Hey, that’s one of my test sheets! It’s still pinned to the bulletin board in my studio, and some of the samples have continued to change. I keep it around out of academic interest, though I’ve decided that as a practical test, it has some problems. For one, the mediums are applied in a manner that is not consistent with how they are actually used. Medium is typically mixed into paint, or in some instances they are used to oil out the painting and are then painted into (in which case you are mixing them with paint as you work). So applying them by themselves is already in a situation that doesn’t match any real world use case. Additionally, because mediums can have different “tinting strengths,” so to speak, seeing how dark they are on their own is less helpful than you might think. A medium might appear quite yellow, but that doesn’t necessarily mean that it will yellow paint to any noticeable extent. Additionally, because all of these mediums have different viscosities, some of those sample swatches are thicker than others. There’s just no avoiding it. So shortly after making that Wetcanvas post, I started a new test, which I think is more useful in a practical sense (more on that below).
That said, those samples have changed a bit more, and the rankings today are a bit different. Damar is still the lightest sample–it’s completely clear, with no yellowing. Second place now belongs to Liquin, which has not yellowed any further in the past three years. Third place is a tie, between mastic (which has taken on a very faint yellow tinge), cold-pressed linseed oil (no further change), and the stand oils (which seem to have sort of “caught up” to regular linseed oil). Fourth place goes to Winsor & Newton’s Drying Linseed Oil, which is linseed oil with a manganese drier added. It is only very slightly more yellow than regular linseed oil and stand oil.
Even though I think this test is flawed, I find these ongoing changes interesting, for a few reasons. People are often cautioned that metallic driers cause considerable yellowing/darkening, but that doesn’t seem to be the case at all. At least within the duration of this test, though research indicates that metallic driers have a catalytic effect on paint only during the initial oxidation phase, which means that the drier in this sample should be effectively inert at this point.
Painters are also commonly told that stand oil yellows far less than cold-pressed linseed oil, but that doesn’t seem to be true, either. It certainly yellows more slowly than regular linseed oil, but that may simply be a product of the fact that it oxidizes more slowly (stand oil is a very slow drier). Given enough time, it seems to catch up to regular linseed oil in terms of yellowness. Part of me wonders if the same might be true, in the other direction, with the sun-thickened linseed oils. Those samples are quite yellow, but could that simply be because they are such fast driers? Will the regular linseed oil (and stand oil) catch up to them eventually? Maybe. Jury’s still out.
Anyway, I started a new mediums test in 2017, as a sort of addendum to a white test I was doing–comparing different brands of titanium white to see which ones yellowed the most. You see a lot of white-yellowing tests on the Internet. Most of them are problematic. For one, they tend to get stored in drawers, in the dark. All oil paint yellows in the dark, to varying degrees. Linseed oil paints yellow a lot. This yellowing is completely temporary, though–with enough exposure to light (even regular room light–doesn’t have to be direct UV), it will revert entirely. But it can take a while, and I’m not convinced that all the people doing these tests gave their samples adequate time to bleach out before photographing their results. There are some suspiciously dark yellowing tests out there–way darker than anything I’ve ever seen in my own testing, even using the same paints.
Additionally, a lot of those tests feature thick swatches of paint applied with a knife. Drawing down paint with a knife has a tendency to pull oil to the surface (I have a source for that, in some conservation article somewhere, which I can try to track down again if anyone wants it). Which means: more yellowing. But not yellowing that you would get in the normal process of painting. Well, unless you paint with a knife.
So I figured for my test, I’d apply swatches of paint with a brush, in two layers, to a surface that I’d actually use for painting (in this case, lead-primed linen that has been toned with some raw umber). And then I’d keep it in the light, on a shelf in my studio. And the results, 2+ years in, are a series of paint samples that all look practically identical. There are differences between them, but they are incredibly subtle. I wouldn’t describe any of them as “yellowed.”
Then I did a similar test with mediums. I chose a single titanium white (Old Holland), and painted a swatch of unmodified paint, and then six other swatches of that same white with various mediums added in an approximate ratio of 5%. And I must stress “approximate”–I don’t have the equipment for scientific precision here. But that’s enough medium to loosen up the paint, and is roughly the amount that I would use while painting, so. This is a smaller test, limited to mediums that I’d actually use: Sun-thickened linseed oil (Holbein), sun-thickened poppy oil (Holbein), a 1:1 mix of sun-thickened linseed and cold-pressed linseed, Graham’s Walnut Alkyd, Gamblin’s Solvent-Free Gel, and Liquin.
Note that some of these yellow considerably more than others in my medium-only yellowing test. But when added to paint, the results are very similar to my white test: a row of samples that all look basically identical. No noticeable yellowing, at all, in any of the samples. And there’s a swatch of Old Holland cremnitz white on the end (no medium), just because I had a square left over and was curious. It’s more transparent than the titanium swatches, but not any more yellow.
My takeaway is two-fold: 1) test conditions matter, and 2) people spend way too much time worrying about yellowing.