I've noticed that some of the test tubes included in the various test kits I occasionally use have become dirty with use. They appear to have a bit of a hazy build-up on the walls of the tubes. Does anyone know how best to clean these? I always rinse them with fresh water after use, but it does not appear to be sufficient to keep them clean. Can they be run through a dishwasher? Would boiling them cause any problems? Your insight will be appreciated.