At the Art Treehouse, we often receive questions about the drying time of paints. Some artists prefer to keep the drying time “open” so that they can work on a painting over several days. Some may even add Clove Oil to the paint medium in order to further extend the drying time. Other artists prefer a quick drying time, if for example the paintings are to be sold within a short period of time. In their natural states, walnut oil and linseed oil dry with very solid films in three to ten days, depending on the pigment. To speed the drying time, some manufacturers add cobalt alkyds to their paints and mediums. Recently there has been significant concern about cobalt driers, due to their toxicity as well as ethical issues related to the mining of cobalt in the Democratic Republic of Congo. The entire paint industry has been working overtime to find a less toxic alternative to cobalt alkyds.
The use of cobalt became widespread following the removal of lead in paints. However, recently the Cobalt Reach Consortium chose to self-classify 12 cobalt substances under the EU CLP Regulation as Toxic to Reproduction. In the US, the National Toxicology Program has written several reports on cobalt, and has issued a report concluding that cobalt and certain cobalt compounds are reasonably anticipated to be human carcinogens. This is in addition to the international concern over the usage of child labor to mine the toxic cobalt. Two alternative approaches have been proposed: manganese and iron. These two alternative driers show promise, but have drawbacks such as adding color to the paint. At this point, their greatest usage is therefore with dark paints. More recently, one iron-based drier, originally found to remove stain bleaching in laundry cleaning, shows good paint drying activity at very low usage levels, outperforming cobalt. In France, a consortium has been organized to develop a vegetable-based alternative to alkyd cobalt, and they expect to complete their work within 36 months.
WHAT YOU NEED TO KNOW: Toxic cobalt driers are still widely used by many artist paint manufacturers and are very common in commercial paint mediums (they are not used in any of The Art Treehouse materials). In the near future, cobalt alkyds will be replaced by iron-based, manganese-based, or vegetable-based alkyd driers. In the meantime, painting with walnut and linseed oil without added driers is the traditional archival method for creating beautiful paintings that last for centuries.