Editor’s Note: Lewis Dartnell’s book The Knowledge: How to Rebuild Our World from Scratch (the-knowledge.org), distills the crucial information a group of postapocalyptic survivors would need to recover if society were ever to collapse, be it from a pandemic outbreak, asteroid impact or other disasters. This basic knowledge on the key technologies and central scientific principles that underpin our everyday lives would serve as a quick-start guide for civilization itself to help survivors rebuild as quickly as possible and avert a prolonged dark age. Below is a basic primer on photography from Chapter 11.



The following is excerpted with permission from The Knowledge: How to Rebuild Our World from Scratch, by Lewis Dartnell. Available from Penguin Press, a member of Penguin Group (USA), LLC, a Penguin Random House Co. Copyright © Lewis Dartnell, 2014.





Photography is a wonderful technique—a way of harnessing light to record an image, capturing an instant in time and preserving it for eternity. A holiday picture can trigger vivid reminiscences even decades later, but a photograph records the visual world with far greater fidelity than memory can ever offer. Yet beyond drunken party snapshots, family portraits, or breathtaking landscapes, the incomparable value of photography over the past two hundred years has been in presenting what the eye cannot see. It represents a key enabling technology across numerous fields of science, and will be vital in accelerating the reboot of society after a disaster. Photography allows investigators to record events and processes that are exceedingly faint or occur over timescales too rapid or too slow for us to perceive, or at wavelengths invisible to us. For example, photography offers extended exposure times to soak up feeble light over far longer periods than the human eye can offer, allowing astronomers to study a multitude of dim stars and resolve faint smudges into detailed galaxies and nebulae. Photographic emulsions are also sensitive to X-rays and so allow you to create medical images for examination of the body’s interior.



The crucial chemistry behind photography is simple enough: certain compounds of silver darken in sunlight and so can be employed to record a black-and-white image. The trick is to create a soluble form of silver that can be spread evenly in a thin film, but then convert it into an insoluble salt that sticks on the outside surface of your photographic medium and doesn’t get washed away again.



First, coat a sheet of paper with egg whites containing some dissolved salt, and allow it to dry. Now dissolve some silver in nitric acid, which will oxidize the metal to soluble silver nitrate, and spread the solution over your prepared paper. The sodium chloride will react to create silver chloride, which is both light-sensitive and insoluble, and the egg albumin will prevent the photographic emulsion from soaking into the paper fibers. If you scavenge in the post-apocalyptic world, a single solid-silver teaspoon will contain enough of the pure element to produce over 1,500 photographic prints.



When light rays hit this sensitized paper, they provide the energy to liberate electrons in the grains and so reduce the silver chloride back to metallic silver. Large lumps of silver, such as a polished platter, have a bright luster, but a speckle of tiny metallic crystals scatters the light instead and so looks dark. On the other hand, areas of the sensitized sheet not exposed to light remain the white of the paper behind. The key follow-up step after the exposure is to kill this photochemical reaction and so stabilize the captured shadows. Sodium thiosulfate is the fixing agent still used today and is relatively easy to prepare. Bubble sulfur gas through a solution of soda or caustic soda, then boil with powdered sulfur and dry for crystals of “hypo” (a nickname derived from its old name, hyposulfite of soda).



Using a lens set into the front of a light-tight box to project an image onto sensitive paper on the back wall produces a photographic camera, but even in bright sunshine it can take many hours for this rudimentary silver chemistry to take a “photo.” Luckily, you can increase the sensitivity of your camera enormously with a developer—a chemical treatment that completes the transformation of partially exposed grains, reducing them entirely to metallic silver. Ferrous sulfate works well, and can be synthesized easily enough by dissolving iron in sulfuric acid. And as the chemical proficiency of the post-apocalyptic society improves, in place of chlorine salt you can substitute that of one of its atomic siblings, iodine or bromine, which produce far more light-sensitive photographic emulsions.



However, the fact that light exposure turns the photosensitive grains dark with silver metal, whereas shadows in the scene remain pale, means that the photo comes out tonally reversed from what your eye sees—you’ll get a “negative.” There is no fast-acting chemical reaction that produces a permanent positive image—no initially black substance that rapidly bleaches in sunlight—and so photography is burdened with this negative outcome. The necessary conceptual leap to is realize that if this reversed, negative image is created in the camera on a transparent medium, all that is needed is a second stage of printing out using the negative as a mask on top of sensitized paper, so that the pattern of highlights and shadows reverses again back to normal. The wet collodion process uses guncotton dissolved in a mixture of ether and ethanol solvents—all substances we have already come across in this book—to produce syrupy, transparent fluid. It’s perfect for coating a glass plate with photochemicals, and then exposing and developing the image before it dries into a tough, waterproof film. And if instead you use gelatin (boiled out of animal bones) it is possible to create a dry plate that is even more photosensitive and allows much longer exposure times.



Photography is a fantastic example of a novel application created by the fusion of several preexisting technologies, and using relatively straightforward materials and substances. Build a fireclay-lined kiln to produce your own glass by melting silica sand or quartz with a soda ash flux. Take one dollop to grind into a focusing lens and another to flatten into a rectangular pane for a negative plate, and draw on your papermaking skills to produce smooth prints. The chemistry underlying photography uses the same acids and solvents marshaled time and time again in this book, and you can take a primitive photo using substances derived from a silver spoon, a dung heap, and common salt. Indeed, if you fell through a time warp back to the 1500s you’d readily be able to source all the chemicals and optical components you needed to build a rudimentary camera, so you could show Holbein how to take a photograph of King Henry VIII rather than create an oil painting.



Filling in the periodic table of the elements, exploiting explosives, and using photography as a tool for rediscovery will all be important activities for a civilization restarting after the Fall. But as the post-apocalyptic society recovers and begins to flourish, it will need ever-increasing amounts of the basic substances we’ve discussed throughout this book. And to meet these demands, civilization will need to develop the more advanced processes of industrial chemistry.



We often hear about the Industrial Revolution and the innovation of ingenious mechanical contraptions for alleviating the toil of humankind, thereby greatly accelerating the pace of progress and transforming eighteenth-century society. But the transition to an advanced civilization is as much about the invention of chemical processes for the large-scale synthesis of the necessary acids, alkalis, solvents, and other substances critical to the running of society as it is about machinery for automating spinning and weaving and building roaring steam engines.



Author’s Note:



One of the things I tried hard to ensure while researching and writing The Knowledge was that I got first-hand experience in many of the processes and skills I talk about, and this includes even the author’s photograph on the book cover, shown here.



With help from Richard Jones, a photography historian, I took this photo myself by mixing together the rudimentary silver chemistry and using a primitive single-lens camera, resurrecting techniques that date back to the 1850s—the earliest years of photography.



There are a few interesting things to note with this photograph. Firstly, the primitive silver chemistry we used is very slow to react to light—it is hugely less photosensitive than modern ISO 400 films—and so correspondingly long exposures are needed. The image here is a 16-second exposure (hardly a snapshot), and the slightest, imperceptible movement during that time results in a horribly blurred photo. To help solve this problem, hidden conveniently out of sight behind me is a wrought-iron stand and skull brace for holding my head perfectly stationary!



Such long exposures also mean that it is exceedingly hard to smile naturally and hold the expression perfectly still for a good fraction of a minute without it looking like a rigor mortis snarl. This goes a long way toward explaining the ridiculously stern look common in early portraits of Victorian ladies and gentlemen.



Also, the simple photochemical system used here is more sensitive to ultraviolet than visible light, as UV rays deliver more energy to drive the silver conversion reactions. This means that these photographs aren’t quite recording the world as the human eye sees it. As you can see, primitive photos make the lips look unusually pale (because they reflect more UV) and the skin appears more textured and blotchy in the UV as you are looking into a deeper level of the dermis.



To explore further material, including videos and behind-the-scenes views, and to join the conversation visit the-knowledge.org. Lewis Dartnell is a research fellow at the University of Leicester in England.



To talk to Lewis directly about the themes of The Knowledge, join his Reddit Ask Me Anything today (Friday) from 11a.m. EDT at http://redd.it/24jndm

