub.1, R.sub.2 and R.sub.3, together with the linking nitrogen atom, are a pyrrole, pyridine, picoline, pyrazine, quinoline or isoquinoline radical, and m is a value from 0.2 to 3. When used in varnishes, pigment mixtures of this kind are distinguished in particular by increased color strength, improved rheology and high gloss.
