Yakisugi (焼杉) is a traditional Japanese method of wood preservation by slightly charring the surface of the wood to increase durability. In the West, it is also called Shou sugi ban (焼杉板).
For hundreds of years Yakisugi was primarily used in Western Japan for exterior siding and fencing on traditional Japanese houses. However, this charring technique was not transmitted to Eastern Japan.
In the Kanto region they were using black paint to achieve the same look finish.
True black color finish is rare in the world and to this day it is still not clear why this technique is only transmitted to Western Japan.
The technique of burning the surface of the wood improves the weather resistance performance, prevents decay, rot, and insect infestation, and makes the wood more resistant to fire than a non-heat-treated cypress plank.
It is often said that this authentic traditional Japanese heat treatment process is a gift from mother nature rather than a scientific construction method technique.
Since the 1970s, mechanization has progressed, and the Yakisugi boards currently in circulation have been charred by high-temperature burners.
Originally all the process was done by hand and nowadays the mechanization process is now generalized.
In Japan, it is generally recognized as for exterior use and is rarely used in interiors.