It's a modern coating of the chamber walls. With very old pipes, at the century mark or more, the surface of the briar in the chamber may not be as resilient as it once was. It may have suffered some damage from an earlier owner, or it may have been charred, so the coating acts as a sort of "instant" insulation, returning the pipe to a safely usable condition. The coating doesn't interfere with the flavors, and a little cake over it provides added protection.I’ve seen your references to these coatings in the past, Jesse. Could you please enlighten me about these coatings? Are they a modern coating, or historical? Were these coatings original to the pipes, or applied more recently as a protective measure? Are they a bowl coating, or a finish?