The Knowledge: How to Rebuild Our World From Scratch

by Lewis Dartnell

  Lime mortars have been used for millennia, but it was a substance first mass-produced by the Romans that changed the nature of building. The Romans noticed that cementum made by mixing slaked lime with volcanic ash, known as pozzolana, or even pulverized brick or pottery, sets far more quickly than lime mortar and is several times stronger. And with the fabulously strong mineral glue that is cement, you can do far more than just stick together ordered rows of bricks. You can also bond jumbled aggregations of rocks or rubble—that is, you can make concrete. This revolution in construction technology allowed the Romans to build awe-inspiring structures such as the Collosseum and the vast bulging roof of the Pantheon in Rome, which is still the largest single-piece concrete dome in the world.

  But it is another, almost magical, property of cement that really helped build the trading and naval prowess of the Roman empire: concrete made with pozzolana or crushed earthenware sets even when completely submerged in water. Unlike lime mortar, cement is said to be hydraulic and sets along a different chemical route. The volcanic ash contains alumina and silica—already discussed above as constituents of clay—which chemically react with the slaked lime to form an exceedingly strong material as they hydrate.

  Hydraulic materials led to an important technological advance. Pozzolanic cement spurred a revolution in Roman marine construction, for rather than simply sinking large stone blocks into the water, the Romans could now pour concrete for freestanding structures directly into the sea to create quays, breakwaters, seawalls, and lighthouse foundations. This technology enabled them to build ports wherever needed for military or economic reasons, including in regions with very few natural harbors, such as the north coast of Africa. Thus, Roman ships came to dominate the Mediterranean.

  This crucial knowledge about strong cements, versatile concrete, and watertight plasters was nearly lost to history with the fall of the Roman Empire. No medieval sources mention cement, and the great Gothic cathedrals were built using only lime mortar. However, knowledge seems to have been preserved in some places, as hydraulic cement was used in a number of fortresses and harbors constructed throughout the Middle Ages.

  But it was in 1794 that the modern method for producing cement was invented. “Ordinary Portland cement” does not exploit volcanic heat like the Roman pozzolana variety, but bakes a mixture of limestone and clay in a specialized kiln at around 1,450°C. The hard clinker produced is then ground up with a small amount of the soft, pale mineral gypsum—also used for plaster of Paris, and in setting broken limbs in a plaster cast—which helps to slow the curing process and gives you more working time with the wet cement.

  Now, I know that concrete is a horrifyingly dull and gray building material, and that there have been some architectural abominations constructed with it. But let’s step back and consider for a second what truly remarkable stuff it really is. Concrete is essentially man-made rock. And the recipe is beguilingly simple: stir together one bucket of Portland cement with two buckets of sand or gravel and enough water to make a thick gloop. Pour this liquid stone into a wooden shuttering constructed to make any shape you fancy, and then wait for it to set into an incredibly hard and durable material. It’s not difficult to see why concrete allowed rapid urban regeneration following the devastation of the Second World War and is still the prime ingredient for city-building today—an icon of the modern age, even though the basic process was invented more than two millennia ago.

  The problem with concrete, however, is that while it’s incredibly strong when compressed in foundations or columns, it’s very weak when under tension. It catastrophically cracks when forces act to stretch it, which stops it from being used for large structural elements like beams, bridges, or floors of multistory buildings. The solution is to embed steel rods in the concrete. Their individual material properties perfectly complement each other: the compressive strength of concrete combines with the tensile strength of steel. This reinforced concrete was hit upon in 1853 by a plasterer who inserted straightened metal barrel hoops into concrete floor slabs as they set. And it is this final innovation that really unlocks the potential of concrete for aiding reconstruction after the apocalypse.

  Concrete is a wonderfully versatile construction material, but it is ceramic bricks, with their refractory properties, that you will need to use to contain high temperatures and so achieve the skills of metallurgy.

  METALS

  Metals offer a suite of properties not provided by any other materials. Some are exceptionally hard and strong, making them ideal for tools, weapons, or structural components like nails or whole girders. But, unlike brittle ceramics, they also exhibit plasticity—if stressed they deform rather than shatter, and they can be pulled into thin wire suitable for fastening, fencing, or conducting electricity. Many metals can also resist very high temperatures, making them ideal for high-performance machinery.

  What you ideally want to be able to reattain as rapidly as possible after the Fall is the mastery not just of iron, but of its carbon alloy, steel. Steel contains a blend of iron and carbon atoms and is so much more than the sum of its parts. The inclusion of carbon changes the metal’s properties substantially, and by varying the proportion of carbon soaked into the alloy recipe, you can control the strength and hardness of the steel to suit different applications.

  We’ll see later how to create iron and steel from scratch, because in the immediate aftermath you’ll be able to easily scavenge them. Salvaged items can be repurposed by relearning the traditional skills of the blacksmith: working by an open hearth, or forge, to keep the workpiece glowing hot as you reshape it between hammer and anvil. The reason that we’ve been able to exploit hard iron through the history of civilization is that it temporarily changes its physical properties when hot, softening to become malleable enough to be beaten into shape, rolled into sheets, or drawn into pipes and wires. This is a fundamental point, because it means you can use iron tools to work on iron to produce more tools.

  The crucial knowledge for fully exploiting iron for tools is the principle of hardening steel—quenching and tempering. Steel is hardened by heating it red-hot so that the internal structure of iron-carbon crystals converts to a particularly rigid conformation (which is nonmagnetic, allowing a simple test during heating). If allowed to cool slowly afterward, though, this crystal reverts back to its previous form, and so you need to chill it rapidly to essentially freeze the desired structure: quenching the piece by dunking it, sizzling, into water or oil. However, a hard substance is also brittle—and a steel hammer, sword, or spring that shatters is useless—so after quenching a piece needs to be tempered. You do this by reheating to a lower temperature for a period of time so that a proportion of the crystal structure relaxes: you are deliberately trading off some of the strength to return a little flexibility to the material. Tempering allows you to tune the material properties of steel, a crucial ability for designing the appropriate metal for the intended task.

  Another key technology, only more recently available, is welding: gluing metal together with molten metal. Acetylene yields the hottest flame of any fuel gas, reaching over 3,200°C when it is burned in a stream of oxygen. A welding torch can be created by separately controlling the pressurized flow of oxygen and acetylene gas through a lit nozzle. Pure oxygen can be produced by electrolysis of water (Chapter 11), or later in the reboot by distilling liquefied air. Acetylene is released by the reaction of water with lumps of calcium carbide, which is itself made by heating quicklime and charcoal (or coke) together in a furnace: substances that we have already covered. As well as being useful for metal joining, an oxyacetylene flame can also be wielded as a cutting torch for steel, a jet of oxygen combusting the hot metal out in a neat line.

  An even higher temperature of around 6,000°C is generated by an electric arc welder: brandishing the power of lightning. Rigging up a row of batteries, or a generator, will produce enough of a voltage that a constant spark, or arc, will leap between the ta
rget metal and a carbon electrode, to weld or cut as the electrode is drawn across the surface. Such jury-rigged oxyacetylene torches or arc cutters would be indispensable equipment for salvage crews sent into the dead cities to disassemble the ruins and scavenge the most valuable materials. One very effective way for melting down scrap steel for recycling is the electric arc furnace. It’s essentially a giant arc welder: large carbon electrodes surge electricity through the metal to melt it, with limestone flux used to remove impurities as slag on top and the molten steel being poured as if from a kettle. Running an arc furnace from renewable electricity would be an important technology to try to leapfrog to in order to relieve demands on fuels for thermal energy in the post-apocalyptic world.

  But retaining access to metals as a class of materials is only half of the story: you’ll also need to be able to adeptly work them into the forms you need. If you can’t salvage any working versions of the essential machine tools for this, how might you construct them from scratch?

  An incredibly elegant example has been provided by a machinist in the 1980s who created a fully equipped metalworking shop—complete with lathe, metal shaper, drill press, and milling machine—starting with little more than clay, sand, charcoal, and some lumps of scrap metal. Aluminum is a good choice, as it has a low melting point for easy casting and is very corrosion-resistant, so it will be scavengeable even long after the apocalypse.

  The heart of this phenomenal project is a small-scale foundry, consisting of a salvaged metal bucket fitted with a refractory inner lining of clay, and fired by charcoal intensified with a stream of air through the bucket side. This backyard furnace is more than sufficient to melt the scavenged aluminum, and the molten metal can then be poured to cast a whole variety of machine components. Casting molds can be constructed from fine sand mixed with clay as a binder and a little water, packed around carved patterns in a two-part wooden frame.

  RUDIMENTARY FOUNDRY: MELTING SALVAGED ALUMINUM IN A SMALL-SCALE FURNACE (TOP) TO BE CAST IN A SAND MOLD (BOTTOM).

  The first machine to create is a lathe. A simple lathe is composed of a long, flat beam called the bed, with a headstock fixed at one end and a tailstock at the other that can be unlocked to slide left and right along the bed track. The workpiece is attached to the spindle on the headstock—perhaps by bolting to a faceplate, or gripped in a chuck with movable jaws—and then the whole piece spun around this center, driven by a pulley or gear system from whatever motive force you’ve harnessed (waterwheel, steam engine, or electric motor). The tailstock can be used to support the other end of the workpiece, sliding along the bed to accommodate different lengths, or else to bear a tool like a drill to bore through the center of the workpiece as it turns. A carriage also slides along the bed, mounted with a cutting tool on a cross slide so that it can be precisely positioned around the workpiece, shaving into it as it turns to create any desired profile. Astoundingly, not only is the lathe capable of duplicating all of its own components to create more lathes, but starting from absolute scratch, you can even produce, during the rudimentary stages of construction of your first lathe, the remaining components needed to complete it.

  LATHE WITH THE HEADSTOCK AND ROTATING SPINDLE TO HOLD THE WORKPIECE ON THE LEFT; TAILSTOCK ON THE RIGHT; AND, IN BETWEEN, THE MOVABLE CARRIAGE BEARING THE CUTTING TOOL.

  In order to cut precisely spiraling threads on your workpiece, you would want to fit a long lead screw alongside the bed to smoothly move the carriage, and ideally couple this with gears to the headstock spindle to perfectly coordinate their motion. In a post-apocalyptic world you would really hope to be able to scavenge a ready-made long, threaded screw, as cutting a thread with a constant pitch is fiendishly difficult otherwise. In our history it took a long process of reiterative refinement to make the first precise metal screw thread, from which many others can then be constructed, and you would want to avoid having to repeat this.

  Once you have a lathe you can use it to construct the parts of other, far more complex machine tools, such as the milling machine. Whereas the lathe applies a tool to a rotating workpiece, the milling machine bears a rotating tool against the workpiece, and is exceedingly versatile—once you have a milling machine you can create pretty much anything else. So this demonstration is a microcosm of the history of technology itself: simple tools making more complex tools, including more precise versions of themselves, and repeating this cycle to ratchet upward.

  But what if you can’t find any ready-purified metals for forging or casting, or you’ve already used all that was scavengeable? How do you get metal out of rocks in the first place? The general principle of smelting is to remove the oxygen, sulfur, and other elements the metal is compounded with in the ore. This requires a fuel to attain high temperatures, a reducing agent, and a flux. Charcoal (or coke) performs admirably for the first two functions: it burns fiercely, and as it combusts in the smelter it releases carbon monoxide, a powerful reducing agent that strips the oxygen away to leave pure metal. The overall blueprint for a rudimentary iron-smelting furnace is similar to that of a kiln for burning lime. The furnace is charged with layers of charcoal fuel and the crumbled iron ore rock. Some limestone mixed in with the ore serves as a flux, lowering the melting point of the refractory gangue (the worthless rocky stuff) so that it turns fluid in the furnace, absorbing the impurities away from the metal. The flux forms a slag that is drained away, and your metallic prize can be extracted from the furnace.

  If your furnace doesn’t operate hot enough to melt the iron formed, you’ll need to retrieve the solid metal as a spongy lump and then batter and pummel it on an anvil to fuse the iron together and squeeze out the remaining slag. To become hard enough to be useful for tools, this pure, wrought iron must be fiercely heated once more with charcoal to make it absorb some carbon—to form steel—and then worked on the anvil again. Repeatedly folding and reflattening it, you’ll essentially stir the solid material to create a uniform steel that can then at last be forged into its final form. This is backbreaking work for the smith, and the rate of steel production is severely limited. The key to modern civilization is efficiently making steel in bulk. Here’s how you do it.

  The solution is to force a powerful stream of air up the furnace stack to ferociously intensify combustion. The Chinese invented the blast furnace by the fifth century BC (more than 1,500 years before it appeared in Europe), and later improved the design using waterwheel-driven piston bellows. For even greater efficiency at achieving high temperatures, preheat the ingoing air blast using the hot, combustible waste gases escaping out of the furnace flue. The freshly smelted iron in the blast furnace absorbs lots of carbon, which acts to lower its melting point to about 1,200°C. The metal liquefies completely and can be run out of the bottom of the furnace, along channels cut in the floor, to cool in a row of ingot molds. The result is pig iron—so named because medieval foundry workers thought the molds looked like newborn piglets suckling on a sow.

  This high-carbon iron, with its lowered melting point, can be remelted and poured into a mold like hot wax. Cast iron is therefore greatly convenient for quickly fabricating items like cooking pots, pipes, and machinery parts, and the Victorians mass-produced cast-iron girders. But cast iron also has one great disadvantage: the high carbon content makes the metal brittle, and cast-iron bridges, for example, have the nasty habit of collapsing if their structural components are bent or stretched.

  BLAST FURNACE FOR SMELTING IRON. THE ORE, FUEL, AND FLUX ARE FED IN AT THE TOP, AND AN INTENSE STREAM OF HOT AIR IS FORCED UP THE STACK FROM THE BOTTOM.

  The innovation that really made possible the later stages of the Industrial Revolution was a means for easily transforming blast-furnace pig iron into steel. In terms of carbon content, steel lies in between pure wrought iron (usually less than .01 percent carbon) and brittle pig or cast iron (3–4 percent carbon): from about 0.2 percent carbon for tough steel for machine gears or structural members, to about 1.2 percent for
particularly hard steel for ball bearings or the cutting tools of your lathes. So how do you decarbonize pig iron?

  The Bessemer converter is a giant pear-shaped bucket, lined with refractory bricks and mounted on pivots so it can be tipped. The vessel is charged with molten pig iron, and then air is pumped in through holes in the bottom, not unlike the action of a bubbling aquarium aerator. The excess carbon reacts with the oxygen and escapes as carbon dioxide gas, and other impurities are also oxidized and scrubbed out into the slag. One lucky outcome is that as the carbon burns it releases enough heat to keep the iron molten throughout.

  The difficulty is that it is hard to judge the operation accurately enough to remove almost all of the carbon, but leave just under 1 percent. The trick for nailing the final composition, obvious in retrospect, is to run the conversion long enough that you are sure absolutely all of the carbon has been removed, and then simply mix exactly the final percentage you want back into the pure iron. This Bessemer process was the first method in history for cheaply mass-producing steel, and you’d want to leapfrog back to this point as quickly as you can.

  GLASS

  While iron and steel are the celebrated building stuff of the modern industrialized world, humble glass, so easily overlooked (or at least looked through), has also been critical in our progression. Glass, one of the first synthetic materials to be made by humankind, was invented in Mesopotamia, the cradle of the first cities, some time in the third millennium BC. We’ll see in a bit how glass, and its unique combination of essential properties, forms the linchpin of science. But let’s start with the fundamentals of how to make the stuff.