There was another medium experiment that I came across on the wetcanvas forum, this one was carried out by Chicago-based artist Kyle Surges (https://www.kylesurges.com/).
He writes: "I recently did a test with a few different mediums to see how much they yellowed by drying in the dark. Then I allowed them to receive indirect sunlight for a few weeks. I was surprised at how well the yellowing can be reversed.
Below are some swatches of titanium white and a tint of ultramarine blue mixed with common drying oils as well as alkyds. Each color swatch was mixed with a generous, but consistent amount of medium throughout. For the last row I allowed the pure medium to form a film on top of the white acrylic gesso. This test was then left to dry in complete darkness for 1.5 years; the results can be seen in the first image below. As you can see, some mediums resisted yellowing much more than others. The only inconsistent outcome was with the pure walnut oil. On it’s own, it didn’t appear to yellow at all. I think this must be an error because both the white and blue swatches containing walnut oil did in fact show yellowing. I believe it is safe to say walnut oil will not yellow as much as linseed oil, but more so than stand oil.
After I photographed the results of these tests samples left in the dark, I allowed them to sit in a north facing window sill for about 1 month where they received indirect light. Then, I photographed them once again with the same camera settings and lighting as before. The second image shows how yellowing can be reversed by exposure to sunlight. The time spent in the light really helped clear up both the paint swatches and oil films. Even the worst examples improved greatly.
Brands of Oil Mediums Used
- Utrecht Alkali Refined Linseed Oil
- M. Graham Alkali Refined Walnut Oil
- Utrecht Stand Oil
- Grumbacher Sun Thickened Linseed Oil
I thought that this test would be insightful here as well.