Chemical Interactions
The drying of oil
When oil paint dries, there occur chemical interactions between
oil and pigments. Actually, the drying of oil is not a physical
phenomenon like the drying of watercolors, where water evaporates,
leaving a lighter color film on the paper. Oil drying is a chemical
reaction between the oxygen of the air and some constituents of the
oil. We call this an oxidation. The paint film becomes
heavier and harder by absorption of the oxygen.
Some pigments have the power to accelerate this chemical
reaction, other ones to slow it down.
That’s why I recommend to avoid some pigments for
oil painting, although they are perfectly permanent and can be
excellent for other techniques, like watercolors or acrylics. (This
could not be true with alkyd fast drying oil colors, of which the
manufacturer seems to have been able to standardize the drying time
of all pigments.)
For example, it’s better not to use several
blacks like Lamp Black, Carbon Black, Ivory Black for oil painting
because they slow down the drying of oil. That’s why I
consider it’s better to use Mars Black.
On the contrary Raw and Burnt Umber contain a
siccative chemical substance which accelerates the drying too much,
what makes the paint film brittle and subject to cracking. So
it’s better not to use these pigments when painting in
oil.
Low qualities of French Ultramarine can react to
the oil and become grayish and brittle. This is known as the
“Ultramarine sickness”.
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The Refractive Index
Another phenomenon: when drying, gradually the refractive
index of the linseed oil film becomes higher (because it
becomes heavier and harder). That means that some pigments (those
with a refractive index near the one of the film) seem to be more
and more transparent, what has the effect of darkening the tone of
these colors, which becomes more and more brown, because the oil
“yellows” when ageing.
A striking example. Raw Sienna has a bad reputation of darkening
with age, but it’s not this semi-transparent pigment that
darkens (it’s an absolutely permanent one), but the oil film
which becomes brown at the very moment when the pigment becomes
more and more transparent.
So you have to be particularly cautious when glazing colors:
always choose the less yellowing possible medium.
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Chemical interactions between pigments
The Old Masters knew very well how to manage the few colors they
possessed. They knew that some mixings were dangerous because the
pigments could chemically react to each other.
Nowadays, this problem is no longer so important because the
most chemically dangerous pigments have been eliminated from the
painters palette. For example Flake White (= White Lead)
reacts to some pigments like Cadmium Yellow and Vermilion.
If you limit yourself to the palette I recommand, you are at no
risk of chemical reactions between pigments.( Back to “Best Palette”)
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Chemical reactions between pigments and air pollution
Sulfur, a pollutant contained in the air, has blackened many
lead or mercury containing colors, among which Lead White and
Vermilion are the most known. The reason is that mercury sulfide
and lead sulfide are black. Now you understand why Zinc White and
Titanium White are safe from this point of view: their sulfides are
white!
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Conclusion
Lead and mercury (quicksilver) are the most dangerous components
of the pigments that can be used in oil colors. That’s why I
recommand to leave aside the concerned pigments, in other
words:
- Lead White PW1 (= Silver White = Flake White);
- Chrome Yellows PY34 and Chrome Orange PR34
(contain lead);
- Vermilion PR106 (contains mercury).
The sulfur of some pigments (the Cadmiums, Ultramarine) cannot
be dangerous for permanence if not in the presence of lead.
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