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New Worlds: We Built This City

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For durability of construction, paper does not beat rock.

Or things that are rock-adjacent. I recognize there are flaws in me grouping all types of earthen construction as inorganic materials; after all, dirt is partially organic, and things like brick often include organic additives to help bind them into blocks. But in terms of how we build with them, they behave more like stone than they do like most organics, and so this is the approach I’ve chosen to take.

Before we get into that, though, let’s rewind the clock.

Our very ancient ancestors sometimes used caves as shelter. These are good for keeping the rain off, but also the heat or the cold; they tend to remain at a stable temperature, albeit not a very warm one (as anyone who’s ever visited a cave knows). Underground living isn’t only a prehistoric practice, though: sites ranging from the Nabataean city of Petra in Jordan and the Cappadocian region in Turkey to the Ancestral Puebloan cliff dwellings in the Southwest United States to the Mogao and Guyaju Caves in China show that under the right circumstances — like stone soft enough to cut without too much difficulty — you can have quite extensive settlements of that kind.

It’s also possible to have a structure that’s only partially underground. Dugouts of this kind take advantage of the insulating properties of the earth without requiring as much excavation as a fully subterranean room, and they present a lower profile to the wind. In some cases there’s a symbolic element underlying the choice to work downward into the earth . . . but caves and semi-caves are important enough on that front that they’ll need an essay of their own on some future day.

Back to the pragmatics! If you’re going up instead of down, earth can be used in a variety of ways. One option is simply to pile it up, as see in the mounds of the Mississippian culture or earthworks in other parts of the world — but I do it a disservice to say “simply,” because it takes more than a shovel and a lot of labor to make that work. If you want the mound to endure and not gradually slip under its own weight, you need to plan its stratigraphy with some care.

Or you can instead pack it really really hard. Rammed earth is a method that involves laying down successive layers (mixed with a stabilizer, which could even be animal blood) within a framework, then compacting each one to something like half its original volume. Once it’s “cured,” this is remarkably durable — you can even build defensive walls this way — and it has the advantage of working basically everywhere: not all environments have suitable stone or wood, but they all have dirt.

That’s super labor intensive, though, and much of the time we’ve taken an easier route. Mudbrick/adobe is dead simple to make; you just need suitable soil, water, and something like straw, rice husks, or dung to help them dry more evenly and avoid cracking. You don’t even need to fire them, which is good news for arid regions where fuel is in short supply — and that aridity is good news for the mudbricks, which are more vulnerable to erosion in the rain than the fired kind. You can extend their lifespan, though, by covering their exterior with stucco: essentially the same thing as plaster or mortar, with the differences of terminology based more on where the material is used than what it’s made of.

(Heck, you can even skip the dirt and binder and just make your bricks out of water . . . assuming the weather is cold enough! Igloos are the same basic concept, using blocks of tightly packed snow.)

Scale up that stucco/plaster/mortar thing, and you get concrete. We often think of that as being modern, and certainly the scale on which we employ it is new, but the stuff itself goes back thousands of years. The beautiful dome of the Pantheon in Rome? Concrete. It was revolutionary stuff, allowing for shapes not easily achieved with stone or brick, and Roman concrete is actually less vulnerable to erosion than the modern kind; scientists have been studying why, in the hopes of making our own buildings and other structures more durable.

And then, of course, there’s stone itself. Rough versions of this literally just involve piling up rocks, though dry stone construction — using carefully placed uncut stone, without mortar — definitely requires skill and patience. Most of the time we use mortar to fill the gaps and increase stability, and/or cut (“dress”) the stone, whether into regular blocks or the beautiful jigsaw puzzles of Incan architecture.

That latter is the stuff of elite construction: castles, holy sites, defensive walls, and so forth. It’s simply too expensive for the average schmoe, who can’t pay the skilled masons for the months of work required. And even for the elites, it wasn’t always the material of choice; after all, stone buildings are essentially man-made caves, and warming them in cold weather can be difficult. They might prefer a nice snug wooden manor instead.

In fact, cut stone is expensive enough that even the fanciest buildings often didn’t use it exclusively. If you’ve ever seen a ruined castle that looks like a big pile of rubble, that’s because the dressed blocks that formed the exterior of the walls have long since been stolen away by people who want to re-use them elsewhere. What remains is the core: usually a mix of mortar and smaller, unshaped rocks, poured between the two surfaces to fill in the rest of the wall’s thickness with something much cheaper. The same principle gets applied to wooden buildings, too, with less expensive and polished material used for internal structural members the residents won’t be looking at every day.

Capping this off — literally — yes, you can use these materials for a roof. Our best theory for how Egyptian temples got their stone-slab coverings is that once the walls were built, the whole thing was filled and surrounded with sand or earth, the slabs were dragged up and across, and then all that fill was dug out again. Most cultures, though, have opted for a lighter approach and a wooden framework (temporary or permanent) to assist with construction. Ceramic tiles are popular in hotter countries, because they trap less heat than thatch, or you can also make shingles out of a stone like slate which easily cleaves into flat plates. Careful climbing on those, though: if they start coming loose, not only can one slip out from under your foot, but it could kill someone in the street if it hits them in the head.

See? Construction materials aren’t just about a descriptive background detail. Whether it’s a flying roof tile as an impromptu weapon, an adobe house collapsing in an earthquake, or a whole neighborhood of half-timbered houses going up in flames when a fire starts, this can actually be the stuff of life and death!

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