You can soak in raw vinegar and wash it all the time, and then gently grind it off with matte paper after 5 minutes. Be careful not to wash the raw vinegar for too long, otherwise it will be harmful to the bedside, and the sooner you grind it, the lighter it will be. After grinding off a large area of debris, wipe off the residual impurities with a rag stained with salt, and then scrub it with clear water.