OneAndOneIs2

I wrote a post a while ago on the importance of securely deleting files before passing your hard drive on to somebody else.

A few of the commenters were of the opinion that, if you don't want people to be able to access your data, you shouldn't sell your hard drive, you should hit them with thermite.

I suspect that most of the people who throw thermite around as something clever have never actually done it themselves. But hey, I did chemistry up to degree level, and taught a fair amount of it when I was doing my science teaching bit. So I thought I'd talk about it.

So. Thermite. It's the name of a chemical reaction based on powdered aluminium. Typically mixed with iron oxide (AKA rust) but other metal oxides can work. It revolves around the varying reactivity of metals. Specifically, how much they want to lay their hands on Oxygen atoms.

A brief series of reactivity from high to low:
Aluminium
Carbon
Iron
Tin
Lead
Copper
Silver
Gold

Aluminium is, significantly, more determined to grab onto Oxygen than Carbon is. This is why it took us so long to discover & refine it: All the other metals in the list can be refined simply by heating them over a Carbon-based fire and letting the heat allow Carbon Monoxide to steal the Oxygen atoms from the metal ores, resulting in pure metal and Carbon Dioxide.

So, obviously, if Carbon can steal Oxygen from Iron, Aluminium, which is even more reactive than Carbon, should be able to do so even more effectively. And indeed, it can.

Those of you who are thinking "If Aluminium is so hot on Oxygen, how come it doesn't rust?" - it does, but Aluminium Oxide is clear and colourless and forms a protective film over the metal. Rubies and sapphires are, in fact, Aluminium Oxide with impurities. It's clear, and it's tough.

Anyway. What does this mean? It means that if you mix Aluminium and Iron Oxide, the Aluminium will try and steal the Oxygen from the Iron Oxide. You need some heat to kick-start the reaction. The easiest way to do this (At least in school labs) is using a strip of Magnesium, which burns with a white flame, as most people will have seen.

This very hot flame causes the Iron Oxide/Aluminium powder mix to start reacting, and the Aluminium promptly burns, using the rust as a source of Oxygen. Having stripped the Iron atoms of their Oxygen, you are of course left with pure Iron. Which, at thermite temperatures, is very molten, and being heavier than both Aluminium and its oxide, sinks to the bottom.

How hot is the stuff? Hot enough that thermite can be used to weld railway tracks together. Also hot enough to completely destroy a speed camera if it's performed in a flowerpot with a hole in the base placed on top of said Gatso. Or so I've heard...

Certainly more than hot enough to destroy a hard drive beyond all chance of recovery. Particularly if you decide to get adventurous and try using some of the more exotic metal oxides for a bigger gap - I hear that Copper-based thermite is quite a frightening thing...

Will it securely destroy your hard drive files? Oh yes, it'll melt the platters into a small puddle. Totally secure. But then, an ordinary campfire will wipe them just as effectively. And it doesn't have nearly as many huge safety problems, such as insanely high temperatures and vision-damaging ultra-violet light emissions.

So frankly, why bother? Thermite is nasty stuff and using it to wipe hard drives isn't clever. It's pointless overkill.