Tips for working with catagory II and III lightfast-rated pigments

Hi Bob –
As you might imagine this topic can have many layers of complexity so will try to address some of the basic issues. I also have never done testing on NR9 – it is not a pigment we use – but can share results from Alizarin Crimson which is generally seen as the weaker between the two. As you intuited the lightfastness for all these colors would be worse in the tint and in fact lightfastness ratings are based on the performance of a specific tint let down with titanium white until you achieve 40% reflectance in the wavelength of maximum absorbency. For a sense of what that looks like, and a description of the lightfastness test procedures, see the following article:
http://www.justpaint.org/alizarin-crimson-now-you-see-it/
and for how to interpret those results based on what is called delta E, see:
http://www.justpaint.org/delta-e/
Many but by no means all lightfastness II and III colors can perform markedly better in the masstone. A classic example would be Alizarin Crimson, and by extension NR9 as well. See the following where we show some actual results:
http://www.justpaint.org/alizarin-crimson-now-you-see-it/
As you can see the deeper masstone in the oils holds up quite well – it actually barely budges and on its own would be considered  a strong LF I. But as a tint there is a major bleaching out. Fanchon Red is similar in this way, but not so much with PY 3, where masstone and tint scored very close to a LF II.

I should also point out in the Alizarin Crimson piece you can see how effective a UV protective Varnish was in preserving what would otherwise be a light watercolor wash. Or see the results of applying a varnish to fugitive dye-based inkjet prints towards the end on the piece on Lightfastness. That said, applying a UV varnish or UV filtering Museum Plexi or Glass, does not always work that effectively. If a pigment is sensitive to light within the visual spectrum then the varnish and glass with have far less impact.

In truth, outside of historical interest, there really are few reasons not to seek out more permanent alternatives that would be safer to use. For example, Pyrrole Reds are rock solid and can replace napthols, while Bismuth Vanadate lines up fairly well against PY3, and finally PR177 is much more permanent that either PR83 or NPR9, but is not a perfect match being a bit cleaner and higher Chroma. However, if wanting to use them, keep them as close to full strength as possible and applied generously, and if aesthetics and finances allow, use a UV protective spray or UV Plexi at the end.

Hope this at least answers some questions.

I have very little to add here other than to mention that in addition to the difference already mentioned, quinacridone red is also a little less transparent and, therefore, lighter in mass tone than either alizarin or rose madder lake. As stated, though, these small differences should not stop one from using the vastly more permanent alternative for anything other than a historical reconstruction.