This type of re-usable heat pad has been around for a good few years now. The science is based around an old chemistry laboratory experiment with salt and water that was assured to wake up even the most bored of students.
The simple hand-warmer pads looked at here are something a little different from the commonly found one-time use pads that promptly become a waste product. These pads are based around a saturated solution of sodium acetate. This solution is non-toxic (it is used in food) and sealed in a soft flexible pad.
Salt- or Sodium Acetate (NaAc), is dissolved by the manufacturer into hot water to create a supersaturated solution. A little colour is added and it is then sealed into a flexible pouch along with a small convex/concave metal disc, The solution remains in a liquid form inside the pad until it is ‘shocked’ into crystalising. Flexing the small titanium disc with the fingers causes it to click. This is enough of a trigger to convert the nearest sodium acetate molecules into crystalline form, acting as nucleation sites that allows the rest of the solution to re-crystallize. This why this type of product is sometimes referred to as a ‘crystallisation-type’ warmer. Flexing or kneading the pad during this quick process will soften the pad and it is best to mould it at this point into any desired shape or conformity as it will later harden into an unyielding block. The formation of sodium acetate crystals is an exothermic action- heat is released as a result. This is why these small sealed silicone envelopes can work so effectively as short use, reusable heat pads.
The heat from these pads does not last long. The pads warm to as much as 130°F/54°C and will stay warm for half an hour and are still knocking out a little heat up to an hour and sometimes beyond, depending on the temperature. So pretty much ideal for a dog walk, brief hike, will warm the foot of a sleeping bag, or provide some welcome, if brief, heat on the terraces. If heat is required for longer periods, you need to look beyond this simple technology. There is plenty of choice.
The pads are ‘reset’ by placing them in boiling water and returning the sodium acetate to its previous supersaturated liquid state (an endothermic reaction). This must be fully done as the presence of a single crystal can start the process. This type of heat pad gets a lot of scathing reviews. Invariably these are due to the hardened, heat exhausted crystalline form not being completely returned to a liquid form. Just one crystal, not easily seen, can start the reaction. I have the small pads which are a doddle to reset, fitting into a small saucepan.
Larger or awkwardly shaped pads can be more challenging to reset due to the difficulty in fitting them into a pan and ensuring that all the crystals are converted to liquid form. The little pads are placed into a small saucepan and slowly bring to a boil, occasionally prodding and flipping them in the water with a wooden spatula. Keeping them off the bottom of the pan will lessen the chance of the outer covering being damaged. Once at a boil, each pad is lifted out, held in a towel (to avoid burning fingers) and shaken and moulded briefly, then placed back into the water. After a minute or so the heat is turned off and the water cools, with the pads still in the pan. Once lukewarm, pads are removed, dried off, making sure not to click the metal disc, and that is it until a heat pad is again required. If they re-crystalise on removal from the heat, they were not evenly heated throughout and the process has to be repeated, properly.
There are many uses for heat packs but I will not go into in any depth here as those people who use heat pads, or require them, are all too aware. Suffice to say that a heat pack is not simply to warm fingers on a cold day, but can also be used in sports therapy and to relieve the pain of arthritis. Alternatively, these pads can also be used as cold pads. Put them in the fridge (not freezer) until cold, them use them as you would any other cooling pad. When used as either hot or cold pads it is best to include a textile barrier between pad and skin.
This type of heat pad is not for everyone. They are not a particularly cheap product for the period of time that heat is produced, though cost is relative to how much use you get from them. The cheapest I have seen are sold by LifeSystems at less than a fiver for two. They are reusable, safe and very simple, if a little involved to reset for reuse. These pads come in various sizes from various manufacturers. No doubt quality and effectiveness will vary. Note that the saline solution also has some added colouring and will make a sticky mess should the tough silicone outer covering split. The ones shown here were made in China and imported by H&Z Corp. Ltd. I purchased mine from a UK distributor. That is by no means any form of endorsement.
Three Points of the Compass 