Some tried straw, others the stalks of various plants, the fibers of nettles, or the bark of trees. Some of these experiments produced usable sheets, but none offered a supply that was both large and cheap. Straw found some use for coarse papers, and in the nineteenth century a grass called esparto, imported from Spain and North Africa, became an important raw material for fine printing papers in Britain. The most abundant source, however, was wood.
Wood is made largely of cellulose fibers, but they are held together by another substance, lignin, which binds the fibers into a rigid structure. Separating the fibers of wood is much harder than separating the fibers of rags. In the 1840s, a German inventor named Friedrich Gottlob Keller developed a method for grinding wood against a rotating grindstone while water was sprayed onto it, producing a mass of fibers that could be made into paper. This mechanical pulp, often called groundwood, was cheap and could be produced in large quantities. It contained the lignin of the wood, however, and the paper made from it was weak and tended to darken and become brittle when exposed to light and air. Groundwood pulp came to be used for newspapers and other papers not meant to last.
Chemical methods offered a way to remove the lignin and free stronger, cleaner fibers. In the 1850s, the soda process was developed, in which wood chips were cooked in a solution of caustic soda. In the following decades, the sulfite process was developed, in which wood was cooked with a solution containing sulfur compounds. The sulfite process was first patented in the 1860s and became commercially important in the 1870s and 1880s, producing a bright, fairly strong pulp that could be bleached. In 1879, a chemist working in Danzig, Carl Dahl, developed a further method, now called the kraft process, from the German word for strength. It used a solution of sodium hydroxide and sodium sulfide and produced a dark but very strong pulp. Because the chemicals used in the kraft process could be recovered and reused, and because it worked well with many kinds of wood, it eventually became the most widely used method of making chemical pulp.
The shift to wood transformed the paper industry. Mills grew larger and were built closer to sources of timber. Regions with abundant softwood, such as Scandinavia, parts of Central Europe, Canada, and the northern United States, became major producers of pulp and paper. The price of paper fell sharply over the second half of the nineteenth century, and paper became cheap enough for daily newspapers, inexpensive books, magazines, catalogs, and a growing variety of packaging. By the end of the century, wood had largely replaced rags as the main source of fiber, though rag paper continued to be made for documents and other uses in which permanence and strength mattered most.
Bleaching also changed during this period. Early papermakers had depended on clean, white rags and on exposure to air and light to whiten their sheets. The discovery of chlorine in the late eighteenth century led to its use for bleaching pulp, which allowed mills to use colored or soiled rags and, later, to whiten wood pulp. Early bleaching with chlorine sometimes damaged fibers when it was not carefully controlled. Over time, methods were refined, and in the late twentieth century many mills adopted bleaching methods that used chlorine dioxide, oxygen, or hydrogen peroxide instead of elemental chlorine.
VII.