It was a quasi-religious moment. There in front of me, in a display case at the British Museum, lay the original copy of The Adventure of the Missing Three-Quarter, in Sir Arthur Conan Doyle’s own hand. Like any other Sherlock Holmes fan, I have read and reread the detective’s adventures numerous times, but never before had I gazed upon an original version. Unfortunately, the hallowed moment was a little tainted by the aged appearance of the manuscript. It was a brownish yellow in color! Of course, one would expect a 100-year-old piece of paper to show its age. That was no surprise. But the appearance of the Missing Three-Quarter’s neighbor was. A Gutenberg Bible, produced over 500 years earlier, looked as good as new! And it will likely be on display long after the Sherlock Holmes manuscript has crumbled away along with millions of other books stored in the British Library and other major libraries around the world. What is the difference? It all boils down to the paper that was used.
Ah, paper. We don’t give it much thought, but our society would grind to a halt without it. Remember those promises that computers would provide a “paperless society?” Forget it. We use more paper than ever. Rough copies spew out of our printers, and we use reams of paper to feed our Internet habit. Yet most people have no idea of the complex chemistry involved in producing the marvelous product that gives us grocery bags, facial tissues, toilet paper, books, and a myriad of other products, including newsprint.
The earliest forms of paper were not that complicated. Thousands of years ago, the Egyptians scraped out fibers from the inside of the bark of the papyrus plant (our word paper derives from this), and pressed them into sheets. Actually, though, papyrus wasn’t really paper. Not by our modern definition, anyway: paper is the substance that forms when a slurry of disintegrated cellulose fibers is allowed to settle on a flat mold. When the water is drained away, the deposited layer can be dried into paper. The oldest surviving such piece, although devoid of any markings, was discovered in a Chinese tomb in 1957, and dates roughly to 100 BC. The first paper with writing on it is also of Chinese origin, and can be traced to about 110 AD. Supposedly, this paper was made by a process developed by Ts’ai Lun, the “chief eunuch” in the emperor’s court. Why the emperor needed eunuchs isn’t exactly clear, but to guard the ladies of the court would be a good guess. In any case, Ts’ai Lun apparently had some time on his hands, and discovered that macerating hemp fibers, old rags, and scrapings from the inner bark of mulberry trees with water, and then spreading the resulting pulp thinly on a drying frame, resulted in a material suitable for writing.
Amazingly, news of this discovery did not spread to the Western world for about 1,000 years. Europeans recorded their history on parchment, laboriously made from animal skins. When word finally reached Europe through Arabs who had learned about papermaking from the Chinese, one would have expected the Church to jump on the new technology. Such was not the case. Parchment was the only material fit to carry the Sacred Word, the Church maintained, and called papermaking a “pagan art.” Initially there was not much opposition to this curious view, because papermaking was not an easy task for Europeans. There were no mulberry trees, which seemed to be the key to Chinese paper. Finally, they turned to hemp fibers, cotton, and linen rags as raw materials. These were boiled in water to a point of disintegration, and then pounded into a pulp before being poured into drying trays. Treatment with animal gelatin usually followed to prevent water absorption and to reduce the spreading of the ink. Each sheet had to be made by hand, but the paper was of remarkably good quality, as witnessed by the spectacular condition of manuscripts such as the Gutenberg Bible. (Gutenberg printed Bibles both on parchment and on paper, so his work represents the transition from the old to the new.) Soon, as more and more people learned to read, and as the Industrial Revolution began to pick up steam, rags could no longer meet the demand for paper manufacture. This forced the English to pass a law that all burial garments had to be made of wool, a substance that could not be used to make paper. By the mid-nineteenth century, the shortage was so severe that America actually imported linen wrappings from Egyptian mummies to make paper. And then came a breakthrough. Friedrich Keller, in Germany, devised a method of making paper from trees!
This idea that paper really does grow on trees had actually been brewing since the early eighteenth century. That’s when René de Réamur, a French mathematician, physicist, and nature lover, had trouble publishing his research due to a simple lack of paper. Then, one day, while out on one of his nature walks, he happened to take a close look at a nest fashioned by North American wasps. Its light thin walls looked as if they were made of paper! Several months of study led him to the realization that the insects dined on twigs, which their digestive system somehow converted to paper. In 1719, he excitedly reported to the French Royal Academy that the American wasp makes a fine paper by extracting the fiber of common wood. “They teach us,” he said, “that one can make paper from fibers of plants without the use of rags or linens, and seem to invite us to try whether we cannot make fine and good paper from the use of certain woods.”
De Réamur was not an experimentalist, but Jacob Schaffer, a German clergyman, was. He successfully mimicked the work of the wasps and produced paper samples from various woods. Friedrich Keller, another German, capitalized on the idea and devised a papermaking process based on chipping wood and then beating the chips into pulp. The pulp could be mixed with water, and the resulting slurry poured through a fine screen. When dried, the residue from this “mechanical pulping” process yielded sheets of paper.
Joy, however, was short-lived, as the newfangled paper proved to be of poor quality. Chemists soon discovered why. The pulping process degraded the wood fibers into shorter fragments, which weakened the paper, and, unlike cotton or linen, wood pulp contained a substance called lignin, which caused the paper to discolor readily. As revealed by examination under a microscope, wood is made up of vertical stacks of hollow fibers, anywhere from 1 to 3 millimeters long, held together with the glue-like lignin. Because lignin is such a strong binding agent, it was difficult to separate intact fibers through mechanical grinding. The fibers ended up being ripped into smaller fragments, which give a weaker pulp than one made of the longer fibers found in cotton or linen.
And then there was the problem of yellowing. Lignin reacted with oxygen and light to produce colored molecules, which were responsible for the discoloration of the paper. So chemists went to work on trying to dissolve the lignin out of the pulp. They soon discovered that this could be done by “chemical pulping,” a process that involved boiling wood chips in a sulfite solution. It was superior to mechanical pulping, but the process also degraded some of the cellulose fibers. Good enough for newsprint, but not for quality paper. Around 1880, German paper manufacturers introduced the “kraft” process. Digesting wood pulp with a mixture of sodium sulfide and sodium hydroxide yielded paper that was strong (“kraft” is German for strong), but that still yellowed because of residual lignin. It was great for many uses, including grocery bags (we still use kraft paper for these), but had to be bleached if it were to be converted into writing paper.
Bleaching was not a problem, as chemists were already familiar with the ability of chlorine to remove color from fabrics. It had the same effect on lignin still left in the paper. But unfortunately, chlorine also degraded cellulose. (Just think of what happens if you leave bleach on a cotton fabric too long.) As we later learned, it also reacted with components of lignin to produce the notorious dioxins, compounds that are toxic in minute concentrations. This eventually forced the industry to look for alternative bleaching methods. Today, most bleaching is carried out with oxygen or chlorine dioxide, which do not produce dioxins. Another problem that plagued paper manufacturers was the smell produced by the delignification process. Anyone who has ever been around a paper mill will agree that the aromas of methyl mercaptan or dimethyl sulfide do not make treasured memories. Mercifully, modern pollution control equipment has dramatically reduced the emissions.
By the late 1800s, many, but certainly not all, of the paper production problems had been solved. A major concern was cellulose’s natural affinity for water. Paper lacked resistance to moisture, which meant that any ink applied would spread too easily. But chemists were not going to be stymied by this. Somehow, they had to find a way to waterproof the paper’s surface. And they did. The water repellant properties of rosin, a substance that could be extracted from the southern pine tree, were well known. But how could it be applied to paper? Aluminum sulfate (alum) was already used at the time as a “mordant,” a substance that allowed dyes to stick to fabrics, so applying the same chemistry to paper was logical. It worked. This was the very first example of “sizing” paper. The word derives from the Latin “assidere,” to set in place. Basically, water proofing chemicals are set in place on the surface of the cellulosic fibers.
Paper was further improved by the addition of materials such as starch and kaolin (a type of clay), which filled in some of the pores between the fibers, and titanium dioxide, which added opacity and brightness. Beautiful printed pages began to roll off the presses. Everything seemed hunky-dory. But this didn’t last long. Aluminum sulfate, you see, is an acidic substance. And acids break the glucose-glucose linkages in cellulose. This weakens the paper, and discolors it to boot. The fragments of cellulose now can be oxidized by air to molecules that contain “aldehyde” groupings, and such fragments are yellow. That’s why books printed on acid paper begin to turn yellow and crumble after thirty years or so, even if the lignin has been removed. That’s why the death knell now sounds for millions of books and manuscripts stored in libraries around the world.
Intense efforts have been mounted to save these works. Deacidification processes, ranging from rinsing individual sheets in alkaline solutions (calcium hydroxide, for example) to exposing whole books to gaseous bases such as diethyl zinc, have met with various degrees of success, but cannot possibly be applied to millions and millions of aging books. As with many other things in life, prevention is better than treatment. And once again, chemists have taken up the challenge. Sizing materials that do not leave an acid residue have been developed. They have names like “alkyl ketene dimers” or “alkyl succinic anhydrides,” which do not roll easily off the tongue. But water does roll easily off paper treated with them. And instead of an acid residue, these compounds leave an alkaline one. That also means that the expensive titanium dioxide whitener can be replaced by cheaper calcium carbonate. This cannot be used in acid papers because it reacts with acids to liberate carbon dioxide gas. Alkaline paper that uses this technology was introduced around 1990, and is taking over from acid papers. Tests show not only that alkaline paper is stronger, is more readily recycled, and can last for hundreds of years, but that its manufacture is less polluting, requires less energy, and leads to less machine corrosion.
There have also been dramatic developments in mechanical pulping. It turns out that if wood chips are heated with steam, the lignin softens enough so that the wood fibers can be pulled apart without much damage. The lignin is left in, but can be decolorized with hydrogen peroxide. Whereas, twenty-five years ago, mechanical pulp had to be blended with bleached kraft pulp to make paper strong enough for newsprint, today it can often be made with 100 percent “thermomechanical” pulp. Paper is clearly constantly being improved. So save this book and read it again in 100 years. You’ll be amazed by how some of the problems described here will have been solved.