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February 27

Magnesium purity

Harbor Freight sells a magnesium firestarter bar for $2[1] which is about 20% of the price of the name brand item (Doan's). People who might or might know what they are talking about, say that the HF one is crappy because the magnesium is heavily cut with aluminum to reduce the cost.

Is that plausible? Is there a way to check, beyond checking the density by measuring and weigh it? Not worried about tiny impurities, basically want to know if it's really a magnesium bar or some counterfeit alloy. Thanks. 2601:648:8200:990:0:0:0:BDFA (talk) 07:14, 27 February 2023 (UTC)[reply]

At $2 why not just buy one to do a density test like you say? I suppose you could check the color of the flame, with a diffraction grating if you're not sure. But really I can't see why anyone would bother, magnesium is cheap enough that the amount used won't make much difference to the price. NadVolum (talk) 09:09, 27 February 2023 (UTC)[reply]

If it does exactly what it says on the tin (i.e. start fires), then worrying about the chemical composition seems a bit unnecessary. Alansplodge (talk) 11:17, 27 February 2023 (UTC)[reply]

Checking the density will involve separating the 'magnesium' from the striker bar on the side. I suggest you do that under water. Mg costs about $8 per kg, AL $l so replacing Mg with denser Al would save $5/kg, one of these things weighs about 10g, so 100% substitution would save 5c. Your call. Greglocock (talk) 23:05, 27 February 2023 (UTC)[reply]

It only weighs 10g? I didn't realize they were that light. Yes I do plan to buy a HF one and will try a density test. The Doan's besides being 5x as expensive are now hard to find since they are discontinued (some are still in the supply chain) so I'd rather not buy one of those just for comparison. I hadn't thought of a flame test. With magnesium that might be a little too vigorous, but maybe. Yes I can try separating the striker bar. It is just glued on. Thanks all. 2601:648:8200:990:0:0:0:BDFA (talk) 23:37, 27 February 2023 (UTC)[reply]

Wear dark sunglasses if you do the flame test. NadVolum (talk) 23:58, 27 February 2023 (UTC)[reply]

Yes, definitely. I have dark safety glasses but will see if HF has any cheap welders' goggles. Any suggestions about low rent spectroscopy? Thanks. 2601:648:8200:990:0:0:0:BDFA (talk) 04:59, 28 February 2023 (UTC)[reply]

So, I seriously think we're running into an XY problem here; OP: You've asked several times about how to check the purity of an off-the-shelf magnesium firestarter; however unless we know why you need pure magnesium it's hard to provide good advice. What is the actual problem you are trying to solve such that you need pure magnesium?--Jayron 32 18:39, 28 February 2023 (UTC)[reply]

Indeed, the specific application is going to have an impact on what method is needed to determine the purity. There are a lot of considerations when picking analytical methods. --OuroborosCobra (talk) 18:58, 28 February 2023 (UTC)[reply]

Jayron32, it's just a consumer value comparison or mythbusting effort. There is an expensive brand of magnesium bar (Doan's) and a cheap brand (HF). Some users claim that Doan's costs 5x more because it is real magnesium while HF's is some kind of adulterated crap. Others say magnesium is magnesium. There is a youtube of a guy trying to light fires with both brands of bar, and he has an easier time with Doan's so he pointedly throws his HF bar in the garbage, but it wasn't anything like a blinded experiment. There are other youtubes finding them to be about the same. National pride is involved since Doan's is US-manufactured while HF's is imported.

Each brand of bar has a flint striker on the edge, so another possibility is that both bars are similar magnesium but the Doan's has a higher quality striker, making a difference in firelighting ability that is unrelated to the magnesium itself. The youtube HF critic noticed that the HF bar had a rough texture, making him think it was impure, but it occurs to me that I could just try polishing it to see if I can make it shiny like the Doan bar.

Why don't I compare both bars directly? Basically because I don't feel like dealing with the cost and hassle of obtaining a Doan's bar, while I can get an HF bar next time I'm at one of their stores. 2601:648:8200:990:0:0:0:BDFA (talk) 20:33, 1 March 2023 (UTC)[reply]

Honestly, the quickest and easiest way to make this comparison is a simple density test. Assuming they aren't working so hard on the adulteration to try and come up with similar bulk property density, you should be able to take a piece of the "magnesium," weigh it, then submerge it in water to determine its volume. If you know what the density of magnesium should be, then this test will tell you whether it is just magnesium, or whether it is a mixture/something else entirely. That said, to make this test at all useful, you'd have to obtain Doan's bar and compare with it directly, since you don't actually know its purity anymore than HF. If you want more specific information, such as what the adulterant is, you could do something like atomic absorption spectroscopy or atomic emission spectroscopy, and that could tell you what other elements are present. You'd probably also need something for digesting the bar material into solution, like nitric acid. If you want to quantitatively know how much of those other elements are present, you are going to need a procedure which establishes a calibration curve for each element of interest. I don't recommend doing anything except the simple water displacement/density test unless you are qualified and experienced with the various safety issues involved. --OuroborosCobra (talk) 21:12, 1 March 2023 (UTC)[reply]

The volume measurement by water displacement would be pretty fiddly with an object weighing only a few grams. Also, see xkcd 2681. --174.89.12.187 (talk) 14:38, 2 March 2023 (UTC)[reply]

Depends on how much they are cutting with impurities, and what those impurities are. If it was aluminum, for example, the percent difference on the density compared with magnesium is 55%. So, let's say they cut the magnesium with about 50% aluminum, we would expect a density difference of ~25%, which is easily measurable with the water displacement test. --OuroborosCobra (talk) 15:19, 2 March 2023 (UTC)[reply]

I wonder if any aluminium "impurity" might be included, not to "adulterate" the bar to make it cheaper, but to increase its mechanical strength? As I dimly recall from chemistry lessons half a century ago, magnesium is pretty soft and ductile. However, I am not an expert on alloys. {The poster formerly known as 87.81.230.195} 51.198.55.125 (talk) 12:27, 3 March 2023 (UTC)[reply]