"And some MM cobs are left uncoated and take on a more natural look and seem to breathe better."
Pipes, whether they be cob or briar do not breathe. Yet another myth passed from one to another :roll:
Think about how thick the walls of a pipe bowl are and of course take into account the density of the material involved. There is no way that air is able to pass from one side to the other in any perceivable quantity.
Besides, during combustion a certain amount of water is released from the tobacco, some of which is absorbed by the bowl thereby making 'breathing' even less possible. This is why bottles of wine are stored on their sides to keep the cork moist therefore preventing the ingress of air over time.
Regards,
Jay.