Rinse the metal pipe with hot water to remove any loose debris.
Scrub the inside of the pipe thoroughly with a pipe cleaner.
Rinse the pipe again with hot water.
If the residue is tough to remove, boil some water in a saucepan. Add the metal pipe and boil for five to ten minutes. Remove with tongs, allow to cool, and scrub again with the pipe cleaner.
Rinse well and allow to dry before using.