Unlike disposable hand warmers, these hand warmers are reusable. They’re small plastic pouches with clear liquid and a small metal disc inside. To release the heat, flex the metal disc. Hey, presto! The liquid will turn to crystals that give off heat. The heat lasts about 45 minutes, or longer if you have it in your pocket. To reset the hand warmer, boil it in water for about 10 minutes, until the crystals turn into liquid again. You can boil several at once.
How does the hand warmer work? The liquid in the pouch is made up of water and sodium acetate. When you flex the little disc, it releases tiny bits of metal. These agitate the sodium acetate, an amazing chemical, causing a chain reaction. The sodium acetate becomes crystals that release the stored heat energy. It will heat up to 54°C! You can reverse the process by boiling the pouch until the liquid is clear again.