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  • Oil paint becoming transparent over time.ApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2019-03-14 00:55:45 ... Most recent comment 2019-03-15 01:24:15
    Drying Oils Oil Paint Pigments Technical Art History

    ​I'm wondering if the mechanism for oil paint becoming more transparent over time is well understood?

    More specifically, if I paint a layer of paint over another layer, will they both become more transparent at the same rate, or will the top layer become transparent more quickly?

    I know some pigments will fade more quickly than others, but for simplicity, let's make the pigments in both layers identical.

Answers and Comments

  • EditDeleteModerator Answer

    Really fading is not the issue in this instance. Increased transparency of oil films over time is a result of a couple of factors but primarily it is the conversion of certain pigments, primarily lead white and zinc white, into metal carboxylates (more commonly known as metal soaps). This occurs because of the interaction of fatty acids and mobile metal ions in the paint film.  This same can happen with other pigments but these are the most effected. In essence, basic lead carbonate has a refractive index of 1.94 and 2.09, which makes it relatively opaque in linseed oil which starts out with a refractive index of 1.48 -1.49. Lead carboxylates have a lower refractive index and if enough of the lead white is converted the film become less opaque. It is believed that this mechanism is exacerbated when the film is exposed to moisture and heat.

    Another less important factor is that aged drying oil films have a slightly higher refractive index than do newer films. This may cause a very slight increased transparency but the above mechanism creates a far more pronounced impact.  

    Given the above, I am not sure that your layering would really have any effect one way or the other. If it were an issue of fading the pigments at the surface would be far more sensitive than those below. We know this from cross-sections taken from historic paintings where the uppermost gradient of a fugitive color has little color but that same pigment below still retains its original hue.

    Brian Baade
    2019-03-14 13:57:32
  • ApproveRejectUn-ApproveUser Comment

    ​OK, great.

    The reason I asked was because many artists tone their canvas, or work from a medium value, placing lighter paint on top.
    My thought was that the light paint would gradually become more transparent and therefore become darker as the underpainting shows through.
    Correct me if I'm wrong, but from what you've said, this would still be the case if using lead because, as there would bemore lead in the lighter layer, it would possibly form more lead carboxylates.
    (Also, being on the surface, it would be exposed to more heat and/or moisture than the underlayers).

    I gather this issue isn't as much of a concern for titanium white?
    Do metal ions also convert titanium into titanium carboxylates?
    And if so, do they have a higher refractive index?

    2019-03-14 17:22:52
  • EditDeleteModerator Answer

    What you describe is one of the reasons why it was always problematic to work on a dark ground. A transparent toning layer is probably a better choice but unless the highlights were applied quite heavily, they could lower in tone over time.

    I do not know of any studies on titanium dioxide converting to a soap over time. Certainly, zinc oxide does and it appears to cause more deleterious effects.

    Brian Baade
    2019-03-14 19:08:32
  • ApproveRejectUn-ApproveUser Comment

    Yes, I'm well aware of problems with zinc!
    Thanks Brian.​

    2019-03-15 01:24:15

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