Paper is made by breaking wood down into individual fibers, mixing those fibers with water to form a thin slurry, then spreading that slurry across a screen so the water drains away and the fibers bond together into a flat sheet. That core process hasn’t changed much in over a thousand years, but modern mills do it at enormous scale and speed. Here’s what happens at each stage.
Turning Trees Into Pulp
Papermaking starts in the forest. Softwood trees like pine and spruce are the most common source because their long fibers create strong paper, though hardwoods like birch and eucalyptus are also widely used for smoother printing papers. After harvesting, logs are stripped of bark and fed into one of two pulping processes.
Mechanical pulping physically grinds logs against a rotating stone or feeds wood chips between steel refiner plates to tear the fibers apart. This method converts a high percentage of the wood into usable pulp, wasting very little raw material. The tradeoff is that it uses a lot of electricity and produces weaker fibers. Paper made from mechanical pulp also yellows over time because it retains lignin, the natural glue that holds wood cells together. Newsprint and magazine inserts are typical mechanical-pulp products.
Chemical pulping takes the opposite approach. Wood chips are cooked in a pressurized vessel called a digester with a solution of chemicals that dissolve the lignin and free the cellulose fibers. The most common version, called the kraft process, uses sodium hydroxide and sodium sulfide. Chemical pulping yields less pulp per ton of wood, but the fibers that come out are longer, stronger, and more resistant to aging. Cardboard boxes, office paper, and book pages typically start as chemical pulp.
After pulping, the raw fibers are washed and screened to remove leftover wood fragments and chemicals. If the end product needs to be white, the pulp goes through a bleaching stage using oxygen, hydrogen peroxide, or chlorine dioxide to strip out remaining color.
From Pulp Slurry to Paper Sheet
Once the pulp is clean, it gets diluted with a large amount of water until it’s about 99 percent water and 1 percent fiber. This thin slurry, called furnish, is pumped into the headbox of the paper machine. The headbox is essentially a pressurized chamber that spreads the slurry evenly across a fast-moving mesh screen known as the forming fabric or wire.
Most large-scale mills use a machine called a Fourdrinier, named after the brothers who financed its early development. On a Fourdrinier, the wire forms a continuous belt that picks up the fiber slurry at the breast roll, then carries it over a series of table rolls, foils, and suction boxes. Each element pulls water down through the mesh by gravity or vacuum, and the fibers start settling into a thin mat. By the time the web reaches the couch roll at the end of the wire section, enough water has drained away that the fibers hold together as a fragile, damp sheet.
The wet sheet then enters the press section, where it passes between heavy rollers lined with felt. These rollers squeeze out more water mechanically, bringing the moisture content down further. From there, the sheet travels through the dryer section: a long series of steam-heated cylinders. The sheet wraps around one hot cylinder after another, evaporating the remaining water until the paper reaches its target moisture level, usually around 5 to 8 percent.
Near the end of the machine, the paper may pass through a calender stack, a set of polished steel rollers that press the surface smooth and control its thickness. For glossy paper, a separate coating station applies a thin layer of clay or calcium carbonate before calendering. The finished paper winds onto a large reel at the end of the machine, often moving at speeds exceeding 60 miles per hour on high-speed machines.
How Much Water It Takes
Papermaking is one of the most water-intensive manufacturing processes. Modern pulp mills typically consume around 20 cubic meters of water per ton of paper produced, but the range across the global industry is enormous, spanning from 10 to over 300 cubic meters per ton depending on the mill’s age, location, and technology. For perspective, at the start of the 20th century, producing a single ton of paper required 500 to 1,000 cubic meters of water.
Mills have cut consumption dramatically by recycling process water internally. Closed-loop water systems identify every point in the mill where water enters and exits, then route used water from one stage to supply another stage that can tolerate a lower purity level. The most efficient mills in countries like Germany have driven consumption down to about 13 cubic meters per ton. Still, more than 15 percent of paper mills worldwide use over 100 cubic meters per ton, so there’s significant room for improvement industry-wide.
How Recycled Paper Is Made
Recycled paper follows the same basic forming and drying steps, but the front end of the process is different. Instead of starting with wood, mills start with bales of collected wastepaper. The first job is turning that wastepaper back into usable fiber.
The bales are dumped into a large vat called a pulper, where water and agitation break the paper back down into a fiber slurry. From there, the pulp goes through screening to remove staples, tape, plastic film, and other contaminants, followed by centrifugal cleaning to spin out heavier debris like sand or glass fragments.
The trickiest step is de-inking. In flotation de-inking, air bubbles are passed through the diluted pulp while a foaming agent is added. Ink particles are hydrophobic, meaning they repel water, so they cling to the rising air bubbles and float to the surface in a layer of foam. That foam is scraped off as a reject stream, leaving cleaner fibers behind. Depending on the grade of paper being produced, the pulp may also go through a washing stage and a round of bleaching to brighten it further.
Each time paper is recycled, the fibers get shorter and weaker. Most fibers can go through the recycling process five to seven times before they’re too degraded to form a usable sheet. That’s why recycled paper is often blended with a percentage of virgin fiber to maintain strength.
Specialty Papers Use Different Fibers
Not all paper comes from wood. U.S. currency, for example, is printed on a blend of 75 percent cotton and 25 percent linen. Cotton and linen fibers are longer and more durable than wood pulp fibers, which is why a dollar bill can survive a trip through the washing machine while a receipt disintegrates. The currency paper also has small red and blue security fibers embedded throughout the sheet during manufacturing to help prevent counterfeiting.
Other specialty papers draw on different raw materials for specific properties. Cigarette paper uses flax or hemp fibers for slow, even burning. High-end stationery and art papers sometimes use cotton rag pulp for texture and longevity. Tea bags often incorporate heat-sealable synthetic fibers so the bag holds together in boiling water. In each case, the fiber source is chosen for a physical property that wood pulp can’t easily provide, but the basic process of suspending fibers in water, forming a sheet, and drying it remains the same.