Warning: this is deep in the weeds for any artist for morning, so if you're reading this before noon, make sure your coffee is close!
In my last post I showed you a "study" I put together to increase my knowledge of my blue pigments and various mediums. Update on that soon. At the end, I mentioned the post by Carol McIntyre that shows something like I've done in the above photo to help me judge the relative temperatures of cobalt and ultramarine blues. On the first row, squares of cobalt are surrounded by ultramarine, and on row 2 it is the reverse. I used Vasari paints in column 1 and Gamblin in column 2.
In Carol's post, she asks readers to tell her which blue jumps forward to them. Which does to you? I am almost firmly convinced there is no right or wrong, only a bias each of us has for feeling that red or yellow is warmer. Cobalt leans more green (therefore has more yellow) and Ultramarine leans red, so if you think red is warmer, feel it is warmer, ultramarine will logically seem warmer to you, and the reverse is true. Gamblin's site also states that Ultramarine is warm. I have run into many artists I respect that think the opposite is true, but there are arguments on both sides. I have used Ultramarine as a cool blue years ago, and switched to using it as my warm blue a while back. Based on this study, though, and several other factors, it's now migrated back to the cool side of my palette.
It is important to note in this that all color is relative, and compared to another blue, ultramarine may be warm, but as compared to cobalt, my current assessment is that it is cooler. There is a little variation in intensity of the colors in my study, which I did my best to avoid, but the cobalt advanced to me more in Carol's post and here. That surprised me, but it stands to reason (and I struggled with that until I saw Carol's study and became a believer). As we know, yellow drops out first as colors recede, and we're taught that warm colors advance and cool colors recede. John Sloan, in his book On Drawing and Painting, states that "The most projecting point on the spectrum is yellow-orange; the most receding, blue-purple." So the yellow in cobalt would mean that it advances over ultramarine, which has less yellow and more red (feeling that blue-purple thing). Additionally, why is it that I would use ultramarine for the intense blue of the sky above, but cobalt closer to the horizon (where it seems all things go to be warm)?
I know there are other points on both sides out there (I've put links to three articles on the subject below), and I'd love to hear any you have for the sake of completeness. For now, through this and my other study, I've determined that I want to push cobalt off my palette for the time being, subbing in Cerulean (a much more obviously yellow-blue) like my friend, David Boyd, and see where that takes me. I still have many tubes of cobalt and it's too pricey a color to go to waste, but it'll keep.
To further confuse you, find three interesting articles on the subject:
1. Sharon Hick's interesting take here
2. and Bob Bohr's article here
3. and Lisa Marder of ThoughtCo.'s writeup here
*I'll get an update on the study of my blue pigments and mediums in my next post--too much to cover in one post!