I have a very old book called *Lees Priceless Remedies* which is pretty much a DIY guide for the 19th century. In it is a recipe for “everlasting fence posts” which recommends you soak your posts in a mixture of raw linseed oil and pulverised charcoal. The Japanese use a technique where they char the outer face of timber so it’s basically pure charcoal. Apparently this makes the timber underneath less liable to rot or insect attack.
The most important reason is resistance to rotting. Charcoal does not biodegrade for hundreds of years. When you char the wood there is a protective layer that prevents the bit that you bury from rotting. Untreated wood lasts about 10 years in the ground, treated wood can last much longer but is expensive.
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