Thermos flasks are quite hard to clean as the main problem is you cannot get access to the inside.

Here are a few ideas that will help you get your thermos sparkling again......

1. Some people do have success with denture tablets. However this depends on the type of tablets that you use. What can be done is pop two denture tablets in the flask with hot water, close the lid and let it soak overnight. Rinse out in the morning

2. Alka Seltzer tablets. Pop 4 tablets into the flask and fill with warm water, leave it to soak for an hour or so. Then rinse out.

3. Bicarbonate of Soda (Baking Soda). Place 3 teaspoons of baking soda into the flask and then fill with boiling water. Leave to soak for 30 minutes and then rinse clean. This will also remove any smells that have accumulated within the flask.

4. Pour a tablespoon of salt into an empty flask and drop in a few ice cubes.  Close the flask, and swirl it so the salt and ice scour the entire inside. Pour out the ice and thoroughly rinse.

In addition to these tips you can also try the following that I personally have not tried. Save your egg shells and dry them out in the oven for a few minutes. Crush them up and pour into the flask with any of the above tips and shake. The shells will help break up any encrusted residue or stains. I have done the same with little stones however you have to be careful as these can break the glass inside the vacuum.

Another piece of essential kitchen equipment is a bottle brush. You can get them anywhere where they sell home wine making equipment. The bottle brush is excellent for scrubbing out the inside of thermos flasks.