Primer: How Do Hand Warmers Work?

Jan 29, 2018

Category: Gear

Every time the weather gets a bit chilly, we manage to go out into the world with little or nothing covering our hands – only to quickly remember how uncomfortable it is to have cold digits. And whenever that happens, we find ourselves turning to the same tools year after year: hand warmers.

Only something occurred to us: while we were thankful for the little wonders, we weren’t entirely sure how they worked. Yes, we understood that it had something to do with chemical reactions, but that was about the extent of it. This year, rather than being left wondering, we took it upon ourselves to do a little research as to how, exactly, hand warmers really work. And we’ve distilled it down into the following primer – you know, just in case anyone out there was wondering just the same as us.

Types Of Warmers

There are five fairly common types of hand warmers and each of them works a little bit differently.Before we get started, it’s worth noting that not all hand warmers are created alike. Sure, there are some that are more widespread than others, but they don’t all work the same. In fact, in our research, we discovered that there are five fairly common types of hand warmers and each of them works a little bit differently. The benefit of this knowledge is twofold: it allows us to discern the risks and rewards of any one type of hand warmer and it gives us the ability to make an informed decision as to which types we want to spend our money on and which we’d rather avoid. The five most common types of hand warmers are as follows: air activated, supersaturated solution, lighter fuel, battery-powered, and charcoal. We’ve broken down how each functions, as well as some pros and cons in the following sections.


An Iron Reaction

Probably the most common of all hand warmers – and also perhaps the most interesting – are the disposable air-activated variety. These are the kinds you see while waiting in the checkout line of grocery stores in the colder parts of the world. And while they seem so everyday, they’re actually quite interesting because of the chemical reactions through which they function. You see, these little packets of warmth just waiting to be unleashed are filled with 4 ingredients that makes them work: iron powder, activated charcoal, The heat made by these hand warmers is actually a byproduct of another natural reaction: rusting.sodium chloride, and something called vermiculite. The one other necessary ingredient is air (oxygen), but it’s also the catalyst that gets the whole process started.

Believe it or not, the heat made by these hand warmers is actually a byproduct of another natural reaction: rusting. What happens is, when you remove the small microporous packets – which allow the free flow of air through their structure – from their protective packaging, the iron powder inside begins to rust, thereby creating heat. But the powder and air mixture isn’t enough to do it alone. The process of oxidation actually requires water, as well – which is housed inside the naturally porous activated charcoal. The presence of water also does something else: it acts as a conductor that allows the heat to be spread out more evenly.

Then, it’s sodium chloride’s turn. Salt – in much the same way it functions in the cooking of food – helps amplify the overall reaction, accelerating the whole process. But, the addition of salt also has the potential to speed up the reaction and its intensity by too much (we’re talking serious combustion, flames and everything). And that’s what vermiculite takes care of. This this hydrated magnesium aluminum silicate – derived from the latin term for ‘little worm’ – acts as an insulator. This is because, when exposed to heat, it expands, absorbing the excess head as it does. It’s also fire-resistant, chemically inert, odorless, and lightweight. That’s all there is to it. The biggest downside to this clever little bit of chemistry, however, is that it’s definitely a one-time thing.

Pros: Hours of Warmth, Inexpensive, Small
Cons: Wasteful, One Time Use

Purchase: $16

Supersaturated Solution

Rechargeable Pads

The main principle behind supersaturated solution hand warmers is not dissimilar to air-activated disposable ones, but for one major conceptual difference: they can be reused. The reason for this is due to the chemicals within the gel filling of this variety. The gel is comprised of a sodium acetate mixture – one capable of cooling to room temperature without crystallizing. Within each of these packets, there’s a small pliable metal disc that can be bent or “popped,” breaking off microscopic bits of metal which kickstart nucleation – the first step in a thermodynamic reaction.In order to make these reusable, the solution must be turned back into a liquid. That process causes the gel solution to crystallize back into solid sodium acetate trihydrate – a byproduct of which is, of course, heat.

Once that reaction has occurred, however, bending the metal disc will not cause any further reaction. In order to make these reusable, the solution must be turned back into a liquid. Luckily, that process is very simple: just put the packets into boiling water. The heat will cause the solution to dissolve and remain a liquid as it cools back down to room temperature. Then, simply pop the metal disk again and the heating process should repeat itself. Eventually, the chemical bonds will break down and the packets won’t continue to work, but you should be able to get plenty of use out of them before then.

Pros: Reusable, Versatile
Cons: Time-Consuming To Reheat, Less Portable

Purchase: $25

Lighter Fuel

Combust A Move

One of the most important things to know about lighter fuel hand warmers is that, while they state that they are “flame-free,” this does not mean they are free of combustion. Yes, they can create heat without a visible flame, but to produce that heat, they still require the same chemical reaction that burns fuel and creates energy and heat. Take, for instance, the Zippo hand warmer pictured above. It houses a lighter fluid reservoir – with an odd cotton-like mesh inside of it – that needs to be filled prior to operation. Once the reservoir is filled, a “catalytic burner” unit is placed atop it. That unit draws the fuel into it and, when exposed to fire (yes you need a separate lighter or matches to start the reaction), begins the combustion process without a visible flame.While they state that they are “flame-free,” this does not mean they are free of combustion. Then, you simply put the protective casing back over the top of it and you are free to put the warmer wherever you need it.

There are a couple big downsides to this kind of hand warmer. For one, it takes a little bit of time to prepare. You can’t simply grab it and go. You have to fill up the reservoir prior to each use and then wait a couple minutes before igniting it in order to give the fuel a chance to spread throughout the reservoir. Second, the fact that it requires fuel at all can be perceived as a downside, as fuel costs money and, by virtue of needing to fill it up, there’s a potential for spillage with each use. Finally, there’s no safe way to stop the internal combustion reaction. That means, each time you fill it up, you have to let the hand warmer run its course and burn all of the fuel. All told, however, this is still a more economical and less wasteful option that disposable hand warmers. It is worth noting that this type of warmer, as a result of combustion, creates carbon monoxide, which can be hazardous – though shouldn’t be a problem with decent ventilation or outdoors.

Pros: Refillable, Hours Of Heat
Cons: Requires Lighter Fuel, Can Be Messy

Purchase: $15


Plug It In

Generally speaking, these types of warmers have a functional life of about 6-hours per charge.Battery-powered hand warmers are fairly straightforward from a functionality standpoint – though the actual science behind it we’re sure is more complicated. In them, a rechargeable battery sends power to an onboard electrically resistive heating device, which converts that energy into heat. On simple devices, the level of heat is fixed. On more complex ones, the user can select the level of heat put out by the device. Then, once the battery is spent, all you have to do is plug it in and charge it back up for subsequent use. Generally speaking, these types of warmers have a functional life of about 6-hours per charge – which changes depending on the level of heat output. But they are arguably the most economical from a cost-after-purchase angle and are the most environmentally friendly when it comes to usage.

Pros: No Chemicals Or Combustion, Adjustable Heat Settings
Cons: Long Recharge Time, Somewhat Bulky

Purchase: $28


Pocket Barbecue

The downside to this type of hand warmer is pretty obvious: you have to burn charcoal.Calling these ‘pocket barbecues’ might sound a little tongue-in-cheek, but it’s a fairly accurate description. That’s because, in order for a charcoal hand warmer to work, you have to actually burn charcoal – exactly like a BBQ. The case itself is typically a felt-covered unit, similar in shape and format to a sunglasses case, built from metal (to distribute the heat without burning). For it to function, the user takes a rod of charcoal and strikes the ends of it to start the burning process, then places it inside of the unit and closes it. This makes it safe to put in your pockets. The downside to this type of hand warmer is pretty obvious: you have to burn charcoal. It’s wasteful, dirty, and not sustainable in any way. Plus, since it’s actual combustion, it creates carbon monoxide – which can be hazardous to the user’s health.

Pros: Long-Lasting Heat, Reusable Case
Cons: Requires Combustion Fuel Rod, Not Travel-Friendly, Bulky

Purchase: $13

How To Build A Campfire

Say you find yourself stuck in the cold, but you don’t actually have any hand warmers to keep you toasty. Well, then it might be useful to know how to build a campfire.

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