The Science of Cracking Glass
Cold glass introduced into hot water is a recipe for shattering glass. Removing a hot pipe and putting it into a cold temperature environment can crack the glass as well. Any drastic temperature changes from hot to cold suddenly could also shatter the glass. Glass expands when it is heated and contracts when it is cooled.
Safe Boiling Instructions
Place the glass pipe in a pan of water that just covers the pipe. Turn the burner to low heat and begin the heating process. Allow the water to heat up and boil at a slow pace so as not to heat up the pipe too fast. When you see the water begin to boil, you should also see any dirt from the pipe floating. Turn off the burner. Let the pipe and water cool to room temperature before removing the pipe.
Alternative Cleaning Solution
Instead of boiling the glass pipes, you can use alcohol, Oxi-Clean or any eco-friendly substances designed to clean germs and bacteria. Use a pipe cleaning brush with a long handle and bristles at the end. Dip the brush in the cleaning solution and then dip it into your pipe. Rinse the pipe with hot water.