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Do the FRH's lose ALL heat-emitting properties after a one-time use from cooking an entree, or are they (in sufficient quantity) reusable as hand-warmers later on, so long as you have enough of them?

They work a second time just as well as matches.

They work by a chemical reaction, once the primary chemical is used up there's no more heat.

The FRH expends itself when activated, and I don't know of any way to even temporarily shut down the reaction. It could potentially be used as a hand warmer if you waited for it to cool down (which can take a while if you're not wrapping it in something to help insulate the heat), but you'd be restricted to whatever heat was left.

It's pretty much been said. However, there is a possibility here. While it's true, once the reaction finishes its' done. But it's an oxidation reaction.
If you put it in a glove or similar quick enough and can do so without burning yourself (which is the hard part here!) it could work while it is still reacting, as it needs oxygen to keep going. They are largely identical to the handwarmer packs, they don't burn you though, because they do not have enough oxygen so it's a much slower reaction. If you want to try it, GO FOR IT! lemme know how it is.... but I suspect once it is going full .... stream... you are probably not gonna have a lot of luck.

magu2k wrote:It's pretty much been said. However, there is a possibility here. While it's true, once the reaction finishes its' done. But it's an oxidation reaction.
If you put it in a glove or similar quick enough and can do so without burning yourself (which is the hard part here!) it could work while it is still reacting, as it needs oxygen to keep going. They are largely identical to the handwarmer packs, they don't burn you though, because they do not have enough oxygen so it's a much slower reaction. If you want to try it, GO FOR IT! lemme know how it is.... but I suspect once it is going full .... stream... you are probably not gonna have a lot of luck.

They do not need oxygen out of the air for the reaction, they take it out of the Water, thats why hydrogen gas is produced in the reaction

Eistee wrote:

magu2k wrote:It's pretty much been said. However, there is a possibility here. While it's true, once the reaction finishes its' done. But it's an oxidation reaction.
If you put it in a glove or similar quick enough and can do so without burning yourself (which is the hard part here!) it could work while it is still reacting, as it needs oxygen to keep going. They are largely identical to the handwarmer packs, they don't burn you though, because they do not have enough oxygen so it's a much slower reaction. If you want to try it, GO FOR IT! lemme know how it is.... but I suspect once it is going full .... stream... you are probably not gonna have a lot of luck.

They do not need oxygen out of the air for the reaction, they take it out of the Water, thats why hydrogen gas is produced in the reaction

Eistee hitting us with a bit of his Chemistry background..lol

They will work again for a second time, but it's not really a good idea because they won't be nearly as hot and will only be warm for probably a few minutes. Also, the palm of your hands would be covered in a solution of water; salt; iron; and magnesium.

After cooking an MRE meal, I used them while squid fishing this season and stuck it into an outer coat pocket. It kept warm for quite a while.