Since pigments come from a wide variety of materials, some of them dry slowly and some dry quite fast. This varying rate of drying speed can cause problems, both for the artist and the paint maker. To overcome this, paint makers add driers to some pigments, so that there is a balance between different colors. Knowledge of the different driers is helpful to the artist who is concerned with maintaining quality, without excessive exposure to toxins.
A dryer works by catalyzing or accelerating the curing and hardening of oil paint. Often, they are based on metals, and are sometimes called metal soaps or salts. In the process of drying, oil-based paints first absorb oxygen, then they form peroxide which decomposes. Driers accelerate this process.
There are three general categories of driers, classified according to process:
1) Oxidisers – Since the surface of paint is exposed to the air, these driers are most active on the surface of the paint. They include cobalt, manganese, vanadium, cerium and iron.
2) Polymerizers – promote drying internally to the paint. They include zirconium, lanthanum, neodymium, aluminum, bismuth, strontium, and barium.
3) Auxiliaries – secondary additives that help with the “wetting” of the pigment, a necessary part of the paint making process where pigments are prepared to accept oil. These include calcium, potassium, lithium, and zync.
A short summary of drier catalysts:
1) Cobalt – the fastest, most active drier. Can cause wrinkling and color changes in light-colored paints.
2) Manganese – also very fast, but has a dark color which makes it questionable for use.
3) Iron – promotes rapid drying by polymerization. High content of iron in iron oxide colors accounts for their rapid drying time.
4) Rare Earth – useful under conditions of high humidity or low temperatures.
5) Lithium – sometimes added to other driers, most often alkyds.
6) Calcium – Performs well overall, often added during the pigment grinding process as an auxiliary dispersant. Basically marble dust.
7) Zirconium – Used with calcium as a replacement for lead. Has better gloss and color retention than lead, but may not do well under high humidity or low temperature.
8) Aluminum – Great polymerization and yellowing resistance without negatively affecting viscosity. Probably the number one drier in overall usage today in the form of aluminum stearate.
In general, multiple driers are used together because of the efficiency is greatly increased, and the proper balance of driers is essential to the stability of the paint. Also, driers in paints that have a high pigment load must be carefully selected and measured due to possible problems of resin viscosity, and yellowing. At the Art Treehouse, we avoid driers such as the cobalts, due to concerns about toxicity. We most often use calcium carbonate for that reason.