Reusable hand warmers are small plastic packets filled with a clear gel and a metal disc. The gel is a supersaturated solution of sodium acetate --- a safe, food-grade salt. The solution stores energy like a battery, and clicking the disc causes the solution to crystallise and releases the energy as heat. Unlike air-activated hand warmers, reusable gel hand warmers can be renewed for reuse by reheating in boiling water. Campers and hikers can renew hand warmers in camp, while sports fans may want to have several recharged hand warmers at home.
Place a dishrag in the bottom of the pan of water.
Heat the pan of water to a steady simmer or a gentle boil.
Gently place the hand warmer in the pan of water, resting on the dishrag. Use tongs. Be careful to not splash yourself.
Boil the hand warmer for five to 15 minutes until all the white crystals are gone. Once you are absolutely certain you cannot see any crystals, leave the hand warmer in the water for another minute.
Remove the hand warmer from the hot water, using the tongs. Lay it on the dishtowel to dry. Turn off the heat.
Check that there are no crystals in the gel. If there are any white crystals present, the gel may reactivate.
Store the hand warmer in its clear-gel state.
If you have hard water, use bottled or distilled water to prevent build-up on the outside of the hand warmer. Some hand warmers can be recharged in the microwave. Follow the manufacturer's instructions carefully.
Keep the water at a simmer or a gentle boil and remove the hand warmer once it is recharged. Overheating the hand warmer could cause it to explode. Check the hand warmer for weak spots and leaks regularly. Do not try to use or renew it if there is a leak.