Hand warmer
From Wikipedia, the free encyclopedia
Hand warmers are small (sometimes disposable) packets which are held in the hand and produce heat on demand to warm cold hands.
They are commonly used in outdoor activities such as hiking and skiing to keep the extremities warm and assist insulated clothing. Other types of warmers are available to provide soothing heat for muscular or joint aches.
Some hand warmers contain cellulose, iron, water, activated carbon, vermiculite and salt and produce heat from the exothermic oxidation of iron when exposed to air.
A second type generate heat through exothermic crystallisation of supersaturated solutions (usually sodium acetate) and are reusable. These can be recharged by boiling the warmers and allowing them to cool. Heating of these pads is triggered by snapping a small metal device buried in the pad which generates nucleation points which initiate crystallisation. Heat is required to dissolve the salt in its own water of crystalisation and it is this heat that is released when crystalisation is initiated.
A third type uses lighter fluid (lighter fuel) or LPG which is reacted with a platinum catalyst to release heat by oxidation reactions. These can be used on many occasions by simply refuelling.
Depending on the type and the source of heat, hand warmers last between 30 minutes (recrystallisation) to 12 - 24 hours.
