Brooks Tanner replies: Japan drier reduces the drying time of oil and oil based finishes. It is a metallic salt, usually cobalt, that acts as a catalyst aiding in chemical linking of the molecules.
Japan drier should be used in very small quantities since excess use can cause a darkening of the product or even cracking. Use of the drier also increases the gloss of the product and should be tested on a sample if a sheen level needs to be matched. Drier should be added only to the amount of product that is to be used. If Japan drier is added to a can of product, gelling may occur, even if the can is sealed. Since drying time is reduced, leveling is also reduced and brush marks may show more.
Should it be in a finishers cabinet? Absolutely. Sometimes I use it because I don’t have the twelve hours for normal drying; or its late in the day and the customer is coming in first thing in the morning. Reduced dry time also reduces the likelihood of dust getting into the finish. Also adding drier ensures that the product will dry. This is sometimes a concern if dissimilar materials are mixed or if an unknown oil finish has already been applied to the wood.
Bruce Hamilton replies: I’ve been familiar with Japan driers since I started my finishing career thirty years ago. All I had readily available at the time for a table top finish was varnish. At Johnson’s Paint store in Boston, where I purchased my varnishes, it was recommended that I put some Japan drier in my varnish. Since I didn’t have much experience with varnishes, I couldn’t tell whether it did any good or not. I had that can of Japan drier until last year when I finally sent it off to our town’s household hazardous waste day collection.
Japan drier is a liquid that speeds the drying of oil finishes such as varnish and oil paints. It is 97% Naphtha and 3% cobalt compounds. Varnishes and paints containing oils such as linseed and tung dry by oxidation. This means they need to absorb oxygen from the air in order to solidify. The cobalt in Japan drier enhances the ability of the oils to absorb the oxygen molecule.
There are other compounds that speed the drying of oils. Lead is one of them but because of its toxicity is no longer used as such. Boat varnishes use Japan drier also perhaps because they are often varnishing outdoors in poor weather conditions and dirty environments. The longer a varnish surface stays wet to the touch, the greater the possibility that dirt and dust are going to land on the surface and stick.
Cobalt compounds are considered very toxic things. On a scale 1 to 6, cobalt compounds are considered a 5 according to the Scorecard (www.scorecard.org), a pollution information site.
In my opinion, I don’t see any purpose for adding Japan drier to your oil finishes. There is an excellent article by Charles S. Tumors and Marion F. Mecklenburg in the Review in Conservation Journal #6 2005 on the whole subject of driers and oil finishes.
Herm Finkbeiner replies: Japan drier refers to metal salts that are oxidation catalysts and usually is a cobalt salt, cobalt linoleate or cobalt naphthanate, that is added to drying oils like linseed, walnut oil, safflower oil or lacquer to accelerate the curing. Painters use them for the same purpose and probably much more commonly than furniture makers. Without a drier, a lacquer such as used on Japanese lacquerware can take 3-4 months to harden. However, it can take just a week or less with a cobalt salt accelerant. I have no idea how effective they are in an application like wood finishing where a significant portion of the oil is absorbed into the wood and its oxidation is intrinsically slower as a result.