Cleaning Dark Coloring In Metal Vessels

Add the following and heat in a cooker. Soak overnight. (Don't forget to immerse the stained metal vessel in this mixture inside the cooker).

1. White vinegar
2. Lemon
3. Salt
4. Hot water
5. Club soda or baking soda
6. Onion outer layer
7. Mild detergent

Or heat with warm cola (aerated drink)

Additional Reading

1. https://recipes.timesofindia.com/articles/kitchen-hacks/8-ways-to-clean-tough-burn-stains-from-utensils/photostory/63985942.cms
2. https://www.boldsky.com/home-n-garden/improvement/2014/ten-easy-ways-to-clean-a-burnt-vessel-039687.html
3. https://food.ndtv.com/food-drinks/1693827