01 August 2006

Pthalo pigments

The family of pthalocyanine pigments, commonly called pthalos or thalos, come in a number of blue and green shades. They are beautiful, lightfast, transparent, and high in chroma. But I don't use them.

The reason is that they are too bloody strong. Pthalo blue is something like 40 times as strong a tinter as ultramarine blue. That means, for example, that in order to change the color of titanium white by 10%, you would add 40 times as much ultramarine blue as pthalo blue. That sounds like a good thing (efficient!), but in fact it's infuriating. In mixing, it is extraordinarily difficult to add a small enough amount of a pthalo color to get the effect you're looking for. Paint manufacturers reduce this problem somewhat by adding colorless extenders to some pthalo paints, but that only goes so far. Some artists learn to manage with pthalos (they are, in fact, quite popular artist's colors), but I hate trying to work with infinitesimal amounts of paint when trying to make subtle changes to mixtures, so they drive me nuts.

Fortunately, there are good substitutes. Prussian blue is almost exactly the same hue and transparency as a neutral (not green or violet) shade of pthalo blue, but a lot less strong. And viridian is very similar to a neutral pthalo green. So if you have trouble mixing with pthalo colors, try those instead.


Dorothy said...

But isn't real Prussian blue a fugitive pigment? I prefer it too, but I've been led to believe that pthalo is its replacement.

I don't care to use the pthalos, either. I don't even own pthalo green.

David said...


Great to hear from you. I've enjoyed your blog (it would be great if you'd post more often--hint, hint).

Your point regarding Prussian blue is a good one. The ASTM rates Prussian blue as having the highest lightfastness rating (excellent or I). They are usually very reliable for tests of this sort. There is a good discussion of Prussian blue as a watercolor pigment at the excellent Handprint website:


In summary, the author's lightfastness tests indicated that some brands show a slight discoloration within two weeks, but no fading after that. Other brands demonstrate no fading at all.

Generally (but not always) pigments display their worst lightfastness in watercolor and show much better results in heavier media such as oil and acrylic. Overall, I am very comfortable using Prussian blue from good manufacturers in those media. I'd be more careful with watercolor, but it seems that the better brands produce good results in that medium as well. I don't have any in raw pigment form and would be very careful about selecting the company I purchased it from if I did.