I. What is Tung oil? Where does it come from?
Tung oil is the oil extracted from the seeds of tung tree(Vernicia fordii, pictured to the right)[wiki]. Tung oil got its spelling from its native name in China. This oil is a kind of drying oil which means after a long exposure to air, it will harden to a tough and solid film, due to reaction with oxygen in the air. It was brought to US for cultivation in 1905. Even though it is hard to raise on US soil and climate, it was still widely planted on US Gulf coast due to its value and peak domestic production of tung oil reached 45 million pounds in 1958. Afterwards, Atlantic hurricanes turned out to be a deadly foe. Hurricane Betsy(1965) and Camille (1969) dealt a devastating blow to tung oil plantation from which the industry never recovered. Now it is only grown in China and South America.
2. What is the use of it? Why do I care?
The ink on US dollar bill is made from tung oil. Tung oil was not long ago quite common in US. US consumption of tung oil was about 160 million pounds in 1942[TungOilConsumption], which translated to 1.2 lb per capita. It was used to make compounds to coat cables, telephone wires, generators, fans and other electrical equipment. The automobile industry once used large quantity of tung oil. For instance, it used tung oil as the binding agent in every brake band to hold it together. It is used to print US paper currency. Its non-toxic nature makes it particularly appropriate for children’s toys and furnitures[TungOilHistory]. Tung oil is considered the best penetrating drying oil available due to its unique ability to wet the surface, allowing it to penetrate even the densest woods. Unlike linseed oil, it will not darken with age. Recently, it emerges as one of the least toxic finishes for hardwood flooring and also gives a deep, warm and aesthetical appearance.
A. What is hardwood floor finish?
Hardwood floor finish protects hardwood floor from water, spillage of household liquids and normal wear and tear. It also should enhance the beauty of the wood and make it more pleasantly looking. The most common hardwood flooring finish today is oil or water-based polyurethane. Polyurethane, whether oil or water-based, is a film-building finish. It does not penetrate into the wood. It works by depositing a thin layer of clear plastic (polyurethane film) onto the surface of the floor. That layer of plastics usually is transparent and highly durable and acts as the main barrier between the room and floor underneath[hardwoodinstaller].
B. What are oil and water based polyurethane finishes?
Oil based polyurethane has been around since 1936 when Fabulon Co. first started to make it for bowling allies. It dries in 12 hours after application and takes 3~7 days to cure to a layer hard enough to withstand furniture. To do its job, it contains a large amount of solvent which will evaporate into the air(So called volatile organic compound or VOC) during the drying and curing process. Those VOCs usually have a strong odor and will linger for weeks or even months, especially with poor ventilation. The exact chemical composition of the finish are proprietary information which means it is commercial secret. Their health effect is not being discussed as we do not know even how much of what is inside.
Water based polyurethane finish has gained wider acceptance during last two decades due to a few advantages over oil based ones[WaterBasedAdvantages]. It emits much less VOC due to large portion of oil based solvent was replaced by water. However, that does not mean it is VOC free. It just has less than oil based ones. For example, Bona water based polyurethane finish Bona Traffic has VOC content of less than 180 grams per litre while most oil based ones have more than 350 grams per litre. Again, we do not know exactly what is inside it.
C. VOC content and Greenguard certification
VOC has got a bad and rightly deserved reputation. It is especially dangerous to children as their respiratory system is still under development and much less robust[VOC-Children]. Respiratory, allergic and immune effects among children are associated with VOC pollutants. Asthma is one of the common implications.
To promote healthier indoor air, Greenguard set up a certification program which will certify products with minimum VOC pollutions to indoor air. This is how it works. For residential environment, Greenguard has set up a test chamber which mimics the typical single family house in US[Greenguard], such as 8’ ceiling on the 2nd floor and 9’ on the first one. For every product, it applies it to the chamber and wait for a period of time. At the end, it measures the concentration of VOCs as well as a few more specific bad stuff, such as formaldehyde in the air. If it meets a certain standard, it may earn that specific kind of certification. Currently Greenguard has Indoor Air Quality, Children and Schools and Synthetic Turf three certification programs[Greenguard]. The most stringent one is Children and Schools Certification. I feel it is a good way to test a product for indoor air pollution contribution if they perform their test according to their technical documents. It is the only organization I know of which measures indoor air quality impact of commercial products and provides certifications.
Currently, there is only ONE brand of polyurethane finish which carriers Greenguard Indoor Air Quality certification. It is made by Swedish company BONA[Bona]. The common ones are Bona Mega and Bona Traffic. According to Bona, Bona Mega VOC content is less than 150 grams per litre and Traffic is less than 180. No finish has met the more stringent Children and Schools Certification yet.
D. What are advantages of Tung Oil finishes?
First, it is food grade safe. As product of tree nuts, pure tung oil is completely free of man-made chemicals. It is non-toxic and absolutely zero VOC. It does not contain any of the nasty stuff such as formaldehyde. Though not suitable as food, pure tung oil is food grade and is ideal for surfaces which will come in contact with food[TungOilFood]. It does not emit highly toxic hydrogen cyanide upon burning, like polyurethane based flooring finish.
Second, it functions well as a flooring finish. It forms a hard film which is waterproof and impervious to dust, alcohol, acetone, and acid.
Three, it soaks into the wood and highlights its grain. It slightly darkens the wood and gives a warming and pleasant looking.
Four, it is easy to maintain. Once worn out in one place, polyurethane flooring in nearby area has to be removed completely through sanding and then re-apply. For Tung oil flooring finish, you only need to apply more Tung oil on that specific spot. No need to sand the floor to expose underlying wood. Sanding process generates large amount of polyurethane dusts which, if inhaled on site or later, poses significant hazard to respiratory system.
E. What are the disadvantages of Tung oil finishes?
As a finish, it is still in its early stage and very few flooring companies, if any, offers Tung oil finishes for residential flooring. While Bona Mega takes 7 days to cure and reach Greenguard air quality target, Tung oil takes longer to completely cure. It takes about 15~30 days, depending on the ventilation and temperature, for Tung oil to completely cure. It is possible to walk on it with soiled shoes after 7 days. Tung oil also has a faint nutty smell which will exist for weeks or months. Some people may not like that kind of smell.
3. My Own Experience
In the end, it comes down to a choice between Bona Mega and Tung oil. Even Mega carries Greenguard Indoor Air Quality certificate, it still falls short of Children and Schools certificate requirement. To make it as safe as it can be, I decided to go with Tung oil.
Very few places sell Tung oil and tell you in details how to apply it. I checked with my flooring contractor who has extensive experience with Bona products. He currently does not offer Tung oil finish and have never used it. So, I have to be sort of on my own to figure out how to apply Tung Oil to the floor.