Thermos bottles have a vacuum space between the inner flask and outer wall that allow the beverage inside the flask to remain at a near constant temperature, whether hot or cold. Most Thermos bottles are made from a type of plastic called polypropylene, which can hold water up to 100 degrees Celsius, which is the boiling point. Although it can hold boiling water, you must follow a specific method for boiling the water in a Thermos to avoid melting the bottle.
Fill the Thermos flask with water until overflowing to ensure no air remains in the bottle.
Screw the lid on tightly without squeezing the bottle. This will ensure no air enters the bottle.
Put on oven gloves and clasp the neck of the bottle with metal tongs.
Hold the base of the bottle 2 inches above the flame of a fire.
Rotate the bottle gently while maintaining a 2-inch distance above the flame.
Remove the bottle from the heat once you can hear and feel the water inside bubbling consistently. This can take over 25 minutes because the vacuum between the walls of the flask makes it difficult for heat to penetrate the liquid.
Remove the lid of the bottle carefully while still wearing oven gloves. Hold the bottle over a mug or other heat resistant container as you do this to avoid spillage.
Pour the hot water immediately into the heat resistant mug or container and use as desired. Leave the lid off the Thermos bottle so that the bottle may cool down.
Tip the Thermos bottle of water upside down before heating it to check for leaks. The bottle will melt if there is any air left inside.
Always use extreme caution when handling hot items and dealing with fire. Have a bucket of water handy in case of emergency.