About 15 years ago, forest product folks in Finland developed a high-temperature processing treatment for softwoods with the goal of increasing its durability. They succeeded quite well. By heating the wood from 415° to 450°F. in anoxic (oxygen-free to avoid combustion!) conditions, the sugars and other organic components of the wood break down, with the result that bacteria and mold do not grow on the fibers well at all. In other words, in exterior uses, rotting does not occur for an exceptionally long time — and, properly sealed and cared for, never. After the softwood process was successful, they expanded it to hardwoods.
Working with this heat-treated wood required a bit of adjustment. It is more brittle and the fibers on the edges break off easily ... into your hands as splinters, for example. It also needs to stay in clamps longer to achieve a rock-solid bond. Because the treatment leaves it more dense as well as brittle, drilling properly sized clearance and pilot holes for screw fasteners is a must. And use sharp tools!
But those details aside, the impressive thing about this product is that it remains simply wood. No chemical treatments to leach into the environment and no special tools are required. It is darker than non-treated wood of the same species, as you can see in the photo. Cost? It is only a dollar or so per board foot more expensive than its non-treated counterpart. If you would like to try it on this project, it’s available here at rockler.com.