What causes spiderwebbing is a bit of a controversy as it's impact on a pipe's long term health. I think SP may have softened their stance, or altered their definition of spiderwebbing, based only on the condition scale Tyler linked.
Anyway, here's what they said about spiderwebbing a few years back (this may still be on their site):
From Smokingpipes.com:
"Soft spots in the bowl, which char faster and are noted as spider webbing, are often the cause of uneven reaming if done by hand. If using a knife, these softer areas could concave and cause a bigger problem in the future, so great care must be taken to ream the chamber evenly."
Also this:
"Spider webbing is from a 'hot-spot' where the coals were well on their way to a burn thru in the bowl. It's a weak area now and can be mudded over or filled in but best is to smoke cool (as cool as you can, so even char, gentle even light and easy does it sippin') may render a cake forming over the area. Usually can be found at the bottom around and just above the draft hole."
And this:
In general, we use both "Spider webbing" and "charring" to refer to mild damage that has been done to the inside of the chamber by smoking the pipe too hot. It's up to the discretion of the person evaluating the pipes after they've been cleaned to select which term best describes the condition of the pipe. For my own part, I think of "spider webbing" as surface cracks in the wood. I think "charring" is more applicable when a small piece of wood has become a wee bit soft due to overheating. We always do our best to disclose these sorts of issues with estate pipes, and the condition is reflected in the price. I have purchased estate pipes where we had noted either spider webbing and charring and haven't had a problem. We reject all pipes that we believe are at high risk of burnout. And if we don't discover a serious problem until after we've already purchased and cleaned the pipe, they get tossed into the infamous Pipe Science box. (30 years of unreamed cake covers a multitude of sins.)
If you tend to puff a little too zealously, you might want to pass on an estate that has any sort of heat damage in the chamber. But as long as the pipe is smoked gently until you build up a cake of your own, spider webbing is usually not a cause for concern.
Hope that helps.
Joshua Burgess
Smokingpipes.com
Cigrmaster, who's opinion I have great respect for, had this to say:
...is correct that spider webbing is on the inside of the bowl. I would never go near a pipe that has it. I think that people selling these things are doing a disservice by not explaining exactly what it is and how the integrity of the pipe has been compromised. I would also stay away from any pipe that is described as over reamed, that can mean so many things and again the pipes integrity is questionable. There are so many estate pipes that are in great shape, there is never a reason to take a chance with one that has those kind of issues.
If it was my pipe I would smoke it gently even if fully caked, and enjoy the heck out of it until it decides it's time to move on to the next stage of pipe-volution.
