Thermally Modified Wood – Thermal Wood Treatment – Heat Modified Wood and Heat Treatment of Wood are all essentially the same thing for the purposes of this article.
Heating wood for a long enough period in the proper way can improve the woods performance and appearance significantly.
Air Heat Treatment
Oil Heat Treatment
These are the two main methods of heat treating wood. Of the two I prefer the Oil Heat Treatment because it doesn’t need to be heated for as long a period of time. It also introduces a greater degree of water repellency throughout the wood. Oil heat treatment is done in a big stainless steel vat that can treat thousands of board feet in one pass.
Air Heat Treated boards are more readily available. This is a good choice for deck boards and siding.
REALISTIC EXPECTATIONS: No wood or composite wood will be immune to the elements. So if it’s used outside, eventually it will need to be coated. This principle has held true throughout time. Just in the last 100 years we have seen numerous products marketed as “no maintenance”. There may be some high quality metals that actually achieve this but nobody has them on their house, unless you live in the International Space Station!
Asbestos was great but still needed painting. Aluminum siding was great but still needed painting. Vinyl Siding was great but still stains and needs painting and mostly ends up looking horrible after a while. Composite wood (plastic, yuck!) was never great but still needs painting. Ipe and exotic hardwoods are great but still need finishing. Lots of things have come and gone but nothing can change the fact that exterior wood is going to face the natural laws of degradation and regeneration. Life cycle isn’t something to be mastered but it can be worked with, sustainably.
Thermally treated wood is a great way to work with the natural lifecycle of wood. Local woods can be used for interior and exterior purposes with great results.
Not just any oils can be used for oil heat treatments. Using a refined linseed oil is probably the best, although I have personally had even better results when a well filtered beeswax is introduced. This requires expertise since beeswax will harden when it cools. That could be a problem for mass scale operations that require a cool temp flow rate. So, all in all, a high grade, well refined linseed oil is probably the best. Using a low quality oil would probably do more harm than good.
There is a ton of great information available but here is a paper from Germany looking at air vs oil heat treatment methods using a scientific method. oil-heat-treatment-paper
On a personal level, I use a deep fryer outside to cook small wood pieces in Special Linseed Oil or Looking Glass Beeswax Polish. This has worked outstanding for my wooden utensils, bowls, spoons, cutting boards and such. I can throw these things in a sink full of water and the wood stays intact. I even put them in the dishwasher and they still hold up.
Something great happens cooking wood in oil. My most frequently used piece is a cherry spatula. I use this to cook meals all the time and it winds up with eggs and sauces on it every week. I put it in the dishpan to soak and sometimes leave it there for a day or so. It’s worn but still in good shape. When it soaks I can see that the wood cells are protected. This spatula was a gift and appeared to have some sort of oil on it but it quickly began to soak up water and twist. So I cooked it for a couple hours in Looking Glass Beeswax Polish. That was 8 years ago!