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Yellowing of Oils – Update and New Testing at the 5 Year Mark

10 thoughts on “Yellowing of Oils – Update and New Testing at the 5 Year Mark”

  1. Interesting article! Thank you for this research. Do you suspect any relationship between the poppy oil cracking and its protectiveness against yellowing? Something about how it dries or mixes with the pigment, for example? I was thinking, “Well, the poppy oil looks great!” until that last photo, and I wondered if the the same thing that made it less prone to yellowing is what made it more prone to cracking.

    • Hi Lori –

      A great question and kudos on making the connection since, in a very broad sense, the answer is yes! In fact, just as you wondered, all the less yellowing oils, including even our own Safflower ones, form more brittle and less durable films. They also lose more mass and shrink more as they dry. If interested in how and why, I would recommend our article on the Safflower colors which does reference Poppy as well. In this specific case, we are not precisely sure which of all the variables caused the cracking so we are going to have to look into it further. For now we just wanted to point it out as it surprised us as well.

  2. As for the lighting, it is usually the UV (ultra-violet) exposure that yellows plastics, so I would think it would be the same for paints. Sunlight, fluorescent, and LED all emit UV light. Within the UV spectrum, there are different wavelengths, normally designated A, B, and C. Generally, we expect UVA to affect color perception the most, while UVC tends to accelerate skin damage faster. It would be good to accurately measure the amount of UV actually landing on the paint sample surfaces. (Note that the angle of the lighting also makes a difference, so be sure to keep the light meter’s sensor parallel to the surface, not angled toward the light source.)

    • Hi Pamela –

      My immediate response was just how much I love the questions and observations that people like you share, and the dialogs that ensue from that. You are absolutely right that this type of data would be important to know – and unfortunately, it is something we failed to gather at the time but certainly can go back and get some readings to estimate exposure and have a fuller record. That said, the mechanism for yellowing in plastics versus the type of yellowing being studied with oil paints is different. To just point to some examples, oil paints yellow more in the dark, in a process called Dark Yellowing, and the general degree of yellowing of a drying oil is correlated with the degree of unsaturation of the fatty acids rather than UV exposure, which if anything tends to help reverse any yellowing that accumulated in dark storage. The first article covering the yellowing of oils touches on some of this. While certainly UV has to ultimately play some role in the aging and breakdown of oil paints, just like other materials, and thus possibly contributing to yellowing in the long term, I just am not sure it is doing so within the time scale and conditions that oil paints are typically studied under. But will admit I have not looked deeply into the literature – so I also want to keep an open mind. And if you have any references concerning oils, UV, and yellowing, I would be very very interested!

  3. Hi Sarah

    I find interesting the finding about cracking of poppy oil based white. I have thought for some time about asking, or rather suggesting a test you could make; it would be interesting if you did a test focused on strenght of various oils. Just as you tested various color containg zinc white (i.e. zinc buff, naples yellow reddish, king’s blue here: https://justpaint.org/zinc-oxide-reviewing-the-research/) you could test various pigments (natural inorganic, synthetic organic and inorganic) in various oils (linseed, walnut, poppy, safflower) to see whether they crack. I asked about this at MITRA forums in context of one article at naturalpigments.com: https://www.artcons.udel.edu/mitra/forums/question?QID=874


    • Hi Ivan – While we haven’t done a systematic test as you are suggesting, we have looked at different oils (Safflower and Stand) used with Zinc plus a few other pigments, as well as comparing the oils to each other. But nothing extensive or very broad. In terms of Zinc, preliminary data has shown that Stand Oil might be very promising but we need to see if that holds up long term. And of course, making a paint purely with Stand Oil, much less working with it, is not really ideal or practical. However, if the stand oil was used as an additive or medium, it could perhaps help. We will need to see as we continue looking.

      In general, I would expect oils with fewer unsaturated bonds to do better in terms of the specific type of embrittlement caused by zinc. But over and above that, the different oils already dry with differing amounts of strength and durability caused by the loss of volatile components and the corresponding shrinkage that takes place. Both of those, in turn, are linked to the creation of a more porous film. For some sense of that, see Chart 3 in our past article introducing Williamsburg’s New Safflower Colors. At this point, my own speculation is that the craquelure we saw in the poppy-based film was due to the shrinkage and contraction of the paint more than actual embrittlement. But further testing will be needed, and keep in mind, this was made from pigment and oil only, and a more fully formulated paint would likely perform differently. For now, we simply wanted to draw attention to what we saw.

      • Hi Sarah

        I wonder how is it with potential yellowing/darkening of pure oils? If a liqiud oil, e.g. Williamsburg Alkali refined linseed oil, is stored in a glass bottle (whether clear ar amber toned) in shadow or dark place, can it darken too?

        • Hello Ivan,

          Thank you for your great question. The effect of dark storage on wet, bottled linseed oil has not been studied by us, nor elsewhere, to my knowledge. We have not observed dark yellowing with any of our Williamsburg wet products. In general, the effect of day and UV light is that it lightens/bleaches linseed oil, and the effect of polymerizing linseed oil with driers or by heating it, yellows it.


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