1. Let it soak for 1 hour (water + dishwashing liquid).
2. Scrape off the deposit using a wooden spatula or spoon.
3. After emptying the bowl, scrub the bottom using the abrasive side of a sponge.
4. If there are still traces of the stain, scour with steel wool.
Instead of dishwashing liquid, you can also use lemon juice, white vinegar or a dishwashing tablet dissolved in water (leave to work as long as needed).