Voodoo darkening - titanium white.

edited December 2022 in Color Mixing
A few months ago I complained about light sections going dark when I turned my back on them. I discussed it in this thread: https://forum.drawmixpaint.com/discussion/14473/disappearing-highlights-and-light-brushmarks#latest
Well, it turns out I wasn't crazy. It's been christened Voodood Darkening (I think by Julie Beck who put the test together). It felt like voodoo. It turns out in thin layers titanium white tends to go a little darker as it dries. Since I tend paint in thin layers until the end, no wonder I was struggling. This video shows that this person learnt it isn't just happening in organic pigments such as umbers - but in phthalo blue and cad red deep. I can add other blues such as Cobalt to that list based on my experience. The cause isn't understood.
The remedy appears to be thicker layers. A little bodied oil such as stand oil can also address it. However, considering the 'fat over lean' rule - which is really about painting thinner layers over a layer that hasn't fully cured - it might be something we put up at times with in early stages. Then really lay down those highlights and light sections with some generous paint.

Comments

  • Abstraction

    There are two processes at work here. As you add solvent and/or medium the refractive index goes up making the paint brighter and glossier. Then as the paint dries it becomes more transparent, allowing more of the toned canvas to show through the TW, also as it dries the surface becomes more mat and duller in appearance with a lower refractive index.

    Denis
  • So, if I'm understanding this correctly, there should be no problems with TW going dark if we use it straight from the tube or thinned down only very slightly. Of course, when it dries it will be more matte but I guess this can be remedied with varnish if required. I've noticed that great Australian landscape painters like Streeton and Roberts preferred the more pastel matte look in their landscapes.
  • @dencal Since I use neither solvent nor medium neither of those processes are at work. In the link I shared with the test carried out by Julie Beck (Assistant Director of the Academy of Realist Art in Boston) she specifies that she used neither solvent nor medium.
    "The thicker applications, sunk in less, and therefore did not shift value as much. this still confirms my theory that whatever processes happening during the sinking in, is causing the mixture is to darken irreversibly. So, while we still don’t have an exact reason as to why this happens, the only thing that prevents this is to use mediums that stop sinking in."Julie Beck.
    People at Painting Best Practices including George O'Hanlon are unable to isolate the cause. George eliminated refractive index as a cause. Stand oil has virtually the same refractive index as linseed.
  •  It's strange because I have never noticed this problem of TW darkening. Perhaps that's because I tend to use it quite thickly straight from the tube without medium. 
  • Folks

    TW out of the tube is 50% linseed medium already looking bright and glossy. Darkening as it dries, shrinks, becomes translucent, shows canvas toning and mattes, dispersing the light to look duller.

    Denis
  • @dencal, what if you use it impasto, a couple of millimeters thick or more? Is it still going to become translucent? I have paintings that are going on seven years old, and I haven't noticed any shrinking or translucency. Or does it take longer than that? 
  • edited December 2022
    Sinking in means it goes matte right? That will make blacks and dark colours lighten, and lighter colours darken slightly. I have this problem all the time with acrylics.

    You could oil out and then wipe off any excess so that there is just enough oil in place to remove the sinking in (Perhaps with walnut/safflower oil?)

    If you like a gloss finish then a gloss varnish will also restore the true values.
    tassieguy
  • Rob

    Not really in a position to advise about your seven year old work. But, using quality paint in an impasto style suggests there will be no problems.

    Denis
    tassieguy
  • @tassieguy This is not like yellowing - something that happens down the track. It happens as soon as it dries. So if you're not noticing it you don't have a problem. Like you I don't use medium or spirits, even for glazing now. Straight from the tube but perhaps paint thinner than you in early stages.
    Denis do you have a source for your conclusion? The folk at Painting Best Practices/ Natural Pigments don't yet.
  • Adding oil to your whites should make them more glossy and avoid this problem
  • dencaldencal -
    edited December 2022
    Abstraction

    I would think my conclusion is self evident. Fresh, fat oil paint is bright and glossy. Dry oil paint is dull, flat, shrunken and more transparent. The behaviour of light dispersion and refraction is beyond debate. Human perception of colour is profoundly influenced by this chemistry and physics.

    I look forward to Natural Pigments proving me wrong.

    Denis


    Marinos_88AbstractionDesertsky
  • edited December 2022
    @dencal Denis, I suspect you could well be totally correct - the darkening occurs with an accompanying matte appearance. But there remain other possibilities. George O'Hanlon considers there could be multiple factors. The reason i asked the question is that I understood your response to be intelligent speculation as you have a very strong technical grasp of painting and I always appreciate your inputs - but if others took your assertion as gospel I didn't want people who googled thinking we had drawn a conclusion on this DrawMixPaint forum without an adequate evidence base. I've not presented all the discussion and background here as it would be a little boring - I just wanted to flag the issue.
    Acrylic paints are known to darken as they dry. Oil paints are not known to perceptibly darken in this manner in most instances. This particular issue is not titanium white on its own, but in mixes - your answer appears to focus on the white. It's not caused by 'sinking in'. It does not occur with lead white. When we mix paints we mix particles of sometimes quite different sizes and masses.
    The current discussions are around particle size issues.
    "Pigment density packing is the rearrangement of different colored pigment particles in the paint film during drying. Ultrafine pigment particles of different density may flocculate causing a distinct shift in hue. This is related to a well known phenomenon in pigment dispersion and formulation of paint known as syneresis." (Painting Best Practices forum - facebook doesn't allow direct links.)
    “Hess’ Paint Defects” excerpt:

    reaction between pigments or in the drying process and if the pigment particles settle since titanium is a lighter weight pigment compared to lead white
    I would be interested to know anyone else here who has experienced this - I'm interested to compare paint brands and associated oils, additives...

  • @Abstraction, What a fascinating problem. I am sorry to read that you are dealing with it – as it is definitely weird!  A question: just how thin do you apply your PW6? I paint pretty thin and have not noticed this PW6 drying noticeably darker. What do you add to your tubed PW6 to modify it for application? What is the brand? Is the painting you are experiencing this with the big family portrait with the complicated ground chemistry?

    I think that this is a real problem with real causes yet to be determined. What Julie Beck demonstrated on her video seems to be different than the “Hess’ Paint Defects” because of the large time difference in when the problem becomes noticeable: Julie Beck – fast; Hess- slow.

    The Julie Beck Videos: I have watched the video of Julie Beck which you linked for us, and I have some very provisional observations. I need to think about this more, but so far:

    - Julie Beck says that painting thin over a white background “should” dry lighter because the thin paint will be transparent and let the white background show.

    My observation: well, maybe, sometimes. It depends on a few factors she doesn’t get into, but rather glosses over: 1. The pigment of the paint will be some specific degree of semi-transparency or opaqueness. 2. Just how dimensionally thin the paint is – and if this paint layer has been diluted with solvent.  Not every pigment in oil will have the same degree of “transparency” as she suggests. Her logic on this is based on a faulty assumption of how the paint will interact visually with the white ground. I disagree with her that the pink and light blue paint should allow the white background to show through a little. The pink and light blue paint were mixed from red and white, and blue and white. These are not semi-transparent pigments.

    - She says that a matte finish from sinking in should cause a dark color to appear lighter when dry.

    My observation: 1. Not all dark colors sink in. Not all dark colors will appear lighter when dry. I try very hard, and generally succeed, to paint in such a way that the colors don’t cure lighter, darker, or less saturated, compared to when they are fresh.

    Her use of the term organic for classes of paint confuses me, as different painters use the term organic in different ways. This makes it difficult for me to follow her conjecture on how the organic pigments, as a class of paint, react.

    My oil paint shifts value and saturation a very little bit when dried. This holds true for ALL colors because nothing appears as fresh and vibrant as fresh, wet, uncured oil paint. But, for me, this value shift is really small, maybe less than ¼ of a step in a 10 step value scale. The value is a little lighter after the paint is set-up. The saturation shift is also very small – I don’t know how to quantify it as I don’t use Munsell calibrations. But for both value and saturation, the changes are so small that I don’t even adjust the new, fresh paint to accommodate this.

    Without more specific information about the brand, the medium, the ground, how thick the paint layer it, etc., one can only note that it is a weird problem. I think it is some combination of all these factors.

    I have never experienced this in 40-50 years of painting. But, as you know Abstraction, I paint on a completely sized substrate, use no ground with super-absorbent calcium (or else cover it up with a layer of PW1) and do not put on multiple layers of ground, don’t dilute the paint with a solvent, and usually add some walnut oil to retard set-up or Liquin to speed up set-up of the paint. The visual effects of Liquin are similar to stand oil, helping keep the saturation and value mostly unchanging. I also have started using my own mix of 50/50 PW1 and PW6 that I tube and use for the white paint anytime I use white. I find this is easier to keep a consistent white temperature paint. Also, I never paint on a toned surface. I like a white surface – the lighter the better.

    Please keep us posted on this as it could help lots of painters.

    Abstraction
  • Excellent thread and points all! :)

    This might be useful:
    https://justpaint.org/oiling-out-of-dead-colors-in-oil-paintings-3/

    "A third cause needs to be mentioned, which functions independently of a grounds’ particular absorbency; namely film thickness. For many paints, it turns out this alone is enough to cause a matte appearance to develop, as you can see in the following examples."
    Abstraction
  • Very helpful responses @dencal @Desertsky @Richard_P
    Desertsky said:

    @Abstraction, What a fascinating problem. I am sorry to read that you are dealing with it – as it is definitely weird!  A question: just how thin do you apply your PW6? I paint pretty thin and have not noticed this PW6 drying noticeably darker. What do you add to your tubed PW6 to modify it for application? What is the brand? Is the painting you are experiencing this with the big family portrait with the complicated ground chemistry?

    I first noticed it years ago with the lacing foam I put on a seascape. It looked great. Then the slightly greyed whites dropped back and the thicker pure whites mostly stayed ok. I just assumed the oil was subsuming the particles and affirmed my teacher's rule, 'thick highlights. You want them to last.'
    I add nothing to my paint (nowadays). I'm still using Art Spectrum titanium white series II that has a touch of zinc in it (yes, I know, I'm looking for a replacement!!) in sunflower oil*. Currently on portrait the darkening appears to happen with blue in particular. I have noticed it with relatively thick applications as well. It's almost as if the white becomes overwhelmed by the blue as it dries.
    * My plan - a titanium in linseed oil for mixing and a titanium in safflower oil for highlights (probably Langridge). I'm creating a table with all the different titanium whites with %zinc and type of oil. I probably should have a column for other additives. But now interesting to isolate this issue before I invest in a big tube in case it's related to the oil or additives.
    Desertsky
  • Folks

    As paint polymerises it heats up, this exothermic behaviour is seen in two part resins as they cure and more dramatically in oily rags as they self combust. So, here is another phenomenon that May account for, or at least be partly responsible for, Abstraction’s White shift. The phenomenon is called Thermochromism. That is the material changes colour along with temperature.

    Denis


    AbstractionDesertsky
  • Sounds like real Voodoo Denis! ;)
    dencal
  • edited December 2022
    Another possibility is that one just has bought a tube of white oil paint which was from a bad batch at the manufacturer. @Abstraction. have you encountered this in more than one tube of white? 

    I have never used Langridge paint, but have read outstanding recommendations about it.  

    ps - I used titanium-zinc white for decades with no problem at all. I am not arguing that the zinc oxide problem does not exist, just that it is not universal and it probably has other factors involved. You are painting on a hard substrate, not canvas, so no ongoing expansion and contraction forces. 
    Abstraction
  • edited December 2022
    I still think it’s because manufacturers cut white with marble dust to save money. Which in my experience goes slightly more transparent as it cures (no idea why.. maybe the way particles settle and align a certain way). If you’ve ever made a ground using oil+marble dust and forgot to add any titanium at all (oops) you will find that it cures transparent.
    Abstractiontassieguy
  • One thing you could try on a test piece or on one of these paintings is apply a layer of solvent over the dried paint. Does the white go back to it's original values? You could even try this with water..
  • @Richard_P So you mean to create a wet surface effect similar to oiling out or varnish? I see no difference when I do that. I also note that it doesn't happen all the time for me in mixing with titanium white - it's a rare effect.
  • That's interesting. Adding a wet surface makes everything glossy (temporarily) which might have bought back the highest whites for you.

    The fact that you aren't seeing that on these areas that's darkened when dry suggests that indeed something else is going on..
    Abstraction
  • Again stressing that it isn't the pure whites - it's occurring in the mixes.
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