Hand warmer

A pair of air-activated disposable hand warmers, US quarter to scale

Hand warmers are small (mostly 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 extremities warm and assist insulated clothing. Other types of warmers are available to provide soothing heat for muscular or joint aches.

On one side the packets often have adhesive strips so they can be stuck to the underwear (never directly onto the skin as this can cause burns) to keep the lower back warm, for example. There are also types that can be inserted into the shoes or stuck to the socks.

Depending on the type and the source of heat, hand warmers last between 30 minutes (recrystallisation) to 12–24 hours (platinum catalyst).

Types

Air activated (iron)

Some hand warmers contain cellulose, iron, water, activated carbon (to speed up reaction), vermiculite (water reservoir) and salt (catalyst) and produce heat from the exothermic oxidation of iron when exposed to air. This type of heatpads normally lasts from 1 to 10 hours.

Crystallization-type hand warmer with scale showing metal disc trigger

Short clip showing the activation and crystallisation of a crystallisation-type reusable hand warmer.

Supersaturated solution (crystallization-type)

A second type generate heat through exothermic crystallisation of supersaturated solutions (typically sodium acetate) and are usually reusable. These can be recharged by boiling the heaters and allowing them to cool. Heating of these pads is triggered by flexing a small metal disk in the pad, 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. (More at Sodium_acetate#Heating_pad)

This type typically has a shorter heat duration of 20 minutes to 2 hours.

Lighter fuel warmer

Lighter fuel

Another 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.

Battery

There is also a battery operated handwarmer. In this type, electrically resistive heating devices are used to convert electrical energy in the battery to thermal energy. Some use disposable batteries, but others are rechargeable, like cellphones, and can be used for many hundreds of cycles with the same battery. This type is not disposable. Typically they can last up to 6 hours. One of the main benefits is that there are no fumes or other mess with this type.

Charcoal

One of the older and perhaps less used types of handwarmer are made hot through the burning of charcoal in a special case. These can last up to 6 hours and become comfortably hot. The cases for these usually have felt on the outside and have materials inside that do not burn, but spread the heat evenly such as metal. To activate, one or both ends of a stick of charcoal are lit and then quickly extinguished to create a hot ember. The smoldering stick is then placed inside the case and the case is tightly shut. The charcoal sticks are available from most outdoor activity shops and are fairly inexpensive.

See also

• Instant cold pack
• Heating pad
• Hot water bottle
• Sodium acetate (commonly used in exothermic crystallisation hand warmers)