|Use the heat when you need it. Photo: Heat Factory|
First, some basics. Most disposable hand warmers contain a mix of iron, water, activated carbon, vermiculite, cellulose, and salt. Once exposed to air, the iron oxidizes and releases heat in the process. After all the iron has reacted, the hand warmer is done and ready for the trash.
And therein lies the crucial piece of information. In order for the reaction to occur, the hand warmer needs a supply of oxygen. If you cut off the supply, the chemical reaction ceases. And to cut off the oxygen, all you need to do is put the hand warmer in an air-tight zip-lock bag.
Once the small amount of residual oxygen in the zip-lock is used up, the hand warmer is essentially put on pause until you re-expose it to the air. (To minimize the amount of oxygen in the bag, zip it nearly closed, suck out the remaining air with your mouth, and then quickly seal it shut.)
I recommend using thicker, more durable freezer zip-lock bags for this purpose, especially if you'll be stuffing it in your backpack when not in use. Even a small hole can let in sufficient oxygen to keep the reaction going and render your hand warmer cooked.
Keep in mind that this technique doesn't actually cause the hand warmer to produce more heat—there's a finite and specific amount of potential energy contained in a hand warmer that can't be increased. It just allows you to use a hand warmer over a longer time period rather than in just one continuous use.
“Equipped” is an AMC Outdoors blog, written by Matt Heid.