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QoR Lightfastness Testing Update

10 thoughts on “QoR Lightfastness Testing Update”

  1. It’s interesting that your blue shade of PV23 performs better in your watercolors than the ASTM rating of the blue shade in PV23 in oil paint (which is ASTM II, as far as I can see). How can that be? The pigment content in the paint must be at a lower concentration than in the oil paint tested, no?
    (As far as I know the concentration of pigment in the paint in practice generally makes a big difference on the lightfastness of the paint).
    I would like to be able to test the paint I make in the studio sometimes, and this makes me want to do that even more. I tried finding som lightfastness strips around here in Denmark, but haven’t been able to so far, unfortunately.

    Reply
    • Hi Sebastian – We agree there is more to be explored here. Part of the problem is that most of the ASTM ratings trace their origins back to testing done in 1977 and often using pigments from a single supplier. Since then, unfortunately, little in the way of retesting has happened. We are coming across these issue with QoR Watercolors and Williamsburg Oils because their introduction to the market, as well as using quite a few colors currently listed as unrated, prompted us to undertake the type of broad testing you see being reported on. And which happens to also be the first of its kind in literally decades. That said, results such as these is definitely making us take a fresh look at many issues, with Dioxazine Purple being one of them. As new rounds of testing get underway, we will be sure to publish results in a future Just Paint. But realize these things can take time to be completed, giving the complexities involved, so it might be a couple of years before those results are finalized.

      As for the question about how one could have different results in Watercolors vs Oils or Acrylics, the issue of pigment loading is somewhat neutralized since all the tests require diluting the pigment to a similar degree – what is known as a 40% reflectance. However one generally would expect watercolors to be more vulnerable simply because the pigments are far more exposed to the elements and especially UV without the benefit of a binder encasing it. So, that does add to the conundrum that needs to be looked into. For now, at least, the testing of the Dioxazine Purple in QoR looks quite solid.

      Reply
  2. Hi Sarah, Thanks to Mark Golden, to you, and the staff at Golden Paints for your extensive materials, and methods research and education program. It’s extremely helpful, and very much appreciated! I’m wondering if you have continued testing the lightfastness of pigments since the article above?, and if you have, where I might be able to see those results? As you know, I have a current particular interest in PR 264, and I’m also wondering about any further results for PV 23. I have a PV 23 Dioxazine oil paint from another paint maker that insists all their pigments rate at least 7 out of 8 on the wool scale. I’m also wondering if your test results have resulted in ASTM pigment lightfastness rating changes since the publication of your article about 2 years ago? Finally, it does make sense that a pigment in watercolor would generally be less protected than the same pigment in oil, and so testing pigments in watercolors would be the tougher/harsher test. That being said, is Golden/Williamsburg doing any lightfastness testing of pigments in oil paints?

    Reply
    • Hi Jack –

      We definitely do a lot of lightfastness testing for our oil colors and in fact, I believe, just got back a huge trove of samples from Arizona that should help us with getting some of the ASTM ratings updated or having the pigment added to the table. In terms of the specific colors you are interested in, PR 264 has tested consistently in oils as equivalent to ASTM LF I. In fact, its numbers in the 3-month outdoor tests in Arizona would make it a strong LFI, at a Delta E ~2.4, with LF I being between 1-4. In Xenon tests, it had a Delta E of 3.08, which is still a solid I. I would not have any hesitancy in recommending it as a permanent pigment. As for Diox Purple, our Williamsburg Oil Color has tested in both outdoor and accelerated indoor tests as a solid LF I as well, and ASTM provides two different lightfastness ratings for Diox Purple in oils, depending on the shade. We are in the process of updating our labeling and other information to reflect the change from LF II to I on our Egyptian Violet, Payne’s Grey Violet, and Provence Violet Bluish, which all utilize PV23. Expect that to start appearing within the next several weeks, although of course, it takes time to show up on the labels in the stores as they rotate stock through. But rest assured all the PV23 we have used is the same. In terms of getting the ASTM COmmittee to update the tables and add new pigments, it has been a challenge. While we have brought those materials to the meetings, we have unfortunately lacked a quorum, making doing these updates more difficult. But we will keep trying, and in the meantime rest assured that we are in the process of testing or have tested every color we make in all the lines. So never hesitate to inquire about a particular pigment as we are always happy to share what our testing shows.

      Reply
  3. Hi Sarah!

    I see in a reply that we are open to asking for specific pigments! I am very curious about your results for QOR indigo. It is such a beautiful color! It is rated II but if I look at the pigments, they are usually very lightfast on their own.
    Will you also share the rest of the watercolor results at some point?
    Thank you for being so thorough and open about this process!

    Reply
    • Hi Ellen –

      Thanks for asking about the lightfastness of Indigo. Which we agree is beautiful and one of those anchors on a lot of people’s palettes. This is one of those oddities caused by ASTM’s current ratings for pigments used in watercolors, where Phthalo Blue (one of the colors in Indigo) is surprisingly rated as a Lightfastness II. We agree that this does not make intuitive sense, and certainly other respected authorities – such as Bruce McEvoy’s website Handprint ( https://www.handprint.com/HP/WCL/waterb.html ) – has found Phthalo Blues to perform quite well. As the article pointed out, we had a similar case with Quinacridone Magenta and Dioxazine Violet, where our own ASTM compliant testing showed them both as solidly lightfast, where ASTM had given them ratings of III, based on tests from the 1990s. In any case, we will need to include this in a round of testing to see if we can also seek to have the ASTM rating revised. Unfortunately, it is not a color we have tested in the last few years, although we did test Phthalo Green which performed as a solid I.

      As for what to do in the meantime, we are currently stuck with the ASTM rating until we can have our own results in place. But based on the Handprint results, which were across the board for all brands he tested, we think you should feel confident in using colors having Phthalo Blue as being quite durable. Also, keep in mind that even ASTM LF II is considered acceptable for artists work, and watercolors should, of course, be mounted behind a UV filtering glass which will certainly give all the colors a much longer life.

      Hope that helps – and if you have further questions just let us know.

      Reply

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