Wikipedia has an article titled “oil drying agent.” It is useful, but not very extensive. Here, “drying” means “curing by polymerization,” rather than by solvent evaporation.
Plant seed oils are used as food by the developing plant embryo. The oils are not as useful if they become polymerized. So, a drying oil does not naturally dry at a rapid rate, because the seed coat is not perfectly impervious to air and light. From the plant’s perspective, it would be better if the oils did not polymerize at all; but it suffices if the rate is slower than that of seed development.
Drying oils are catalyzed (the reaction runs faster) by certain contaminants, most often divalent metal ions. Divalent metals ions are quite common in mineral compounds, including most traditional pigments. But especially among the modern pigments, there are no divalent metal ions. Some manufacturers will add a touch of divalent metal ions (usually an organic cobalt compound) to those colors that do not naturally have them, in order to make the drying rates of various colors more equal. Other manufacturers don’t do that.
Lead White (Lead Dioxide) will release some divalent Lead ions, and is a very effective drying agent (siccative). Many natural earth pigments, especially those with some Manganese contamination, are also very effective siccatives. Modern synthetic earth pigments may be too purified, thus less effective as siccative. Although Iron is (often) a divalent metal, many forms of Iron Oxide are ineffective as siccative, because the Iron is too tightly bound to the oxygen, and is not released.
One problem with divalent metal ions is that they usually add color. Cobalt adds purple; Iron (if in a suitable compound) would add red; Copper would add green. Lead is more colorless, but subject to long-term change due to atmospheric contamination.
Cobalt is a “primary” siccative, meaning that it directly catalyzes the reaction of atmospheric Oxygen with the oils. Others, such as Zirconium and Calcium, are “secondary” siccatives, meaning that they improve the way that Oxygen is distributed within the curing paint. I have never quite understood that.
The compound Cobalt Napthenate is widely used. This is an organic compound (napthenate) that will readily mix with oils, and will release some of the Cobalt.
I add a little bit of a product named CoZiCa to the inexpensive mass colors (white, etc.) on my palette, and to medium. I do not add it to the expensive colors (PY128, etc.) that are used only as additives to other colors. CoZiCa has Cobalt, Zirconium, and Calcium. This way, only the cheap colors risk drying on my palette before I get a chance to use them. The expensive colors are always mixed with a cheap color at moment of use, so they get their dosage of siccative that way.
If you wish to prolong the lifetime of colors on a palette between sessions, refrigeration is very helpful.
Note that Alkyd oils have different technical considerations. But if you are using an oil medium with “just a touch” of Alkyd added, then the above considerations still apply.
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