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
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
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.
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
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