In the kraft process, wood chips are cooked in a digester with a hot solution of sodium hydroxide and sodium sulfide. The cooking dissolves lignin and hemicellulose, so what is left is nearly pure cellulose fibers. Kraft is the main process worldwide for making the wood pulp used in paper. Kraft cooking is done at very high alkalinity, with a pH of 13 to 14, usually at about 170 degrees Celsius for about two hours. The solid pulp that comes out is about half the weight of the dry wood chips that went in. It is called brown stock, and the used liquid is called black liquor, both for their colors. The kraft process got its start in the 1880s. A mill manager in Sweden stopped a sulfate digester run too early, but processed the half-cooked chips anyway. The pulp made paper that was dark but far stronger than any paper known before, and the Swedish word for strength, kraft, became the name of the process.

Before kraft, the sulfite process was the main chemical method. It dates to the 1840s, and by 1900 it was the leading way to make wood pulp, ahead of mechanical pulping. Kraft later overtook it in the 1940s because it works on more kinds of wood and makes stronger fibers. In the nineteenth century, rags became scarce as more people wanted paper, so papermakers looked for other fibers. The 1843 invention of wood pulp and the Second Industrial Revolution made pulpwood paper the main kind. Canada's Charles Fenerty and Germany's Friedrich Gottlob Keller each developed wood pulping around 1844. The sheet-making machine we use today came from France and England. Louis-Nicolas Robert of France got a patent in 1799 for a machine that makes a continuous sheet. His backers took the idea to England, where the Fourdrinier brothers paid to improve it, and it was installed at Frogmore Paper Mill in 1803.

Recycling begins with water and chemicals. Used paper is mixed with them and chopped and heated, which breaks it down into strands of cellulose called pulp. The pulp is strained through screens to remove plastic, cleaned, de-inked, bleached and mixed with water before it can be made into new recycled paper. Paper can be made from three kinds of recycled material. Mill broke is scraps from making paper that go back into the same mill. Pre-consumer waste left the mill but was never used. Post-consumer waste is paper such as used cardboard boxes, magazines and newspapers. Paper fibers wear out. Each time paper is recycled the fibers are cut and weakened, and after being recycled four to six times, wood fibers become too short and weak to make paper. That is why mills mix fresh fiber, called virgin fiber, into recycled pulp for quality. So paper cannot be recycled forever.

A better recycling process helped save paper. In 1774 a German jurist, Justus Claproth, invented a way to remove printing ink from used paper, which is now called deinking. Before wood pulp, old rag paper was recycled only into low-grade board, so deinking opened up a way to make paper again from paper. Bleaching costs fibers. Bleaching pulp to make white paper uses up about 5 percent of the fibers. Newsprint ranges from 55 to 75 on the ISO brightness scale, while writing and printer paper can be as bright as 104. The Hollander beater was developed by the Dutch in 1680 to make paper pulp from plant fibers. It could produce in one day as much pulp as a stamp mill made in eight days, which made papermaking much faster, though its metal blades cut fibers shorter and made weaker paper.

Watermarks were first made in Fabriano, Italy, in 1282, by changing the thickness of the paper while it was still wet. Since the patterned part is thinner, it lets more light through and looks lighter. Watermarks have been used on stamps and money to discourage counterfeiting. Corrugated board was patented in England in 1856 and first used as a liner for tall hats. In 1871 Albert Jones of New York patented single-sided corrugated board for wrapping bottles, and in 1874 Oliver Long added a liner sheet on the other side, giving corrugated board as we know it. The United States recycled about 46 million tons of paper and paperboard in 2018, a recycling rate of 68.2 percent, the highest of any material EPA tracked in household trash. Corrugated boxes were recycled at 96.5 percent, newspapers at 64.8 percent and other paper packaging at 20.8 percent. The paper industry's group says that in 2025 the paper recycling rate was between 61 and 65 percent, about 45 million tons.

Words to know

• kraft: strength
• digester: pressure cooker
• deinking: removing ink
• corrugated: wavy ridged