A couple of weeks ago, I tried to answer a question about charring the ends of fence posts to preserve them from rot:
There’s a long history of this practice, and it makes sense on the face of it since microbes can’t eat char. An impervious layer of char surrounding the vulnerable wood of your fence post would theoretically provide a protective barrier and preserve the post. Of course it is more complicated than that as I found after a bit of research.
The most interesting tidbit I came across was a post on the permies.com forum from a self-declared Georgia hillbilly describing how his grandfather had treated fence posts:
I know he used oak b/c that's what he had, and I know he burned them to keep bugs out (termite, and whatnot) and I know he soaked them in oil and salt with a coat of red clay painted on after then reheated to harden the clay but that's all I know.
From this, it would seem that charring alone is not enough. The oil, salt and clay are important too. Exactly how and why this works is explained by my friend Stephen Joseph.
Dr Stephen Joseph, professor in the School of Materials Science at the University of New South Wales, Australia, is one of the first people I met in the biochar world, back in 2007 when I attended the first biochar conference in Australia where the International Biochar Initiative was formed. Stephen Joseph and fellow scientist Johannes Lehmann have been the leading lights in biochar for over 20 years now, and their co-edited book, Biochar for Environmental Management is now on its Third Edition. If you want to deep dive into the science of biochar, this book is for you.
Dr. Joseph also has another new book, co-authored with Paul Taylor, that is a more practical guideline for farmers on making and using biochar: A Farmer's Guide to the production, use and application of biochar. Find this invaluable guide here at the Australia-New Zealand Biochar Industry Group website.
Dr. Joseph has always been more than generous with sharing his knowledge and I was very pleased to hear from him once again with a detailed explanation of how the carbon, minerals, and heat can work together to keep your fence posts from rotting. He asked me to share this here, and to invite comments and questions about this explanation. Thanks so much Stephen! I will definitely try this at some point myself.
Note that in his diagram below of the fence post pyrolyzer, Stephen shows a pipe coming off the fire box where smoke can be collected to condense into wood vinegar. I am in the process of making a wood vinegar kiln, and as soon as it is safe to burn this fall, I will be sharing my results with you here. I use a lot of wood vinegar in my garden for pest control and as a bio-stimulant. It’s great stuff and it’s great to know that we can learn to be more self-sufficient by reviving some of the hard-earned knowledge that our ancestors used to improve their (and by extension, our) lives.
Sequestering Carbon and Increasing the Lifetime of Fence Posts — A Great Regenerative Farm Practice
By Stephen Joseph
This is a modified method taken from a farmer’s method from Georgia for treating oak fence posts with biochar and clay. The process can be followed and understood as follows:
Step-By-Step Treatment Process
1. Charring the wood (making biochar on the post). The farmer partially burned or “charred” the oak fence posts by controlled burning (not full combustion). This can be carried out by cutting a hole in a drum that is approximately the same dimension as the hole where the pole is to be placed. A box is placed around the drum. A fire is ignited under the drum (Figure 1). The pyrolysis must be carried out slowly and this creates a hard, carbonized outer layer (biochar-like) on the wood surface, which:
Repels termites and insects by creating a less palatable, dry, and hostile surface
“Case hardens” the wood surface, helping resist rot and moisture ingress
2. Soaking the posts in oil and salt solution. After charring, the posts were soaked or soaked and applied with oil (often linseed or other natural oils) and salt water. This treatment:
Penetrates the wood to add hydrophobicity (oil) and antimicrobial protection (salt)
Helps preserve the wood internally, deterring fungal growth and insect attack
Salt may help in moisture regulation and assists in hardening the post surface
3. Applying a coat of red clay The posts were then painted or coated with a layer of red clay, usually sourced locally:
The clay acts as a physical sealant, filling pores and cracks on the post surface to block moisture and insect entry
When still wet or plastic, the clay adheres well to the biochar-coated wood, creating a dense barrier
4. Reheating to harden the clay Once the clay coating is applied, the posts were reheated gently (near a fire or sun-dried with heat), causing:
The clay to dry and harden firmly, cementing the coating to the wood and biochar surface
A durable, weather-resistant, and abrasion-resistant shell forms around the post
This hardened clay layer further protects the post from rot, moisture, and pests
Why It Works
Additional Notes:
Using oak wood is beneficial due to its natural hardness and resistance, which complements the treatment.
The red clay is likely rich in iron oxide (hence the color), which may also contribute some natural antimicrobial effects.
This method predates modern chemical preservatives but cleverly uses thermal modification, hydrophobic oils, salt antimicrobial effects, and clay sealing to extend fence post longevity.
The biochar surface formed by controlled burning shares similarities with modern surface charring methods proven to improve wood durability.
Please, if you want to learn more about practical biochar tips and tricks for growing your own food, or you are interested in stewardship biochar for restoring natural ecosystems and biodiversity, check out some of my links below:
The Biochar Handbook at Amazon.com
The Biochar Handbook at Bookshop.org
Purchase Kelpie's Practical Biochar Course
Order your Ring of Fire Biochar Kiln®: RingofFire.earth
Check out my YouTube channel: www.youtube.com/@KelpieWilson
Biochar On Site network for practitioners of stewardship biochar
Thank you! Such termite problems here but I want to add that with our super hard Timbers in central Victoria another method is simply (though some labour required) remove the sap wood and soft wood so only the hardship wood remains. Usually with a draw knife.
This has been incredible for me and only has to extend 100 mm above surface level when post in ground.