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Chemical Interactions

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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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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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