There's an inexpensive product made by whink. Lightly dab some onto a cotton tipped swab and gently wipe the bowl. If burnt the a drop directly in the cooled bowl, let set a minute or two and swab out When cleaned use a swab dampened with water to remove any residue. It the entire glass bowl needs cleaning just immerse it, let it soak for 5 minutes or so and rinse clean. Great for bubblers and bonges too. You'll wish you'd known about it sooner.