As we saw in Chapter 1, in the first few decades after the cataclysm, forests will readily reclaim the countryside and even abandoned cities. A small recovering population of survivors will not lack firewood, especially if they maintain coppices of fast-growing trees. The basic principle of coppicing is that once cut down, ash or willow will re-sprout from their own stump and be ready for harvesting again within five to ten years, providing on average five to ten tons of wood every year from one hectare (two and a half acres) of managed forest. Wooden logs are fine for a fireplace warming the home, but for practical applications during the long recovery process you’ll need a fuel that burns much hotter than wood. And this necessitates the revival of an ancient practice: charcoal production.
The process is a simple one. Wood is burned with constrained airflow to limit oxygen availability so that it cannot combust completely, but is instead carbonized. The volatiles, such as water and other small, light molecules that turn to gas easily, are driven out of the wood, and then the complex compounds making up the wood are themselves broken down by the heat—the wood is pyrolyzed—to leave black lumps of almost pure carbon. Not only does this charcoal burn far hotter than its parent wood—because it’s already lost all the moisture, and only carbon fuel remains—but the loss of around half of the original weight also means that it is far more compact and transportable.
The traditional method for this anaerobic transformation of wood—the specialist craft of the collier—was to build a pyre of logs with a central open shaft, and then smother the whole mound with clay or turf. The stack is ignited through a hole in the top, and then the smoldering heap is carefully monitored and tended over several days. You can achieve similar results more easily by digging a large trench and filling it with wood, starting a hearty blaze, and then covering over the trench with scavenged sheets of corrugated iron and heaping on soil to cut off the oxygen. Leave it to smolder out and cool. Charcoal will prove indispensable as a clean-burning fuel for rebooting critical industries such as the production of pottery, bricks, glass, and metal, which we’ll come to in the next chapter.
If you do find yourself after the Fall in a region with accessible coalfields, they will again present an irresistible source of thermal energy. A single ton of coal can provide as much heat energy as a year’s firewood output of a whole acre of coppiced woodland. The problem with coal is that it doesn’t burn as hot as charcoal. It’s also pretty dirty—the fumes can taint products made using its heat, such as bread or glass, and sulfur impurities make steel brittle and troublesome to forge.* The trick to using coal is to first coke it, echoing the practice of turning wood to charcoal. Coal is baked in an oven with restricted oxygen to drive out impurities and volatiles, which, like the products of timber dry distillation, have their own diverse uses and should be condensed and collected.
Combustion also provides light, and while the recovering society restores electrical grids and reinvents the light bulb, survivors will need to rely on oil lamps and candles.* Plant oils and animal fats are particularly well suited to serving as condensed energy sources for controllable combustion, due to their chemistry. The main feature of these compounds is their extended hydrocarbon chains: long daisy chains of carbon atoms with hydrogen atoms stuck on at the sides, adorning the flanks like stubby caterpillar legs. Energy is contained within the chemical bonds between the different atoms, and so the long hydrocarbons represent dense reservoirs waiting to be liberated. During combustion, this large compound is ripped apart, and all of the atoms unite with oxygen: the hydrogen atoms combine to form H2O, water, and the carbon backbone fragments and escapes as carbon dioxide gas. The rapid disassembly of long fatty molecules during oxidation releases a torrent of energy—the warming glow of a candle flame.
An oil lamp can be as basic as a clay bowl with a pinched spout or nozzle, or even just a large shell. A wick, made of plant fiber such as flax or simply rush, draws the liquid fuel up from the reservoir to be evaporated by the warmth of the flame and then combusted. Kerosene has been a common liquid fuel for glass lamps since the 1850s (and today also powers passenger jet planes over the clouds), but it is derived from the fractional distillation of crude oil and would be difficult to produce after the collapse of modern technological civilization. Any unctuous liquid suffices, though: rapeseed (canola) or olive oil, or even ghee from clarified butter.
A candle can dispense with the container altogether because the fuel itself remains hard until melted in a small pool in the vicinity of the flame—thus a candle is no more than a cylinder of solid fuel with the wick running down the middle. As it burns down, more wick is exposed, producing a larger and smokier flame, unless you periodically snip the wick. The fuss-saving innovation, which didn’t occur to anyone until 1825, is to braid the fibers of the wick as a flattened strip, so that it naturally curls over and the excess is consumed by the flame.
Modern candles are composed of wax derived from crude oil, and the availability of beeswax will always be limited, but you can make a perfectly functional candle from rendered animal fat. Boil meat trimmings in salty water, and scoop the hardening layer of floating fat off the surface. Pig lard produces a smelly, smoky candle, but beef tallow or sheep fat are passable. Pour molten tallow into a mold, or even just dip a row of dangling wicks into hot tallow to coat them; allow them to cool and set in the air. Then repeat, building up layer after layer until you have substantial candles.
LIME
The first substance that a recovering post-apocalyptic society will need to begin mining and processing for itself, because of its multitude of functions that are absolutely critical to the fundamental operations of any civilization, is calcium carbonate. This simple compound, and the derivatives easily produced from it, can be used to revive agricultural productivity, maintain hygiene and purify drinking water, smelt metals, and make glass. It also offers a crucial construction material for rebuilding and provides key reagents for rebooting the chemical industry.
Coral and seashells are both very pure sources of calcium carbonate, as is chalk. In fact, chalk is also a biological rock: the white cliffs of Dover are essentially a 100-meter-thick slab of compacted seashells from an ancient seafloor. But the most widespread source of calcium carbonate is limestone. Luckily, limestone is relatively soft and can be broken out of a quarry face without too much trouble, using hammers, chisels, and pickaxes. Alternatively, the scavenged steel axle from a motor vehicle can be forged into a pointed end and used as a drill to repeatedly drop or pound into the rock face to create rows of holes. Ram these with wooden plugs and then keep them wet so that they swell and eventually fissure the rock. But pretty soon you’ll want to reinvent explosives and use blasting charges to replace this backbreaking labor.
Calcium carbonate itself is routinely used as “agricultural lime” to condition fields and maximize their crop productivity. It is well worth sprinkling crushed chalk or limestone on acidic soil to push the pH back toward neutral. Acidic soil decreases the availability of the crucial plant nutrients we discussed in Chapter 3, particularly phosphorus, and begins starving your crops. Liming fields helps enhance the effectiveness of any muck or chemical fertilizers you spread.
It is the chemical transformations that limestone undergoes when you heat it, however, that are particularly useful for a great range of civilization’s needs. If calcium carbonate is roasted in a sufficiently hot oven—a kiln burning at least at 900°C—the mineral decomposes to calcium oxide, liberating carbon dioxide gas. Calcium oxide is commonly known as burned lime, or quicklime. Quicklime is an extremely caustic substance, and is used in mass graves—which may well be necessary after the apocalypse—to help prevent the spread of diseases and to control odor. Another versatile substance is created by carefully reacting this burned lime with water. The name quicklime comes from the Old English, meaning “animated” or “lively,” as burned lime can react so vigorously with water, releasing boiling heat, that it seems to be alive. Chemi
cally speaking, the extremely caustic calcium oxide is tearing the molecules of water in half to make calcium hydroxide, also called hydrated lime or slaked lime.
Hydrated lime is strongly alkaline and caustic, and has plenty of uses. If you want a clean white coating for keeping buildings cool in hot climes, mix slaked lime with chalk to make a whitewash. Slaked lime can also be used to process wastewater, helping bind tiny suspended particles together into sediment, leaving clear water, ready for further treatment. It’s also a critical ingredient for construction, as we’ll see in the next chapter. It’s fair to say that without slaked lime, we simply wouldn’t have towns and cities as we recognize them. But first, how do you actually transform rock into quicklime?
Modern lime works use rotating steel drums with oil-fired heating jets to bake quicklime, but in the post-apocalyptic world you’ll be limited to more rudimentary methods. If you’re really pulling yourself up by your bootstraps, you can roast limestone in the center of a large wood fire in a pit, crush and slake the small batches of lime produced, and use them to make a mortar suitable for building a more effective brick-lined kiln for producing lime more efficiently.
The best low-tech option for burning lime is the mixed-feed shaft kiln: essentially a tall chimney stuffed with alternating layers of fuel and limestone to be calcined. These are often built into the side of a steep hill for both structural support and added insulation. As the charge of limestone settles down through the shaft, it is first preheated and dried by the rising draft of hot air, then calcined in the combustion zone before it cools at the bottom, and the crumbling quicklime can be raked out through access ports. As the fuel burns down to ash and the quicklime spills out the bottom, you can pile in more layers of fuel and limestone at the top to keep the kiln going indefinitely.
A shallow pool of water is needed for slaking the quicklime, and you could use a salvaged bathtub. The trick is to keep adding quicklime and water so that the mixture hovers just below boiling, using the heat released to ensure that the chemical reaction proceeds quickly. The fine particles produced will turn the water milky before gradually settling to the bottom and agglutinating as the mass absorbs more and more water. If you drain off the limewater, you’ll be left with a viscous sludge of slaked-lime putty. We’ll see in Chapter 11 how limewater is used to produce gunpowder, but let’s look here at one particularly useful application of slaked lime: to create a chemical weapon against marauding hordes of microorganisms.
SOAP
Soap can be made easily from basic stuff in the natural world around you and will be an essential substance in the aftermath for averting a resurgence of preventable diseases. Health education studies in the developing world have found that nearly half of all gastrointestinal and respiratory infections can be avoided simply by regularly washing your hands.
Oils and fats are the raw material of all soaps. So, somewhat ironically, if you carelessly splash bacon fat onto your shirt cooking breakfast, the very substance you use to clean it out again can itself be derived from lard. Soap lifts greasy stains from your clothes and washes the bacteria-laden oil off your skin because it is able to mingle comfortably with both fatty compounds and water, which do not themselves mix. It takes a special kind of molecule to display this social-butterfly behavior: one with a long hydrocarbon tail that mixes with fats and oils and a charged head that dissolves well in water. An oil or fat molecule is itself composed of three “fatty acid” hydrocarbon chains all stuck onto a linker block. The key step in making soap, known as the saponification reaction, is to snap the chemical bonds attaching the three fatty acids. A whole category of chemicals known as alkalis are able to do this, “hydrolyzing” the connector bond. Alkalis are the opposites of acids, and when the two meet they neutralize each other to produce water and a salt. Common table salt, sodium chloride, for example, is formed by the neutralization of alkaline sodium hydroxide with hydrochloric acid.
So to make soap, you need to produce a fatty acid salt by hydrolyzing lard with an alkali. While it’s true that oil and water don’t mix, this fatty acid salt can embed its long hydrocarbon tail into the oil and leave its head poking out to dissolve in the surrounding water. Coated with a fur of these long molecules, a small droplet of oil is stabilized in the midst of the water that rejects it, and so grease can be lifted off skin or fabric and be washed away. The bottle of “invigorating, reviving, hydrating, deep clean sea splash” men’s shower gel in my bathroom lists nearly thirty ingredients. But alongside all the foaming agents, stabilizers, preservatives, gelling and thickening agents, perfumes, and colorants, the active ingredient is still a soap-like mild surfactant based on coconut, olive, palm, or castor oil.
The pressing question, therefore, is where to get alkali in a post-apocalyptic world without reagent suppliers. The good news is that survivors can revert to ancient chemical extraction techniques and the most unlikely-seeming source: ash.
The dry residue left behind after a wood fire is mostly composed of incombustible mineral compounds, which give ash its white color. The first step to restarting a rudimentary chemical industry is alluringly simple: toss these ashes into a pot of water. The black, unburned charcoal dust will float on the surface, and many of the wood’s minerals, insoluble, will settle as a sediment on the bottom of the pot. But it is the minerals that do dissolve in the water that you want to extract.
Skim off and discard the floating charcoal dust, and pour out the water solution into another vessel, being careful to leave behind the undissolved sediment. Drive off the water in the new vessel by boiling it dry, or if you’re in a hot climate, pour the solution into wide shallow pans and allow it to dry in the warmth of the sun. What you’ll see left behind is a white crystalline residue that looks almost like salt or sugar, called potash. (In fact, the modern chemical name for the predominant metal element in potash derives its name from this vernacular: potassium.) It’s crucial that you attempt to extract potash only from the residue of a wood fire that burned out naturally and wasn’t doused with water or left out in the rain. Otherwise, the soluble minerals we are interested in will already have been washed away.
The white crystals left behind are actually a mixture of compounds, but the main one from wood ash is potassium carbonate. If you burn a heap of dried seaweed instead and perform the same extraction process, you can collect soda ash, or sodium carbonate. Along the western shoreline of Scotland and Ireland the gathering and burning of seaweed was a major local industry for centuries. Seaweed also yields iodine, a deep-purplish element that you’ll find very useful as a wound disinfectant as well as in the chemistry of photography, which we’ll come back to.
If you follow the process described above, you can collect about a gram of potassium carbonate or sodium carbonate from every kilogram of wood or seaweed burned—that is only about 0.1 percent. But potash and soda ash are such useful compounds that it is well worth the effort in extracting and purifying them—and remember that you can use the heat of the fire for other applications first. The reason that timber serves as a ready-packaged stash of these compounds is that over decades of time the tree’s root network has been absorbing, from a vast volume of soil, water and dissolved minerals that can then be concentrated with fire.
Both potash and soda ash are alkalis; indeed, the very term derives from the Arabic al-qalīy, meaning “the burned ashes.” If you now mix your extract into a boiling vat of oil or fat, you can saponify it, creating your own cleansing soap. You can therefore keep the post-apocalyptic world clean and resistant to pestilence with just base substances like lard and ash, and a little chemical know-how.
This hydrolysis reaction is enhanced, however, if you use a more strongly alkaline solution: lye. This is where we return to slaked lime, calcium hydroxide.
You don’t want to use slaked lime itself for saponification because calcium soaps form a scum on water rather than a lovely lather. But the calcium hydroxide can be reacted with potash or soda so
that the hydroxide swaps partners to produce potassium hydroxide or sodium hydroxide: caustic potash or caustic soda, both of which are traditionally called lye. Caustic soda is powerfully alkaline (it will readily hydrolyze the oils in your skin into human soap, so be extremely careful in handling it) and is therefore ideal for this crucial saponification process, making cakes of hard soap.*
Another alkali that is very easy to produce is ammonia. Humans, and indeed all mammals, get rid of excess nitrogen as a water-soluble compound called urea, which we excrete in urine. The growth of certain bacteria converts urea into ammonia—the distinctive stench of which you’ll be all too familiar with from poorly cleaned public restrooms—and so the crucial alkali ammonia can also be produced by distinctly low-tech means: fermenting pots of piss. This was historically a crucial process for the production of clothes dyed blue with indigo (traditionally the blue of jeans). We’ll return to the diverse uses of ammonia later.
Saponification of fat molecules will also give you another useful byproduct. The chemical component of the lipid that acts as the linker block grasping the three fatty acid tails, glycerol, is left behind after lard is transformed into soap. Glycerol is itself fabulously handy, and can be easily extracted out of a lathery soap solution. The fatty acid salts of the soap itself are less soluble in brine than in fresh water, and so adding salt will cause them to sediment out as solid particles, leaving behind the glycerol in the fluid. Glycerol is a key raw material for making plastics—and explosives (which we’ll come to in Chapter 11).
The Knowledge: How to Rebuild Our World From Scratch Page 11