The Heat Factory warmer is a soft lightweight pouch containing a mixture of iron powder, charcoal, salt, sawdust, and vermiculite. When exposed to the air, an oxidation process takes place that generates heat up to 130 degrees for more than 8 hours. (Bring a heavy-duty zip lock bag to halt the process for use later.) Contents are biodegradable. Unopened, the Heat Factory warmers have a shelf life of three years. Place the heaters in the mesh pockets near the lens and battery, ideally. Toss them...