Medium Yellowing Test by Ben Sones

I recently came across this great experiment that was shared on the Artist’s Network Wetcanvas forum. It was an experiment carried out and shared by artist Ben Sones:

One of my medium yellowing tests–this one has been stored in the light (on the wall of my studio, where it receives daily north light) for about three years. These are swatches of straight medium on a piece of Arches Huile (sized) oil painting paper:

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The mediums are, starting on the top left:

Gamblin Galkyd Lite

Gamblin Galkyd

Gamblin Solvent Free Fluid

Winsor & Newton Liquin Original

Winsor & Newton Liquin Fine Detail

Graham’s Walnut Alkyd

Winsor & Newton Damar

Groves Amber Medium

Studio Products’ Mastic

Rublev Oleoresgel

Gamblin Cold Pressed Linseed Oil

Graham’s Walnut Oil

Winsor & Newton Stand Oil

Holbein Sun-Thickened Linseed Oil

Blue Ridge Sun-Thickened Linseed Oil

Studio Products’ Black Oil

Holbein Sun-Thickened Poppy Oil

Gamblin Stand Oil

Mixture: 50% W&N Stand Oil + 50% Graham’s Walnut Alkyd

Mixture: 50% W&N Stand Oil + 50% Gamblin Solvent Free Fluid

Winsor & Newton Drying Linseed Oil (linseed + manganese drier)

Mixture: 50% W&N Drying Linseed Oil + 50% W&N Stand Oil

Some notes:

  1. When I first made the sheet, the Groves Amber was, by far, the darkest swatch on the sheet. Over time, it has stayed basically the same, while some of the other swatches have darkened a bit, particularly the sun oils, the solvent-free alkyds, and the black oil. The black oil has actually almost “caught up” with the amber, but not quite. They look similar in this photo (it’s hard to eliminate all glare and provide completely even lighting), but in person, the amber is definitely darker. Just a bit, though.

  2. However! One thing this test doesn’t highlight is the fact that, like paints, different mediums actually have different “tinting” strengths. So the colors you see here are really only half the story. For example, on this sheet, the sun thickened oils look like they are almost as dark yellow as the amber medium. In practice, though, the amber medium has a considerably higher “tinting strength.” If you add it to some titanium white paint, it turns the paint noticeably oranger. If you add the same amount of sun thickened oil to some titanium white paint, it sort of disappears into the paint and doesn’t affect the color much at all. When stored in the light, at least–in dark storage, sun-thickened oil will yellow paint noticeably.

  3. Linseed oil gets more of a bad rap for yellowing than it deserves. Probably because many people who test for yellowing store their samples in the dark. Dark storage aging is a pointless test, IMHO (though I do have a similar sheet that I keep in a drawer). Yes, linseed oil-based mediums yellow considerably in the dark. But that doesn’t really represent the typical display conditions for a painting, and the yellowing caused by dark storage is reversible. So it’s not a thing that I worry about. In my light-storage tests, linseed oil actually tends to yellow a little bit less than walnut oil, over time.

  4. Graham’s Walnut alkyd has a bit of a bad rap for yellowing, but in my tests, it is almost indistinguishable from straight walnut oil in terms of yellowing. The alkyd is a touch darker, but it’s close. Both yellow a fair amount, but as with sun-thickened linseed oil, it’s not particularly noticeable when the mediums are mixed into paint.

  5. Alkyds generally seem pretty good when it comes to yellowing. The “solvent free” variations (Graham’s Walnut and Gamblin’s Solvent Free Fluid) tend to yellow more than the traditional alkyd mediums. Liquin Original was the best performer, yellowing almost not at all.

  6. The 50/50 mix of a solvent-free alkyd with stand oil (Graham’s Walnut Alkyd + Stand Oil is a favorite mixed medium of mine) yellows a little bit on this sheet, but is basically non-yellowing when mixed into white paint–even when stored in the dark. This mixture performs a lot like a straight sun thickened linseed oil–viscous and leveling, but with more flow than straight stand oil–and generally dries overnight.

  7. Top five least-yellowing: Damar is easily the least yellowed sample on the sheet, in that it hasn’t yellowed at all (in light storage, or in dark storage). The mastic hasn’t yellowed, either, but was very slightly more yellow to begin with, right out of the bottle. It places second. Winsor & Newton’s Stand Oil comes in third, followed very closely by Liquin Original, followed by Gamblin’s Stand Oil.

  8. Stand oil is amazing. The Winsor & Newton stand sample has yellowed basically not at all–even on my dark storage sheet. The Gamblin Stand Oil sample is a touch yellower; Gamblin’s stand oil seems to be a lower quality product in general, and I wouldn’t recommend it. The bottle that I have was clear when I bought it, but now is full of bits of floating mucilage that have separated out of the oil over time (see my review for Blick’s for more details). My bottle of W&N Stand, despite being years old, is still crystal clear.

  9. The only mediums on this sheet that I would specifically recommend against are Black Oil and Groves Amber. Both are just too dark and yellow/orange, even when mixed into paint. The sun thickened linseed oils are kind of borderline–personally, I used to use sun thickened oil a lot, but this test actually convinced me to start using stand oil instead.

Hopefully that will be helpful to someone.

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Thanks for the intel-report from the frontline! There’s so much to worry about in art that these intelligence briefings are always highly useful.

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Thanks for that, it’s really helpful and eases my mind a little considering I’ll only touch linseed, walnut and the walnut alkyd due to health reasons.

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Great experiment. One thing to bring up in case anyone has any thoughts: I have never been able to find a chemical explanation for either the reversible yellowing that happens in the dark or “long term” yellowing. To my knowledge (and I have seen it stated explicitly in credible sources) we do not now why oil/paint yellows at the molecular level.

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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.

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Excellent! Many thanks for sharing your ideas and research.

I wonder where the limit to how yellow something will get, if after 5 years it has achieved it’s maximum yellow threshold, or if we can extrapolate this through molecular analysis. I mean it can’t continue to yellow its way to cad lemon in 5000 years, even kept in a drawer.

Then could there be any inherent constituent part of a different paint which would act as a catalyst when mixed with titanium white and increase or speed the yellowing effect, as this could potentially warm what was meant to be a cool grey for example. But even so, as you’ve found - it is unlikely to be worth worrying about :slight_smile:

I even used a lefranc & bourgeois ageing varnish recently to artificially produce a unified yellowing effect in a new painting (giving that golden glow) - I enjoy yellowing in certain situations!

We have some long-running “tests” in the form of very old paintings–stuff from the early Renaissance has been around for ~500 years. And while many old paintings have a strong yellow cast to them, this is almost always caused by discolored varnish. Varnish resins discolor over time, and they also collect dirt and atmospheric pollutants that become permanently embedded in the film. (This is why it’s a good idea to varnish your paintings–dirty varnish can be removed, but dirt embedded in paint generally cannot). But strip away the yellowed varnish, and you typically reveal colors that are still clear.

The recent cleaning of Van Eyck’s Ghent Altarpiece is a good example. Here’s a before and after–check out the whites and blues in the highlights of the armor. The binder used throughout this painting (even in the whites) is thought to be linseed oil.

This is another good one–a small varnish removal test they made on the center panel of the Ghent altarpiece at the start of the cleaning process:

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Thank you SOOOO much for your contributions here @bsones. It’s truly a good bit of invaluable info for today’s oil painters. I sincerely appreciate your sharing the fruits of your investigations with us. Looking forward to exploring much more in the coming New Year!!! Hope you all have a wonderful 2020!

:heart:

Thanks for posting this. I admit that I have been scared of Liquin because I’ve seen examples of it turning very dark in the bottle over time (see link below)…also, I’ve had bad experiences with Galkyd darkening and delaminating. But this test is quite persuasive. Why do you think Liquin darkens in a bottle, but stays so clear when painted out? I use a lot of medium, so I’m always on the hunt for the best non-darkening option.

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I think it’s mainly a volume/depth thing. Older bottles do seem darker though, which may just be due to a settling of the contents (it is an emulsion).

I spotted this paper palette in my bin and it happened to have some globs of pure liquin on the underside (the colour is from paint on the reverse which unintentionally helps you see the transparency of the liquin on top):-


1 - is a big fat glob of liquin which after three days has dried to a blister like brown bubble (still liquid inside). this is stood proud 2mm off the palette at it’s full peak.
2 - is a about 1mm high of fully dried liquin
3 - is about 0.5mm tall of dry liquin noticably lighter in colour and more transparent.

it would be unusual to use liquin in any of these impasto quantities - as it would also make the paint very runny, but in my opinion even if you did - the chroma of the paint would likely overpower whatever faint yellowing of the liquin could occur.

I’ve used neat liquin in a thin coat over paint as a varnish without any problem so far. I would say dammar is actually more yellowing (comparing pure titanium white areas of different paintings coated with either).

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Thanks for documenting that, Martin! I would never use it that thickly, but I do paint in many layers, so I’m concerned that the combined effect of the layered liquin might be a problem.

Thanks for sharing that here Paul! For those not on Facebook, Ben addressed the issue of observed, significant aggregate discoloration on George O’Hanlon’s Best Practices: “It’s because when you look at Liquin in a bottle, you are looking through at least an inch-thick mass of Liquin, whereas a film of Liquin on a support (even if that film is pure Liquin) is something like 1/10,000 of an inch thick. And the tiny amount of Liquin that you mix into your paint is even less than that. Liquin does have a brownish color, but that color is so slight that it’s not noticeable at all in the amounts used in painting. Sort of like how linseed oil looks yellow in the bottle, but clear if you brush it out or add it in small amounts to paint.” -Ben Sones.

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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.

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