Since humans began using wood, the material has always been subject to more or less successful methods of wood preservation. Alexander the Great is said to have soaked wood used for bridge building in olive oil. The Romans brushed their ship hulls with tar. Tar was also extensively used by the Scandinavians which makes sense, as their lands to a large extent are covered in dense pine forests. Tar is dry distilled from finely split pine wood in so called “tar kilns”. According to Wikipedia, tar has probably been used in Scandinavia since the Iron Age. It remained the most popular method for hundreds of years, and from the 14th century on, it was one of Sweden’s most important exports. Production didn’t cease until the beginning of the 20th century when the wood tar was replaced with chemicals.
Just to clarify – there are two kinds of tar. One is derived from wood, the other one from coal. When Northern Europeans mention tar, they often mean the wood variety. Southern Europeans, however (lacking the abundance of conifers) refer to the kind that is derived from coal and petroleum. When you further distill either of the tars, you get something called creosote. To this day, the wood derived creosote is used for flavoring foods and other less harmful applications. (Think smoked meats.) The more potently toxic coal-derived version became the wood preservative of choice. When the Industrial Revolution started picking up speed with new technologies and improved pressure treating processes in the second half of the 19th century, creosote took center stage with the preservation of rail road ties for the world’s nascent railroads.
With WWII, the chemical industry started to balloon. From the 1940’s on, Chromated Copper Arsenate (CCA) replaced creosote as the pressure-poison of choice, protecting the wood against insects and microbial agents. In the US, in the 1970’s, pressure treated wood went from being used in mostly industrial and agricultural applications to becoming the budding DYI homeowners’ best friend, as they contemplated building decks and garden projects. According to the EPA, most of the wood used in residential settings from that time and forward has been CCA – treated. It is definitely not good for you. Yet, it took until until December 31, 2003, when in an agreement with the EPA, the US wood treatment industry stopped using CCA. It was replaced with copper-based pesticides, and a variety of other chemicals. As far as I can tell in terms of the chemicals, it seems wood treated with borates is the way to go as it is “Not defined as a hazardous waste nor listed by RCRA, CERCLA, SARA, Clean Water Act, Safe Drinking Water Act or Clean Air Act”. However, in the EPA’s list of viable alternatives, I was saddened to see that they don’t list the one thing that made me want to write this blog entry. You see, there is something MUCH better out there…
It’s not a new idea. In fact, in its crude form, it is very, very old! Records indicate it has been used at least since the Vikings ruled the seas. The Japanese too, have a similar history of weatherproofing lumber – a technique they call shou-sugi-ban. The Finns, however, have taken it to a whole new level of refinement, and several other countries around the world, including our neighbor to the north, have followed suit. In a relatively brief search, I found one producer in Indiana, but I’m writing this from Oregon. Unless I’m totally wrong, I think Oregon’s supply of lumber far outweighs that of Indiana’s. In addition to being a big lumber state, our fair state usually prides itself on its environmental progressiveness. We really should have this kind of production here! Things may have changed since last I checked, but this past spring, when I was working on a project for which I wanted to find a non-toxic, non-plastic, non-tropical wood alternative to redwood or red cedar heartwood. I knew heat-treated lumber was being used in Europe as well as Canada, and thought for sure I’d be able to find it here. So, I called around to a few lumber companies to ask if they carried this thermally modified – or “acetylated” wood. To my great surprise and dismay, the answer throughout, was no.
The clean, easy, elegant solution I’m talking about, is to use steam and heat to seal the wood against the elements. This concept has tremendous potential. One of the great beauties of it, is that it works for all species of wood. That means that instead of using the naturally resistant woods (which, due to demand, are becoming more and more scarce), we can transform more abundant species (softwoods like pine and fir, as well as hardwoods like birch and aspen) into excellent alternatives that perform far better than untreated lumber, without using any toxins whatsoever! Wow – in my mind, I sense the potential of a total revival of the Oregon lumber industry which by expanding into this technology could get a lock on the entire western US market. How about that for green job creation??? But, I digress…
By heat-treating (essentially cooking) the wood in this manner, it becomes more durable. In the process, it looses a number of its extractives, including hemicellulose which in untreated wood would attract fungi and damaging insects. It renders the lumber less moisture absorbent (meaning it won’t rot), and makes it more dimensionally stable. The dimensional stability allows for smaller design tolerances. Another positive is that the wood’s conductivity is reduced by 20-25% which makes it a better insulator. However, as it loses its extractives, it also loses some strength and becomes somewhat more fragile. Its capacity for bending, and its modulus of elasticity is also reduced somewhat. The ThermoWood handbook does not recommend heat-treated wood for structural applications, but research is ongoing. Tests involving glulam using this wood have apparently been very positive.
The heat treatment is a two-phase process. First, the wood has to be dried. It doesn’t really matter whether it is previously kiln dried or green – it’s initial condition only affects the time and energy (thus the cost) it takes to achieve the level of dryness required for phase 2. Compared to the amount of energy used in the process of kiln-drying lumber, the method of acetylations requires 25% additional energy.
“Drying is the longest phase in the heat-treatment process. Green wood contains water in two forms: free water in cell lumens and bound water in cell walls. During drying, some of the water in the cell lumens travels via capillaries in the direction of the grain due to surface tension and steam pressure differences. If the pores between one cell lumen and another enable its free travel, water can travel several metres. Otherwise, capillary drying reaches only a few cells from the ends of the wood. The great majority of the water is removed by diffusion through the cell walls in the form of steam. This occurs through the cell lumens perpendicular to the grain…. Successful drying is important in order to avoid internal checks. Since the wood becomes elastic at high temperatures, its resistance to deformation is better than in traditional kiln drying.“
When the moisture level is at nearly 0%, the second phase begins. During the heat treating process, the wood is further heated up to a temperature of 185 – 215 degrees Celcius (or even higher), depending on the desired properties and quality of the final product. The higher the temperature the higher the durability and dimensional stability will be. At this point, the wood begins to darken in color, and all extractives drain/evaporate. The higher the temperature, the darker the color, but as you can see in the photo, the range is quite remarkable. The steam prevents the wood from splitting and from catching fire. The heat-treatment takes aprox. 2-3 hours. The cooling down process happens slowly in a controlled environment, again to prevent splitting. The finished lumber is then re-moisturized to a level of 5-7%, in order to increase its workability. There are no internal stresses whatsoever in heat-treated wood, which means no cupping, bowing, etc. As it is currently recommended that this wood not be used in structural applications, most of it is used for decking, siding, doors, windows and furnishings. Manufacturers everywhere offer warranties ranging from 25 – 60 years for their lumber. This is amazing, and puts the durability, quality, performance and resistance to decay of our thermally modified, regular, domestic softwoods is right up there with many tropical hardwoods.
I got much of the information I posted here in this document released by The Finnish Thermo Wood Association. It contains all the information you’ll ever need (or want), as well as interesting footage and diagrams. While the Finns are the undisputed leaders of this industry, there are several companies around the world using the technology to create great products. Here are a random few:
As far as workability goes, ThermoWood is compared to that of hardwoods. Sharp machine- and milling tools are needed, or tearing will occur. Pneumatic nailing, or pre-drilled holes are recommended. The sawdust is very, very fine, so using a high-performing dust extracting system is strongly suggested. As far as testing of finishes is concerned, ThermoWood lets us know that “the best coating systems for ThermoWood consisted of the priming oil and solvent-based alkyd or water-based acrylic topcoat.” Because of its reduced ability to absorb moisture, any water-based paints will take longer to dry than on untreated wood. So far, however, no problems have been reported. Untreated, ThermoWood will soon turn gray. Another thing to note is that, if left unfinished, the process gives it a noticeable smokey smell which will eventually dissipate. Painting or staining the wood will seal the smell in. Even though some may perceive it as unpleasant, the vapors have been thoroughly tested. There are no toxins present.
I think it’s fascinating that by altering the basic properties of wood, it essentially can become a new material. As this little tidbit would have it: “ThermoWood has been tested as a bone substitute material (VTT & Surgical Clinic of University Hospital in Turku). Preliminary tests have shown good results: heat-treated birch has similar properties to bone. ThermoWood is sterile, and no toxic substances have been found.” I suppose we might start to expect seeing it in orthopedic clinics around the world, too!
Going full circle, I’d like to touch back on shou-sugi-ban – the traditional Japanese method of sealing wood with fire, that is rapidly gaining popularity in contemporary architecture. Traditonally used with Japanese Cedar (Sugi – which actually is a Cypress – Cryptomeria japonica), the technique can be used with other woods too. The process promises the same infestation- and moisture resistance, as well as the longevity. An article from Materia, featuring the architect Terunobu Fujimori shows it well, as does this from the blog Pursuing Wabi. Note however, that here the claim is that the burning process renders the wood more resistant to fire. I suppose that might be because the wood seems charred on one side only, but I’m only guessing. It looks to me as if the other side is left as is. If opting for the modern, highly engineered heat-treated wood, you should know that according to ThermoWood, the opposite is true. After going through the drying process, the wood is so devoid of moisture (other than the re-introduced 5-7%) that its fire resistance is somewhat lower than regular, untreated wood. Thermally altered wood ranks in fire class D.
Whether your preference is the traditional or the cutting edge – with this technology you really can have it all! I’m looking forward to being able to use this wonderful “new” material soon – right here in timber country! Really, someone’s got to catch on soon…