Tonight I did some mudding, smoke a ball of Le Petit Robin and then mix the ash with saliva to make a coating of mud for the bottom of your pipe.
I can't say I enjoy smoking Le Petit Robin at all other than that it burns absolutely effortlessly.
Given that I almost never smoke to the bottom of the bowl it's highly unlikely that this coating will ever be of practical value other than some peace of mind, except for my Peder Jeppesen Stack (not shown) that had some cracking above the draft hole that is now smoothly mudded over.
Here's a Chacom Canadian that now has a nice uniform matte black coating.