Wood is made of cellulose fibers held together by a natural glue called lignin. To make paper, a mill has to free the fibers. In chemical pulping, wood chips are cooked in a huge pressure cooker called a digester. Heat and chemicals dissolve the lignin and leave the fibers behind.

The kraft process makes very strong paper. Its name comes from the Swedish word for strength. Brown paper grocery bags and heavy wrapping paper are made from unbleached kraft pulp.

Not all pulp is cooked. Some is made by grinding logs against huge spinning stones. Newsprint turns yellow and brittle as it ages, because it is made with ground wood that still has lignin in it. A paper called wood-free paper has no lignin in it. It is not free of wood, though. It is made from wood that was pulped with chemicals.

A pulp mill can make more power than it needs. It burns bark and the used cooking liquid to make steam and electricity. Paper mills also burn bark and other leftovers from making paper to make steam and power for the mill.

Paper has been made for a very long time. A court official in China named Cai Lun is said to have made paper from tree bark, old rags and fishnets more than 1,900 years ago. For hundreds of years most paper was made from rags. In the 1800s, people learned to make it from wood pulp, and wood became the main ingredient.

The Fourdrinier machine is named for Henry and Sealy Fourdrinier, two London stationers who paid for its development in the early 1800s. A paper machine starts with a moving belt of fine mesh. Wet pulp is poured onto it, and the water drains away. The sheet is squeezed between rollers and dried with heat.

The finishing touches matter too. Mills add a coating called size, because without any size, paper soaks up ink like a blotter. Paper is rolled between hard metal rollers called calenders to make it smoother and flatter. Paper made with long fibers is stronger, which is why mills like spruce and other cone-bearing trees.

Words to know

• lignin: natural wood glue
• digester: pressure cooker
• kraft: strength
• brittle: easily broken