5 minute read


Machine-made Paper

Hardly any paper for book printing is made from rags today. Wood now is the main ingredient of paper pulp, though the better papers contain cotton fiber, and the best are made entirely of cotton. The fibers are converted into pulp by chemical and/or mechanical means.

Chemical pulp starts with logs that have had their bark peeled off and that have been reduced to chips. The wood chips are boiled in strong caustic solutions that dissolve away parts of the wood that are not cellulose, such as lignin and resin, and leave the cellulose fibers more or less free. There are two chief processes for producing chemical pulp: the kraft process, and the sulfite process. The kraft process uses the wood of either deciduous (e.g., poplar) or coniferous trees (e.g., spruce, fir, and hemlock) and produces a very strong paper. The sulfite process is less widely used and employs only coniferous wood and an acid solution in paper manufacture.

Mechanical pulp is mostly made by stone-grinding peeled logs in a stream of water so that the wood is broken up into fibers. Spruce, balsam, and hemlock are the woods considered best suited for pulping by this process. The ground wood contains all of the constituents of the original wood, including those that would have been eliminated as impurities in chemical pulp manufacturing. Mechanical pulp is mainly used for newsprint because paper made from mechanical pulp quickly discolors and becomes brittle. It also tends to be weak. A superior, stronger form of mechanical pulp is called thermomechanical pulp. It is made from wood chips treated by steam under high pressure. Mechanical pulp is sometimes added to chemical pulp for making low cost book papers. A paper containing no mechanical pulp is called a free sheet.

Before the pulp can be made into paper, it is necessary to mechanically beat or refine it. It is also usually bleached with chlorine and calcium hypochlorite. Unbleached kraft pulp is used for grocery bags and heavy wrapping paper. Other materials may also be added to the pulp depending on the type of paper to be made. For book paper, fillers such as white clay and titanium oxide may be added to provide opaqueness and extra whiteness. Size may be added for stiffness and smoothness. Dyes are added for tinted papers. The specific combination of pulp and additives used to produce a particular type of paper is called the furnish for that paper. With better grades of paper, care is taken to produce a furnish that is chemically neutral (pH 7 on the acid-base scale). For a paper to have long life, it must be acid-free.

The machine that converts the pulp into paper is called a fourdrinier machine, after Henry and Sealy Fourdrinier who financed its development in England in the early 1800s. The fourdrinier machine takes pulp that is still 99% water and converts it into a continuous web of paper containing only a small amount of moisture.

Pulp is continuously fed into the fourdrinier machine on the surface of a moving endless belt of fine mesh screening, usually made of nylon. Deckle straps prevent the liquid pulp from slopping over the sides. The screening is shaken from side to side as it moves forward to help drain the water. Suction boxes below the screening pull more water through, as a wire-mesh-covered cylinder presses on the web of pulp from above. The cylinder may be covered with a plain wire cloth to impart a wove effect, or with wire in a ladder pattern to produce a laid effect. To produce a watermark, the paper-maker attaches a wire design to the cylinder.

The now very soggy paper is placed on an endless belt of wool felt that carries it between a series of rollers that squeeze more water from it. It then passes over a series of very large, steam-heated, cast-iron drums that complete the drying process. During drying, the web is held tightly against the hot drums by endless belts of fabric.

After the paper has dried, it is usually run through a series of highly polished metal (calendar) rollers that further compact it and smooth its surface. The calendar rolls are arranged in pairs; each pair rolls at a different speed; this effectively polishes the paper. A variety of calendared finishes can be obtained, ranging from antique (softest and dullest), through eggshell, vellum, machine finish, to English finish (hardest and shiniest available without further treatment).

Further treatment may include supercalendaring, surface sizing, or coating. Supercalendaring is a polishing process similar to the calendaring process but done on a separate machine. The final finish of coated papers are brushed or rolled on in liquid form. The finish may be matte or glossy. Most papers include size in the furnish, but additional sizing may be added to the surface by running the paper through a vat of sizing material to provide a harder finish after the paper is made. The paper may be coated with fine clay. The clay is adhered to the surface of the paper with adhesives. The paper is then supercalendared with extremely smooth rollers. Dull coated papers are made with clays that finish dull, and are less calendared. Other papers are gloss coated. Papers may be coated on one or both sides.

Machine-made paper has a pronounced grain, as evidenced by its tendency to tear and fold preferentially in one direction. This is because the cellulose fibers tend to align themselves in the direction of travel as the pulp is laid down on the wire. Shaking does not completely achieve random alignment. In reeled paper, the grain always runs lengthwise. In sheet paper, the grain may run either the long way or the short way, depending on how it was cut from the reel.

Like most fabrics, paper has a right and wrong side. The bottom of the web (called the wire side) next to the screening at the wet end of the fourdrinier machine is slightly rougher than the top (or felt) side. If only one side of the paper is to be used, the smoother side is usually chosen. Paper made on a twin-wire fourdrinier machine has either two felt sides or two wire sides; this is because two webs of pulp are laid down simultaneously and pressed together as the paper is dried and finished. One-sided paper is more expensive than ordinary two-sided paper.

Additional topics

Science EncyclopediaScience & Philosophy: Overdamped to PeatPaper - Hand-made Paper, Machine-made Paper, Paper Categories, Paper Weights