If the water has become dirty, dump it down the drain. Rinse the flask and refill it with clean water and a few drops of dish soap. For severely dirty flasks, allow the soapy water to sit for a few minutes. This will allow more time for the soap to break up buildup.
Specialty bottle brushes are available at most grocery stores and drug stores. Look for these in the “baby” or “infant” section of the store. [2] X Research source If you don’t have a suitable handled scrubber, wrap or tie a washcloth to a wooden spoon. Scrub the insides of the flask with the cloth wrapped spoon.
Storing a vacuum flask while still wet or capped could cause it to smell musty. To remove mustiness, use the process described for deodorizing and cleaning with baking soda and vinegar. [3] X Research source
Most normal sized flasks will only need about an inch (2½ cm) of vinegar in the bottom of the flask to be effectively cleaned. Distilled white vinegar can be used in place of apple cider vinegar. However, apple cider vinegar is more acidic, making it more suitable as a cleaner. [4] X Research source The reaction between the vinegar and the baking soda should cause fizzing and bubbling. This is completely natural. Wait until this reaction has stopped before going further. [5] X Research source
To boost the cleaning power of the soda/vinegar solution, let it sit in the flask for a few minutes before giving it a thorough shaking. [6] X Research source
Capping the flask too soon could cause pressure to build up from the baking soda/vinegar reaction, which may damage your flask or cause the top to suddenly pop free. [7] X Research source
In place of uncooked rice, you can substitute crushed eggshells. Boil an egg to clean the shells. Peel the egg and crush the shells with your clean hands. Add these to the flask in the same fashion and amount as uncooked rice. [10] X Research source
When clean, air dry the flask for about eight hours or overnight, then store the flask uncapped. This will prevent your flask from developing an unpleasant odor. [12] X Research source
For the best results, keep the solution in the flask for at least eight hours or let it sit in the flask overnight. This technique is also especially useful for removing difficult stains and stubborn odors from flasks. Take care to completely remove the Oxiclean from the flask. Remaining cleaner may cause stomach irritation, sickness, or a bad taste. [13] X Research source
Denture tablets are formulated to cleanse ingrained stains, making them useful for flask cleaning. Denture tablets will likely fizz and foam quite a bit. To prevent overflowing solution from making a mess, you clean flasks with this technique over a sink or outside. Flasks with stubborn buildup may require a little scrubbing action. In the same fashion described for general flask washing, scrub the inside of the flask with a toothbrush, bottle brush, or washcloth. [14] X Research source
Be careful when using harsh cleaners on your flask. In some cases, these may cause damage to your flask. Harsh cleaners can be harmful when ingested. To prevent indigestion, sickness, or a bad taste in your flask, be sure to rinse the flask well.
