@rin@post.ebin.club you can also use bleach so long as you don't have a coating on the cups that will get eaten by bleach. A good way to tell is if it was labeled as not dishwasher safe. If it can't go in the dishwasher it probably won't tolerate bleach. (Dilute to 15% bleach by volume and scrub into the cup. Rinse it throughly. Or soak in 3% by volume.) Some dish soaps include bleach which should help prevent discoloration over time. But if baking soda is working for you it's probably a better option. you don't have to worry about damaging you teaware, discoloring designs, or poisoning yourself if your cup is too porous. Just thought I'd share!