Cleaning the inside of a glass carboy is a pain in the ass. Usually you've got to resort to chemical means, because you can't fit your arm through the 1-inch diameter neck to apply elbow grease and steel wool to the dried-on crud.
Or can you?
Take a small sponge, preferably one with a scrubbing pad, that easily fits through the narrow neck of the carboy, and put a slit in the back or side. Insert a powerful magnet (rare earth) such that the north or south pole is pointing at the scratchy side of the sponge.
Seal it up with thread or silicone goop to keep the magnet in place, and drop it into the carboy. Hold a second powerful magnet, wrapped in a cloth, up to the side of the carboy, and attract the sponge to the inside surface of the carboy.
You can flip the sponge around by holding the other pole of the magnet up to the glass. Drag the magnet around the outside, and the sponge will follow along the inside, scrubbing the surface clean!
If you can find a Magnetic Aquarium Cleaner that will fit inside the carboy, and handle the curved surface, you can use that too. I haven't been able to find one, so I had to make my own.
Note that rare-earth magnets can be very powerful and brittle - if they snap together they can pinch badly, or even shatter. If they snap together with the side of the carboy between them, they might shatter the carboy! The inner magnet isn't much of a threat as long as it stays in the sponge, but the outer magnet needs to be padded, too. If the magnet comes out of the sponge, do not try to retrieve it from the outside with metal or the other magnet - dip a steel wire or washer on a string into the carboy to retrieve it.
© 2010 W. E. Johns