Boiling water will work...just be careful, the heat will make it very vulnerable to breaking, especially if it is a cheap glass piece. Also you could get the highest % of rubbing alcohol you can find, 99% is good but not at every store. Pour a bit in the pipe, cover the carb, mouthpiece and bowl with your fingers and shake the shit out of it. You can also add salt, in with the alcohol, it will act as an agitator. You can use any sized grain of salt you want, bigger ones might be tougher to get out. Then rinse it out real well with hot water. If it is still not clean enough you can soak it in alcohol and shake it around occasionally, eventually it will be clean as new.