The pipe residue contains THC and other cannabinoids, solid particles such as ash and pyrolitic compounds found in smoke. It is a concentrate of everything you inhale and more, because of the large solid particles that stick to the viscous tar.
The general consensus is that this material is not good for you to smoke. That's because of the foul taste and the compounds that may be carcinogenic or mutagenic created by the burn. They are collected and concentrated in the residue.
I wouldn't go that far. When you smoke a pipe you are inhaling the very same compounds. It's sort of like smoking a roach joint. I don't know if it makes a difference that both the compounds and the THC is concentrated in pipe residue. It takes less of the concentrate to get high.
One way of reducing the amount of ash and other impurities in the pipe resin is to soak it in warm water. The resin loses much of its odor and it looks like some of the ash falls out, too. For best results, press the resin into a thin sheet so that there is more surface-to-mass and more of the resin comes in contact with the water.
I'm not advocating using bong resin, but I know that sometimes there is nothing left and you're down to the proverbial stems and seeds and bong resin. Consider my suggestions a harm reduction strategy. If you have to use the resin, clean it up.
Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA. You can also email Ed at AskEd@cannabisculture.com, and send queries via his websites at www.ask-ed.net. All featured questions will be rewarded with a copy of Ed's new book, Best of Ask Ed: Your Marijuana Questions Answered. Sorry, Ed cannot send personal replies to your questions.