Many of my paintings are done in thick impasto oil paint using stand oil. I have done some testing (2 years ago) using different stand oils in impasto oil paints. I have noticed the oil rises to the surface and yellows slightly. When the dried oil is cracked open the paint is a bright white inside. I have tried poppy oil and even though that rises to the surface it does not yellow, but I noticed the paint is more brittle than those done with linseed/walnut stand oil. So my question is how to prevent the oil rising to the surface? Would bees wax help, or possibly add other issues.
Thank you for your response. It seems to confirm what I suspected as I used a palette knife. I did a test sample from 2 years ago with several pigments in oil on an acrylic ground/canvas. I found that with thin palette knife applications there was no oil accumulation on the paint surface (less gloss), whereas with quite thick impasto there was oil dried on the surface (glossy and more yellowing). It appeared that when thicker paint was applied, there was a correlation to more oil rising to the surface.
I wonder if the acrylic ground could absorb excess oil from the thinner paint, whereas with a thicker application the excess oil could not be absorbed and consequently rose to the surface. When this thick white paint (titanium and 2 flake whites) was broken open it was bright white on the interior, and either slightly yellow on the surface(titanium) or very yellow (both flake whites).
I revisited a post by Sarah Sands on the yellowing of oils, https://justpaint.org/on-the-yellowing-of-oils/ . Golden Paint found that a small addition of beeswax (about 3%) prevented the oil separating. I will see if this works.
On another thought you are probably correct in saying it has something to do with physics. On a similar but unrelated subject. When a concrete slab is poured, it appears rough at first, but after it is trowled over several times the water rises to the surface. I think the act of moving a smooth blade over the surface causes the denser particles to settle, allowing the less denser vehicle to rise (in this case water or oil).
The problem is likely that your paint is too fat (too much greasiness/oil as compared to pigment load), too thinned with solvent (which immediately facilitates the accumulation of surplus oil at the surface of the paint}, or a combination of the two. There should no super-surplus of oil at the surface of any application of oil paint. Having less yellowing, but poorer paint film formers, like poppy seed oil oozing to the surface does not solve the problem, but just makes a lateral move to another issues.
On an unrelated, but tangentially related issue, I have applied the same paint, without additional medium, using a brush and a palette knife and the paint applied with the knife is both glossier but also greasier than that applied with a brush. I have also seen yellowing associated with knife applied paints where none was observed with the brush applied films. None of this was scientific and I would not swear in court, but I think that there is far more going on in terms of physics and rheology with paint manipulation that we might intuitively think.
This may seem tangential, but I remember that when I worked with Dr. Leslie Carlyle, she observed what appeared to be transparent, seemingly intentionally applied interlayers between layers of paint and a smooth mylar substrate. We knew that no transparent, unpigmented layer could have been applied because we made and scrupulously documented the stratigraphy of those layers. It appears that the smoothness of the mylar substrate influenced the accumulation of binding media creating the appearance of an applied unpigmented interlayer.
This is not directly related to the discussion at hand, but it does speak to the idea that media can accumulate at the interface between certain physical phenomenon.
I totally agree, but remember that in the situation that I quote about the cross-sections taken from the Carlyle research, the faux interlayer is below the paint layer and not at the top. Clearly there is more going than settling in this instance, and more complicated physical dynamics are going on.
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