There may be several subvariants of this pigment, which are mentioned in this article, which mentions a "standard grade" as well as a redder kind that is more opaque. We've also encountered subtypes with differing lightfastness such as PO34 RL01 (Lightfastness V, very poor) and RL70, which has better but still bad lightfastness at ASTM III. For artistic work, the lightfastness of this orange is pretty terrible in tints (possibly as low as 4 on the BWS). For that reason alone we don’t feel it is advisable to use it. However there are other reasons for avoiding its use.
According to Monona Rossol's work, this is a very concerning color from a health standpoint and will likely metabolize into a carcinogen. We advise all artists to research benzidine in artist pigments for toxicity. Surprisingly this color is used by at least three major paintmakers in oil as single-pigment color.
Depending on the supplier the pigment ranges from inadequate lightfastness to poor, and even in the better ratings it's not that good. It's also found in blends by about six paintmakers and sometimes it is paired with other pigments that also have lightfastness issues. Artiscreation notes that Lightfastness may be slightly improved in redder versions. However the performance is reported to be not very good. Unfortunately several paintmakers use this color in their paints.
