Due to small particles, soda not only softens plaque, but also mechanically cleans the thermos from the inside.
To avoid scratching the coating of the inner flask, we recommend diluting soda with water rather than using it as a dry abrasive.
Pour 4 tablespoons of soda into a liter thermos at the rate of 1 spoon per 250 ml.
Fill with hot water and close the lid. Shake and leave for 3-6 hours.
Shake again, drain the water and soda and rinse 2-3 times with clean water.
Acetic acid is another effective cleaning method.
It is necessary to prepare a solution of two tablespoons of acid and a liter of water (if we are talking about cleaning large vessels) and pour it into the flask.
Let stand for 12 hours and rinse with warm water.
Some recipes suggest diluting a weak solution of table vinegar with water in a ratio of 1:2, boiling and leaving for two to three hours, after which you need to rinse the kettle with water, rinse and boil several more times, this time using ordinary water, changing it after each boiling.
The thermos must be washed after each use.
It is best to do this right away, but of course this is not always possible.
If traces of food or drink are firmly lodged on the walls of the flask, fill it with warm water and leave to soak overnight.
Now don't use an abrasive sponge - you can damage the flask.