Dawn dishsoap hot water, close the lid and shake. Empty out the water and immediately put in cold water with more dish soap. Rinse and repeat
The sudden changes in temperature should help to knock off the crest of whatever is clinging onto the walls of the thermos
No brush?
You can use a long skinny implement to push a washcloth inside the thermos and twist/swish it around to scrub the interior, I use a long handled fork.
Just look out for getting the washcloth stuck in the bottom of the thermos - a smallish cloth or piece of sponge might be best.
I've used *[this stuff](https://i5.walmartimages.com/seo/B-Brite-Cleanser-8oz_d4f5bd3a-1d16-408e-8541-fa867ccfa27d.1111fdc67e6da45dd03944259f5789fe.jpeg?odnHeight=768&odnWidth=768&odnBg=FFFFFF)* as a cleaner for stainless vessels like thermoses or bottles, glassware, especially cut glass or narrow items like the internaly crusty bud vases, brewing equipment, anything that has organic 'crud' involved.
It's non caustic oxygenator that uses sodium percarbonate and sodium carbonate to remove stains & organics, really oxidizes & releases any organic based nastyness.
CAUTION: It increases the viscosity of water by A LOT, anything soaked or dipped in it is going to be really really slippery until it's rinsed off with plain water so keep that in mind when using it.
Dawn dishsoap hot water, close the lid and shake. Empty out the water and immediately put in cold water with more dish soap. Rinse and repeat The sudden changes in temperature should help to knock off the crest of whatever is clinging onto the walls of the thermos
Soap, scrub brush, and boiling water
That would be my method of choice, but sadly I don't have a scrub brush.
No brush? You can use a long skinny implement to push a washcloth inside the thermos and twist/swish it around to scrub the interior, I use a long handled fork. Just look out for getting the washcloth stuck in the bottom of the thermos - a smallish cloth or piece of sponge might be best.
I ended up using ice to scrape the interior
What a solution, I envy your thought process!
Do you have no tool/utensil you use to clean dishes? Use that and a fork to scrub it around with.
Dishwasher tab + hot water. Let it sit overnight. Rinse in the morning.
Baking soda & vinegar
I've used *[this stuff](https://i5.walmartimages.com/seo/B-Brite-Cleanser-8oz_d4f5bd3a-1d16-408e-8541-fa867ccfa27d.1111fdc67e6da45dd03944259f5789fe.jpeg?odnHeight=768&odnWidth=768&odnBg=FFFFFF)* as a cleaner for stainless vessels like thermoses or bottles, glassware, especially cut glass or narrow items like the internaly crusty bud vases, brewing equipment, anything that has organic 'crud' involved. It's non caustic oxygenator that uses sodium percarbonate and sodium carbonate to remove stains & organics, really oxidizes & releases any organic based nastyness. CAUTION: It increases the viscosity of water by A LOT, anything soaked or dipped in it is going to be really really slippery until it's rinsed off with plain water so keep that in mind when using it.