Aside from the percolator, the downstem is the hardest part to clean in a bong, especially when it gets clogged. To clean your glass bong downstem, you will need:
90% (or higher) isopropyl alcohol
Long medical Q-tips, incense sticks (the wooden end), pipe cleaner, or a guitar string. (Do not insert anything inside the downstem that is harder than glass. A lot of bong users broke their downstems this way)
Unclogging a downstem
Remove the downstem and pour salt and alcohol from the top.
Cover both ends and shake vigorously.
Place inside a Ziploc bag and pour a generous amount of salt.
Cover the downstem with enough alcohol to make it swim.
Shake the bag as hard as you can and let it sit for several hours.
Take out the stem and rinse with hot water. Inspect the downstem
If there’s no room for the salt to move, poke a hole through the salt and the clog using the bare end of a Q-tip, the end of an incense stick, or even a guitar string. Do this very gently. The goal is to create a hole for the alcohol and salt to pass through. Not break your downstem.
Repeat the soak and shake process until you have most of the gunk out.
Use the smallest, most flexible pipe cleaner you can use and gently scrub against the walls.
Give it one final pass in salt and alcohol. Rinse thoroughly with warm water until you get all the alcohol out.
A dirty, clogged downstem
I have a fixed downstem
Here’s what you can do to unclog a fixed downstem:
Pour salt through the bowl of the downstem. Leave enough room for the alcohol.
Add alcohol and let the solution soak for 10-30 minutes.
If there’s no room for the salt to move, do as procedure #7-10 above.
Some people create negative pressure by sucking through the mouthpiece of the bong after alcohol is poured in. It’s a risky action to take, especially if you happen to inhale the alcohol fumes instead of sucking out the air to create a vacuum. This is not advised, especially if you have problems with fumes.
How about boiling the downstem?
Boiling the downstem can help get all that gunk out of it. But! It can shorten the lifespan of your downstem, especially if it’s made out of cheap glass. High-grade borosilicate glass may withstand the high temperatures, but it’s not recommended so do so with caution.
Prevent your glass downstem from getting clogged by cleaning your bong regularly. It usually takes 5-10 minutes once you get the hang of cleaning your bong. You can use other cleaners like Grunge Off and Piece Water if you wish to try out commercial cleaners.