I'm a convert. Not long ago, I took to corn cobs and away from clays. It's mainly a clench thing. Why not some other material? Price, lack of ghosting, durability, no need for resting or a strict rotation. I could potentially go on but then I'd be drifting from Quick.
Not to mention the fact that it isn't my intent to address that fact here. So. What is pipe mud, and of what use is it?
Pipe mud is a material made from ground cigar ash and slowly-added spit or water* that you bring to a dry pack cement consistency and schmear into the thinning/cracked heel/wall of your cob (or other woody). A Czech tool can be used to apply the mud. A toothpick would also aid in working the mix into place.
Apply in layers and if too many layers are applied, employ an emery board, but do your level-best to avoid that scenario. Also, do be careful not to clog your pipe's stem, so if working near the draft area, run a pipe cleaner through and keep it in place while you do. Allow a couple of days drying time.
Another excellent usage of pipe mud is to lift the heel of a bowl up-toward the draft hole. I highly recommend this to potentially fix a gurgling pipe and also to most definitely allow you to smoke through all the tobacco sans leaving behind wet unlit baccy in the bottom.
::: very :::
*Distilled water if you're of a careful sort.