Tung Oil: debunking the myths
If you’re a woodworker, I bet you have a strong opinion about tung oil. As a rookie, I became preoccupied (okay, obsessed) with tung oil because I heard so many different opinions about it from veteran woodworkers. Some loved it and some hated it. So I started researching and found that the information available about tung oil was often incorrect, conflicting, and/or misleading. Let’s examine and debunk some of the myths.
A Paraguayan farmer checks his tung oil crop. Paraguay is second to only China in worldwide tung oil production. (Photo by Blake Hanson)
China is by far the world’s largest tung oil producer, providing 83 percent of the world’s supply. (Photo by Blake Hanson)
For millennia, China has used tung oil in many ways, including building and waterproofing their traditional boats, called “junks”. (Photo by dreamstime.com)
Tung Oil vs. BLO
To compare the two finishes, Vaughn MacMillan applied boiled linseed oil to the left half and tung oil to the right half of this platter. The tung oil is a bit lighter, and this difference will get more noticeable as time passes. (Photo by Vaughn MacMillan)
Great for Restoration Work
This armoire was in bad condition until it was restored using tung oil. It now has a nice sheen to it and some protection from everyday use. (Photos by Dave Hawksford)
A Nice Finish
For those of you who want wiping varnish instead of a pure oil finish, Waterlox makes one called “Original Finish” that contains real tung oil. This guitar was finished with the Waterlox product. (Photo by Kellie Hawkins)
Myth #1 Tung Oil was invented by Homer Formsby in 1965 (Formsby’s Tung Oil Finish).
Nope. Tung oil has been around for thousands of years. There’s no doubt that Homer Formsby put tung oil on the map in North America in the late 1960s, when he started marketing his special finish. However, according to Bob Flexner in his book, “Flexner on Finishing”, Formsby’s concoction was really a wiping varnish made with (maybe) a little tung oil, a resin and a thinner.
When you buy tung oil, you have to carefully read the ingredients on the package to have an idea what you’re dealing with. I got my hands on several products labelled tung oil that actually contained solvents, and/or metal compounds that speed up polymerization (or drying), and/or other mystery ingredients. Sometimes there was a little tung oil in there too. It’s also possible to buy partially polymerized tung oil, which hardens faster. I’m not criticizing any of those products, but for the purposes of this article I’m referring to pure tung oil with no additives. By the way, Lee Valley was kind enough to send me several samples of the real thing for testing in this article.
The earliest reference I can find to the use of tung oil is in the writings of Confucius around 500 B.C. The Chinese have used tung oil, also known as China wood oil, for at least 2500 years for wood finishing, wood waterproofing, caulking, inks and paints. I also found some references to using tung oil for medicinal purposes in ancient history. I don’t suggest you ingest it or take a bath in it, but apparently some primitive cultures did. In the 13th century, Marco Polo wrote about the Chinese using tung oil to build and waterproof their traditional boats called “junks”.
In 1905, a USDA scientist brought Tung tree seeds to the US to try to cultivate them in Florida. The crop grew well, but bad weather and a succession of hurricanes spelled the end of most of the US production by 1969. Blake Hanson, president of Industrial Oil Products, the only global supplier of tung oil from all sources, told me that there was some US production again from 1998 to 2005 (from his company) until Hurricane Katrina reared her ugly head. Today, world tung oil production comes mostly from China (83 percent), then from Paraguay (about 14 percent), Argentina (2.75 percent), and Brazil (a tiny bit) and it is used in wood finishing, paints, inks, fuels and other things. According to Professor B. Sivasankar, who wrote a recent textbook on engineering chemistry, these drying oils form stable films that protect surfaces from corrosion and weather. This is why tung oil and linseed oil, for example, are essential components in paints. “Without these drying oils, paints cannot function as protective coatings.”
Myth #2 Tung Oil dries in the air by evaporation.
Nope. Tung oil definitely gets hard, but it doesn’t happen by evaporation. Chemists classify oils as “non-drying”, “semi-drying”, and “drying”. The word “drying” is misleading because the oils don’t really “dry” or evaporate; they “harden” or cure.
The most commonly known drying oils in woodworking are tung and linseed oil. They polymerize or solidify by a chemical process that requires oxygen (from the air) to create cross-linked compounds that make the oil get hard little by little, until it is completely hard all the way through.
Myth #3 BLO is just like tung oil, but better and cheaper.
Sorry, but that’s wrong too. Comparing BLO (boiled linseed oil) to (pure) tung oil is like comparing apples to oranges. So let’s look at both:
Linseed oil, which comes from flax seeds, has a long history.
Flax (cloth) fibres have been found from 30,000 years ago, and we know linseed oil was used in oil paints in Europe in the 14th century. Woodworkers have used linseed oil in wood finishes for a long time because it was readily available, flax being grown easily all over the world. Pure linseed oil dries more slowly than pure tung oil, but no one knows that because everyone buys BLO, which dries fast because of all the added chemicals! BLO is definitely cheaper, and it is good; but it’s not better.
Myth #4 Don’t use tung oil on food surfaces (like counters and cutting boards) because it’s risky for people with nut allergies.
Wrong. I heard this information stated adamantly and authoritatively several times in a few different places, but it’s just simply not true.
First, depending on whose statistics you believe, approximately 1 percent of the population in Canada has an allergy to tree nuts. And according to Dr. Gerry Allen, associate professor of biology at the University of Victoria, tung nuts from the tung tree (species Aleurites fordii) are not true tree nuts at all. They are the seeds of the fruit (drupe) like the seed inside a peach pit. So, are allergies to tree seeds as prevalent as allergies to tree nuts? Again, it depends on who you ask, but probably not. The Journal of Allergy and Clinical Immunology says the incidence of allergy to seeds is 0.1 percent in Canada. We also know that allergies to seeds are more common in cultures where the population comes in regular contact with them. Aside from woodworkers, I’d say the general population in North America rarely comes in contact with tung “nuts”, seeds, or oil. So now our risk of allergy to tung oil is down to 0.1 percent of the population.
We know from a study published in the British Medical Journal in 1997 that in a test group with known allergies to tree nuts, none of the test group had a reaction to nut oil that had been refined.
So if tung oil comes from a seed, and if it is unrefined, the probability of an allergic reaction upon exposure is now reduced to 1/10 of 0.1 percent of the population. Is tung oil refined? Sometimes. Blake Hanson told me that often it’s sold as pure unrefined oil but sometimes a solvent extraction (or refining process) is used. So the probability of being allergic to liquid tung oil is now somewhere between 0 and .01 percent of the general population, while the cured hard oil has even less risk.
Myth #5 Tung Oil never dries and you can’t get a good result from it.
Yes it does, and yes you can.
Almost all experts agree that using a cloth moistened with warm water is the easiest way to raise the grain on your project before oiling. This should be done before you apply tung oil. Raise the grain, sand, and then begin. Bob Flexner says that applying oil is simple, “wipe, wait, sand, repeat.” Apply the oil liberally with a soft cloth or brush and then wipe it off like you mean it. Check after an hour or two, and if extra oil has beaded on the surface, wipe it away. Don’t forget that rags used to apply drying oils are highly combustible. When you’re finished with your rag, hang it outside to dry. Be careful disposing of them.
When using pure tung oil, you need several coats. It’s very important that you thin each coat with the first coat being the thinnest (I recommend 70 percent solvent). Each successive coat should be thicker (less thinned), and the last coat must be the thickest. Your thinner needs to be an organic solvent, one that is carbon based like turpentine, mineral spirits or the newfangled “citrus solvent”.
Every layer except the last must be sanded, so the next layer of tung oil will bond to the previous layer. Three hundred and twenty-grit sandpaper creates the “tooth” that grips the next layer. When sanding between coats, you have to go lightly or you will suddenly sand through one or more previous coats and you will have dreaded witness lines.
Getting good results requires using the right techniques and not being in a hurry. I would allow at least a week between coats, although I have heard of people doing it faster with good results. There are many other finishes better suited to a tight schedule; varnishes, lacquers, and even BLO. However, if you want to use oil, and you have some time to devote to the finish, pure tung oil is in a class by itself. There is no other drying oil that has the same resistance to water, mold, bacteria, yellowing, darkening, but offers strength and flexibility.
Well, all that is wonderful, but is tung oil safe? I asked Marc Spaguolo (of internet Wood Whisperer fame and a woodworker with a background in molecular biology) his opinion of tung oil. He said, “It is my belief, that yes it is safe once cured. In general, most of the ‘bad stuff’ in mineral spirits and other petroleum distillates goes away upon evaporation. Any remaining residue can be washed away with soap and water.” He added, “The biggest difference [between BLO and tung oil] is probably cost. BLO is going to be significantly cheaper. But if one is really concerned about chemicals and food safeness, they will be happier with tung oil.”
So let’s recap: tung oil is more expensive than BLO, and it takes longer to dry. The chances of allergy to tung oil are remote, and tung oil has several other advantages over linseed oil. Professor Norm Kenkel, a biologist at the University of Manitoba, reminded me of another reason to use it: “Tung oil is an environmentally safe and sustainable wood finishing product.” There are reasons why tung oil has been used as a wood finish for thousands of years. It’s great stuff. For a traditional pure oil-rubbed finish, it’s the only game in town.