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Review of thermo & charred wood's treatment methods

Updated: Dec 21, 2023

Here we will have a closer look at two often mixed, conversely even thought these two techniques are the same, thermally modified and Charred (aka Shou Sugi Ban) wood. Thus in this article, we will have a closer look and discuss what and how each method preserves wood. Hopefully, after reading this article you will have a better understanding of the similarities and differences between these two techniques.

 

Thermo wood

According to the G Wood Pro, thermal wood is a kind of wood which was treated to a high temperature. The temperature during the treatment is preset to 390-572 Fahrenheit which is equivalent to 200-300 celsius. Thermally treated wood as a building material is popular for sidings and deckings. It is popular due to its durability and ability to handle all kinds of climates, without changing its shape or being subject to rot or decay.

According to Pelaez-Samaniego, M.R. et al., thermal wood could be treated in two ways, moist and dry environments. Moist environments are when the wood is treated at a pre-determined temperature with heat and steam, while dry environment contains no water vapour, (however there could be some moisture in the wood) wood is treated by nitrogen or air.

During the treatment, wood’s chemical composition is changed (Pelaez-Samaniego, M.R. et al.) and its

Charred Wood
Figure 1. Thermo wood (G Wood Pro, 2021)

characteristics are changed irretrievably. Changes of such treatment are the removal of hemicellulose and the degrading of cellulose, components which both contain sugars. In other words thermally treated wood has no sugar in its characteristics, which is the source of food for mold and bacteria, therefore wood becomes rot and decay-free.

Nolan stated in his article, that thermally modified wood could last for 25 years, this kind of wood is classified as resistant to rot. Moreover, as Hasburgh, L.E. et al., suggest in their paper thermally treated wood allows limited hardwearing properties.

During thermal treatment wood is dehydrated too, which does two things, it becomes more stable to heat or moist weather conditions, meaning that it would not expand or contract, however, it also leads that this kind of product is more flammable, it can burn more quickly and easily (G Wood Pro). In addition, thermo-treated wood becomes more brittle, which could be seen both as a benefit and drawback.

Lastly, thermo-treated wood is treated chemically free, which makes this product more environmentally friendly. In addition, all of the benefits of thermal wood can not be ignored, you have to take into consideration that it makes siding look better, compared to regular wood.

 

Charred wood

While thermal wood treatment is in the preset temperature environment for an extended period, charring wood, on the other hand, is exposing wood directly to fire and creates the cover of char on the surface of the planks. Whilst thermal modification is done on the whole lumber plank, Shou Sugi Ban method modification is done on 2-3mm of the surface.

In their paper Ebner, D.H. et al. explain that pH values increased significantly, which indicates that wood treated by the Shou Sugi Ban method reduces wood decay fungi. In addition, many sources are claiming that charred wood is just like thermo wood rot resistance.



Wood is liable to a 3-5mm charred layer, when you are using a traditional method of burning, depending on the time and the temperature of charing. Own experience is also suggesting the same amount of charred layer, it also depends on the depth of brushing (if brushed), however, once wood charring in the stove and brushing is done, the plank could change its measurements for up to 5mm.

Charred wood application to fences, cladding etc. is a rather new concept in the western world, however, the philosophy of charring wood to prolong its life expectancy is used at least since early 1900. According to Hasburgh, L.E. et al., in the US pupils were charring the end of the fence poles. The same is true, at least in the northern part of Europe, as my colleague Vilmantas always shares the same experience and explains, that he and his grandfather were building a fence in the village and his grandpa said: “We need to charr the end of the fence poles to extend their life expectancy”. Thus this philosophy is long live in pupils' minds, especially ones who were living closer to nature.



As was discussed earlier thermo wood makes your siding look more attractive, however, it is miles away, when you compare it to charred wood. Charred wood depending on the kind of wood, could open magnificent wood paths, which are unique too. In addition, charred wood could last, well maintained, for up to 80-90 years (Lyon, 2021), which is three times more, compared to thermo wood.


 

Conclusion

To conclude, there are some similarities when you compare thermo and charred wood. Both of these methods are treating and changing some properties of raw wood, moreover, this is achieved without using any chemicals, which means that you are getting an environmentally friendly solution.

On the other hand, there is a key difference when it comes to these two wood preservation techniques. The key difference in my opinion is the life expectancy for the wood. Charred wood is claimed that could last for up to 90 years, compared to 25 years of thermo wood. Furthermore, the other difference is fire resistance, while thermo offers little to no fire resistance, charred wood is concrete and more fire resistant compared to raw wood planks (Rose, 2021).

I guess in some cases you would have more benefits in using thermally modified wood, however in my opinion, the best of all building materials is charred wood, which has many benefits with minimal disadvantages. This material has extensive track history, according to Lyon, even today in Tokyo you could find homes built 120 years ago, using traditional Japanese wood-burning technique.

 

Reference

Ebner, D.H. et al. (2021) “Surface modification of spruce and fir sawn-timber by charring in the traditional Japanese method—Yakisugi,” Polymers, 13(10), p. 1662. Available at: https://doi.org/10.3390/polym13101662.

Hasburgh, L.E. et al. (2021) “Durability and fire performance of charred Wood Siding (Shou Sugi Ban),” Forests, 12(9), p. 1262. Available at: https://doi.org/10.3390/f12091262.

Lyon, S. (2021) 5 reasons Yakisugi is amazing, according to an expert, The Spruce. The Spruce. Available at: https://www.thespruce.com/japanese-yakisugi-technique-explained-5204149 (Accessed: December 16, 2022).

Nolan, C. (2021) 4 things you should know about thermally modified Wood Siding, Ipe Decking. General Woodcraft, Inc. Available at: https://www.mataverdedecking.com/blog/4-things-you-should-know-about-thermally-modified-wood-siding (Accessed: December 5, 2022).

Pelaez-Samaniego, M.R. et al. (2013) “A review of wood thermal pretreatments to improve wood composite properties,” Wood Science and Technology, 47(6), pp. 1285–1319. Available at: https://doi.org/10.1007/s00226-013-0574-3.

Rose, A.A. (2021) Controlled burns: 7 deftly detailed charred wood residences - architizer journal, Journal. Available at: https://architizer.com/blog/inspiration/collections/charred-wood-residences/ (Accessed: December 16, 2022).

Thermally Modified Wood Pros and Cons (no date) G Wood Pro. Available at: https://www.gwoodpro.com/thermally-modified-wood-pros-and-cons (Accessed: December 5, 2022).

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