Ancient Exploding Cannonballs

Planetes writes "There is a story on MSNBC about some surprise archaeology. Apparently, cannonballs from shipwrecks are "exploding" (more like heating up and cracking open) when they are exposed to air. At least one reacted so violently it reached several hundred degrees. Talk about a booby trap. I'd never have seen this one coming." Heat from oxidation (that's "rusting", if you haven't taken chemistry) has started many fires in cargo ships carrying iron.

Comments

Rust at work

[R2.0]

There's a road in the northwest that was closed because it was effecively on fire. The state specified ground-up tires be used in the fill under the road in an eco-friendly gesture. Groundwater started the steel belts in the fill rusting, the heat started the rubber burning, and now smoke is coming out of the ground.

Like in KSR's Mars trilogy

[MagikSlinger]

One of the theories I've always heard is that if Martian soil was ever exposed to signficant water (i.e., enough to thoroughly wet it), the soil of Mars could start reacting violently because it's mostly iron that hasn't had the advantage of water to ensure the iron is fully rusted.

KSR has some fun with this when he described a flood released onto pristine Martian soil. Snap, crackle, pop, kids.

Contains oxygen: handle with care.

[Observer]

Arthur C Clarke wrote a short piece about conditions on Earth as deduced from observations by astronomers of the long-extinct Martian civilisation (obtained by archeological investigations undertaken by early scientific expeditions from Earth in the early 21st century, IIRC - one of his early pieces). The Martian scientific consensus was that intelligent life could not possibly evolve on Earth, because 20% of its atmosphere was made up of the dangerously reactive element oxygen. Any life that did exist would have to be heavily armoured and shielded from exposure to such a corosive substance, and even then, there was evidence of sporadic large-scale chemical reactions on the surface of the planet, a terrifying natural phenomenon for which the scientists had invented the technical term "fire". Clearly, no life at all could possibly survive such events....

ACC was quietly ignoring the question of what was maintaining the level of such a reactive substance in the atmosphere in the first place, but he was also making a good point: most people do underestimate how reactive oxygen can be outside the everyday circumstances that we're familiar with. Wood burns, but we can make stoves out of iron, so iron doesn't burn easily, right? Wrong. Stuff an iron pipe with iron rods, blow pure oxygen down it and heat the open end for a while with an oxy-acetylene torch and you get one of the more powerful cheap cutting flames around. The cannon-balls had apparently had lots of fine channels corroded into them by years of exposure to sea-water so there was a large surface area unprotected by a covering of rust: in the cases where the iron combusted sufficient area of unprotected iron became exposed as or after the water evaporated which was enough to get the reaction started.

And Primo Levy commented in one of his books (Periodic Table, perhaps? I don't have it to hand to check) about how treacherously ready sawdust could be to spontaniously combust. A more obviously flammable example than iron, but a similar situation: with more surface area and less nearby mass to absorb heat from any reaction that does start, sawdust is that much more liable to behave dangerously that timber in bulk.