Glycerin coils are a great addition to any bong collection, as they guarantee a cool draw. Simply place your coil in the freezer before your smoke session for about an hour and you're ready to go. The coil will remain cold for about two hours. Don't rush to inhale, take it easy and let the coil do its job. Glycerin coils are best for use with herbs. Vapor will condense in cold temperature and gunk up the coil.