[header] How to clean a vacuum thermosflask that has stains at the bottom [title] Put some baking soda and vinegar into the bottom of your flask. [step] Pour about half a cup (120ml) of distilled white vinegar into your stained flask. Then, shake in approximately one tablespoon (15ml) of baking soda for every cup the flask holds. 
The vinegar and baking soda mixture will froth up, so be sure to put the flask in the sink when you combine them. [substeps] Baking soda and vinegar are excellent at killing germs and wearing down stains when acting together.