Cellulose in plant cell walls works like the steel bars in reinforced concrete, with lignin acting like the hardened cement that glues the fibers together. Because wood's strength comes from cellulose fibers set in a lignin matrix, pulping works by dissolving or tearing apart the lignin to free the fibers. Papyrus and paper are different things. Papyrus is made by laying thin strips of the pith of a reed side by side, putting a second layer across them and pounding them together with a mallet. Paper is made from fibers that have been broken apart and then re-formed from a watery slurry. Papyrus had several weaknesses that helped paper replace it. It grew mainly in Egypt, it took more work and money to make than paper, and it was fragile and could break apart in damp conditions. Paper could be made almost anywhere that had plant fibers and water, so it spread where papyrus could not.

Hydrogen bonds hold paper together. Cellulose has many hydroxyl groups, and the hydroxyls on one chain form hydrogen bonds with oxygen atoms on the same or a neighboring chain. The same bonding between wet fibers is what makes a sheet when the water leaves. By the 7th century, paper had reached Japan from China by way of the Korean Peninsula. Japanese papermakers developed washi, tough paper made by hand from the inner bark of the gampi tree, the mitsumata shrub or the paper mulberry. Washi is traditionally made in winter, because pure cold running water is essential. Cold inhibits bacteria, which keeps the fibers from decomposing. A kraft mill reuses almost all of its cooking chemicals. The used black liquor is concentrated and burned in a recovery boiler, which turns sodium sulfate back into sodium sulfide. The molten salts are dissolved to make green liquor, which is treated with lime to regenerate the white liquor used to cook more wood.

Papermaking spread from China to the Middle East by the 700s and to Spain by about 1150, and then through the rest of Europe. The first paper mill in what is now the United States was built in Pennsylvania in 1690. Cai Lun is traditionally called the inventor of paper, but paper-like material is older than he is, so his work is better described as an improvement that made paper cheap and widespread. Wasps and hornets were making paper long before people. Social wasps build their nests from plant fiber, mostly wood pulp, and hornets build theirs by chewing wood into a papery pulp. One history says Cai Lun's papermaking method, using rags and other plant fibers in 105 CE, was inspired by wasps and bees. Paper found everyday jobs early. The Song dynasty government of China was the first to issue paper money, in the years 960 to 1279. Earlier, in the Tang dynasty, paper was folded and sewn into square bags to keep the flavor of tea, which shows how many everyday uses people were already finding for paper.

The wire of a Fourdrinier machine was once really wire. In the forming section, a specially woven conveyor belt of mesh, known as a wire because it was once woven from bronze, carries a slurry of fibers that drains to create a continuous paper web. Today the belt is a woven fabric, but the name stuck. In 1809 John Dickinson patented a different kind of paper machine, the cylinder mold machine. A cylinder covered in wire mesh turns while partly dipped in a vat of dilute pulp, and the pulp builds up on the mesh. A couch roller then picks the wet sheet off the mold. After pressing, the drying section of a paper machine dries the sheet. The pressed sheet winds in a serpentine path partly around a series of steam-heated drying cylinders, which brings the water content down to about 6 percent, the level where paper stays at typical indoor conditions.

The ISO 216 system of paper sizes was first adopted in Germany in 1922 and spread as other countries adopted the metric system. Its largest standard size, A0, measures one square meter, and every sheet in the series is the same shape, with each size half the area of the one before it. Recycled pulp, also called deinked pulp, is recycled paper that has been processed to take out printing ink and unwanted material and free the fibers. A recycled ton of newsprint saves about a ton of wood, and a recycled ton of printing or copier paper saves a bit more than two tons of wood, because kraft pulping needs about twice as much wood. Recycling one ton of office paper can save the energy equivalent of burning 322 gallons of gasoline, according to the U.S. Environmental Protection Agency. The EPA also says paper materials must be empty, clean and dry before being recycled. The corrugated box was first used to package glass and pottery containers, and by the mid-1950s it let fruit and produce be shipped without bruising.

Words to know

• lignin: natural wood glue
• cellulose: plant fiber stuff
• kraft: strength
• washi: Japanese paper