> Coal, yes. Iron, wouldn't the iron we moved to the surface be even more accessible? We didn't destroy it, just rearranged and concentrated it?

Might depend how long it takes: because rust is porous and friable, rusting iron should eventually degrade to nothing, and the rust would be difficult to re-concentrate then reduce back to iron.

Would whatever the rust turns into be any less accessible (ignoring availability of coal) than what we started out with though? I don't pretend to know the entire "iron cycle", but it seems like it ought to just be turning back into the same sort of minerals that we originally extracted it from?

The problem will be a lack concentrated deposits making post-collapse (and new) industrial efforts much harder. A pile of rust from one tractor in someone's back yard is not going to be worth the effort to mine and refine.

Possibly an analogous situation is the history of steel in Japan and their efforts to extract iron from sand, since they don't have significant iron ore to mine on their island.

The pile of rust that use to be a tractor, sure, that's not worth much. The pile of rust that use to be a city though... surely that's more concentrated than the rust mixed with rock (or clay) that we originally extracted it from?

Most likely not: https://en.wikipedia.org/wiki/Iron_ore, it's almost 50% iron in the worst case and can be up past 70% in the best case. The geological processes that form this ore in the mantle do a pretty good job of concentrating iron by density but it takes geological scales of time to do so and have it lifted back up through the crust. We might have to excavate a lot of earth to get to the concentrated vein of ore but when we find it it's relatively easy to chase and turn into usable iron once you know a little bit about mining and smelting as a society.

A city decayed to rust is going to be a thin layer of iron spread out over miles with some hot spots like where a building once stood (but presumably without a map of the city in this distant future scenario), but there won't be a vein of concentrated ore. Distributed rust can definitely be turned back into pure iron but the energy requirements are going to be substantially higher to do so since you're going to have to sift through much more material to collect it, more material to separate and concentrate it, more material to smelt off, and your operations will have to be more mobile to retrieve it over a larger area. That's why I think retrieving iron from our society will be more like extracting iron from ironsands (2-20% iron), and it will have similar effects on that subsequent society that sits between us now and some future point where geology has re-supplied it to the surface millions of years from now.

> Most likely not: https://en.wikipedia.org/wiki/Iron_ore, it's almost 50% iron in the worst case and can be up past 70% in the best case.

FWIW those are the ratios for the oxides themselves but the formations are not necessarily huge piles of pure oxides, if you go a bit lower to the "sources" section the lowest-concentrated formations viable for exploitation are

> Banded iron formations (BIFs) are sedimentary rocks containing more than 15% iron composed predominantly of thinly bedded iron minerals and silica (as quartz).

However that's only for post-industrial societies, at least if you have alternatives, as it requires churning through ridiculous amounts of materials.

When you don't have alternatives the ironsand article (which would be used in places with no good or accessible ore deposits e.g. japan, famously) quotes

> Sand used for mining typically had anywhere from 19% magnetite to as low as 2%.

though much like gold panning the ironsand would be sluice-separated to a concentration of 30-50% before it was further processed.

Most ironsands deposits are not considered financially exploitable to this day though, with the exception of NZ's where the iconic "black sand" beaches of north island are extremely rich in magnetite (up to 40%).

Iron rusts over historical timescales, metal deposits form over geological ones.

Am I wrong to think of metal deposits as just "rust mixed with rock"? It doesn't seem like the rock part (i.e. the details of what it is mixed with) should be critical for the extraction process?

> Am I wrong to think of metal deposits as just "rust mixed with rock"?

They're concentrated rust mixed with rocks, otherwise it's not economically viable to extract.

Like, iron is ridiculously common, relatively speaking: on earth as a whole it's more common than oxygen, for the crust it ranks 4th at 5% by mass, meaning if you went at it randomly you'd need to sift through 20kg of materials to get 1kg of iron.

Currently, we exploit formations as low as 15% iron (banded iron formations / taconite), that's the lower limit of the economically feasible, and those results in absolutely enormous amounts of tailings (waste materials).

Pre-industrialisation, unless you had no other choice (e.g. only had ironsands to work with) you really wanted to exploit natural (or "direct-shipping") ores, in the 60~70% range, the extraction is way too much work otherwise.

Sure, if you have a billion years to wait for it to all run through the rock cycle again.

The process that concentrated the iron ore we rely on does not operate anymore.

Fortunately, despite millions of tons of production every year, we are nowhere near using up the ore.

Stone coal could be, at least partially, substituted by charcoal - and often was in the past, where reachable underground coal reserves were not available.

"So by weight charcoal and anthracite coal have an energy density of about 30 MJ/kg, while poorer kinds of coal range down to half of that." (https://www.reddit.com/r/askscience/comments/udjl5/charcoal_...) However, the density of charcoal is much less than that of coal: 200 kg/m^3 vs 1500 kg/m^3 (for solid anthracite).

Producing 1kg of charcoal requires 3-4kg of wood. (Producing the 900°C for the process is an exercise for the reader.) (https://www.fao.org/3/y4450e/y4450e11.htm)

Guess what we do most of our iron smelting with?

Primitive iron smelting was done with charcoal, I believe. Correct me if I'm wrong.

Yes. Being stuck at that point would be the problem.

Would it? Most new smelting plants today in the US (any built in the last few decades) use a mixture of hydrogen and carbon monoxide as the reducing gases (natural gas primarily as feedstock, but no reason it couldn’t be about 90% hydrogen produced via, say, hydroelectricity or wind).

Coal didn’t overtake charcoal for smelting iron in the US until the latter half of the 19th century, well after the first industrial revolution.

Melting down scrap iron is one of the main sources of steel in the US, and that is done straight with electricity in arc furnaces.

Coal accelerated the second industrial Revolution, but it was not essential. Far more important for enabling the first industrial Revolution was some of the early scientific knowledge about steam and pressure, such as the work of Robert Boyle, a lot of that based on a sort of reaction to the classics that had been revived in the Renaissance. The biggest argument for coal is indirectly in that it helped the viability of British society (after the island had most its tree cut down over the previous 500 years) which played an important role in the Scientific Revolution (Robert Boyle was Anglo-Irish)… although by the time Britain was playing an important role, the scientific Revolution was already underway on the mainland of Europe. As long as our books are not all destroyed, I think we’d have no problem bootstrapping from charcoal the second time around.

(I think a lot about long term data storage… writing in stone or fired clay still seems like one of the best methods for writing that needs to last 10,000 years… it was, after all, preserved Greco-Roman classics that enabled the renaissance and therefore the scientific Revolution.)

I think that vastly underestimates the dependency tree in modern society. Storing hydrogen in useful quantities is tough, requiring fairly sophisticated metallurgy and cryogenics.

Finding out we've got a hard to replace left-pad module somewhere far up the tech tree wouldn't be fun.

The dependency tree of 19th Century or early 20th century society is a lot more straightforward, however.

And no, you don't need such sophistication for storing useful amounts of hydrogen. Storing large amounts of hydrogen (in this case, also mixed with poisonous CO) was solved in the beginning of the 19th Century (well, late 18th century) in Britain and Germany by using very large near-atmospheric storage vessels called Gas Holders: https://en.wikipedia.org/wiki/Gas_holder

Salt caverns can also be used for greater volumes, i.e. for seasonal storage, as are already used for hydrogen storage in a few places in the US and elsewhere. https://en.wikipedia.org/wiki/Underground_hydrogen_storage

Except the Iron Age started around 2000 BC, so the world would largely be without iron for a very long time.

That gas holder article says they contained methane or coal gas. Methane's density is 0.657 kg/m³; hydrogen's is 0.08375 kg/m³.

Coal gas is a mix that (by energy) is about half hydrogen, as I said. And hydrogen gas a specific energy of 142MJ/kg vs 55.5MJ/kg for methane.

Or instead of stone tablets you could simply build a 10,000 year clock.

https://www.businessinsider.com/everything-you-need-to-know-...

Yeah, I think we switched to coke during the Industrial Revolution not because it's a better fuel than charcoal, but because we were running out of trees.

Coke was Abraham Darby's doing (around 1709-1710), and that was mostly to corner the market for cheap pots and kettles. There was no way for the charcoal crowd to compete on iron, and the bronze bunch - the norm for that sort of thing up to that point - was left forever in the dust.

What?