Hand warmer

Air activated (iron)

Air-activated hand warmers contain cellulose, iron, water, activated carbon, vermiculite (water reservoir) and salt and produce heat from the exothermic oxidation of iron when exposed to air. They typically emit heat for 1 to 10 hours, although the heat given off rapidly diminishes after 1–2 hours.^[1][2] The oxygen molecules in the air react with iron, forming rust. Salt is often added to catalyze the process.^[3]

Crystallisation-type hand warmer with scale showing metal disc trigger

Supersaturated solution (crystallisation-type)

This type of hand warmers can be recharged by immersing the hand-warmer in very hot water until the contents are uniform and then allowing it to cool. The release of heat is triggered by flexing a small metal disk in the hand warmer, which generates nucleation centers that initiate crystallisation. Heat is required to dissolve the salt in its own water of crystallisation and it is this heat that is released when crystallisation is initiated.^[4]

The latent heat of fusion is about 264–289 kJ/kg.^[5]

Lighter fuel warmer (Hakukin brand, Peacock model)

This process can be scaled up to provide a domestic heating store developed by Sunamp Scotland^{[6] [7]}

Lighter fuel

Lighter fuel hand-warmers use lighter fluid (petroleum naptha). These can be re-used by simply refuelling. Typical models can generate heat for either half a day or a whole day, depending on conditions.

Battery

A rechargeable electric hand warmer.  This device plugs into a USB port to charge its battery. It can also be used to charge or power other devices via USB.

Battery operated hand warmers use electrically resistive heating devices to convert electrical energy in the battery. Typically hand warmers can heat for up to six hours, with heat outputs from 40-48C. Rechargeable electronic hand warmers can be charged from a mains power supply or from a 5V USB power supply, with up to 500 recharge cycles possible.

Charcoal

Charcoal hand-warmers provide heat by burning charcoal in a special case. These can last up to 6 hours and become comfortably hot. Charcoal cases for these usually have felt on the outside and have items in it that do not produce heat, but spread the heat such as metal. A charcoal hand warmer can start heating when both ends of charcoal are struck and then extinguished to create a hot charcoal. The smouldering stick is then placed inside the case. The charcoal sticks are available from most outdoor activity shops and are fairly inexpensive.