My father has cleared out several black locusttrees, and has tasked me to find out how to turn them into fence posts. He said he remembers his Grandpa making posts by covering cleaned logs in sort of oil and then getting them very hot. I know nothing of the process, but am eager to learn and am having very poor luck finding resources. If anyone can enlighten me or direct me to a helpful resource, I’d be very grateful.
Just summarizing my understanding is that you char the surface of the wood and then scrap off nearly all the char. What's left behind is a layer of fire hardened wood that between the heating process and the mechanical process of scrapping has now been better sealed against the elements. Go ahead and read the thread. There's more in depth discussion of the process as well as the traditional name for the technique. It's been a bit of a decorating fad recently, so armed with the correct terminology, you will probably be able to find more detailed information on how to do it yourself.
Black locust heart wood needs no treatment, but you can char the end that'll go into the soil which will help.
I'm not quite a lumberjack, but that's OK, I sleep all night and I dream all day; I'll coppice trees, I'll grow my food, and compost poo and pee! With a well and off-grid solar, it's a permies life for me! https://www.youtube.com/watch?v=FshU58nI0Ts
Companion Planting Guide by World Permaculture Association