That ash trick works because it's basically the same deal as graphite powder, just way cheaper and you always got it on hand. I've used it on boiler flange bolts and even on the lag screws for pipe hangers when they get that white rust corrosion. The carbon in the soot fills the gaps and stops the galling before it starts, especially on that old cast iron where the threads are already worn down. Key is to use the fine powdery stuff from the top of the flue, not the chunky baked-on creosote chunks, those just act like sand and make things worse. One thing I'll add, if you're in a humid spot like Denver can get in the summer, keep it in a dry coffee can with a lid, moisture kills the whole thing.