Paper

Paper was probably first produced from bamboo and rag fibers about 2,000 years ago. By the eighth century, papermaking technology had spread to the Middle East. By the middle of the twelfth century, the Moors had transplanted the technology to Spain, from where it spread throughout Europe . Rags continued to be the chief source of paper fibers until the introduction of papermaking machinery in the early 1800s, when it became possible to obtain papermaking fibers from wood .

Hand-made and machine-made papers both consist of tiny cellulosic fibers pressed together in a thin sheet. Each of these fibers is a tiny tube, about 100 times as long as it is wide. Today, most fibers come from wood, though in earlier times, the source was more likely to have been rags of linen or cotton . The length of wood fibers from conifers is about 0.13-0.25 in (0.33-0.64 cm), and from hardwoods, about 0.04 in (0.10 cm). Other vegetable fibers are much longer. Cotton fibers, for example, may be one or more inches in length, with diameters of 0.02 in (0.05 cm). The source material is reduced to a slurry of fibers that floats freely in water , and many of the fibers will have been broken or cut when making the pulp. When the water is removed, the fibers form a thin layer of pulp which eventually becomes paper.

Hand-made paper

Rags to be made into paper are first sorted, and any unsuitable ones are discarded. Seams are opened, and nonfabric materials, such as buttons, are removed. The rags are chopped into small pieces and then cleaned by boiling them in strong cleansing solutions. Next, they are rinsed and beaten while damp until all of the threads have disintegrated and the fibers float freely in water. This is the paper pulp.

The very dilute pulp is next sent to the vat where the paper will actually be made. A rectangular mold containing wires running at right angles to each other is used to make a film of the pulp. Traditional molds have thin, closely spaced parallel wires running across the mold at the surface. These are attached to thick, widely spaced wires beneath them that run in the opposite direction. Paper formed on this type of mold typically reveals a ladder-like pattern when held up to the light , and is known as laid paper. Woven paper is formed on a mold of plain, woven wire screening. Thin wire forming a design may be attached to the mold's surface wires to produce a watermark in the finished paper. A rectangular frame, called the deckle, is placed over the mold to convert the mold into a sort of tray.

The papermaker then dips the mold with the deckle attached into the vat of dilute pulp and draws up a small amount of pulp on the surface of the wire. The mold is then shaken and tilted until most of the water has drained through the wire. The deckle is removed, and additional water is allowed to drain off. A second worker takes the mold and transfers the film of pulp to a piece of damp felt, laying a second piece of felt across the top.

This process continues until a stack of alternating wet paper and felt has built up. The stack is placed in a press to eliminate any residual water. Then the paper and felt are separated, and the paper is pressed by itself and hung up to dry. When dry, the paper sheets are dipped in a tub containing size (essentially gelatin or very dilute glue) and dried again. This gives the paper a harder and less absorbent finish than it would otherwise have had.

All paper was made by hand until the early nineteenth century. Artists use most of the handmade paper consumed today, though hand printers can still be found who believe it the finest printing surface available.

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.

Paper categories

Paper is available in a wide variety of weights, colors, textures, and finishes for a multitude of purposes. Book papers are intended for book and journal printing. Almost all bookpapers are surface-sized for offset lithography . The sizing resists penetration by the water and ink used in offset printing. Book papers are mainly made from Kraft pulp, sometimes with machine pulp added. Text papers are available in many colors and textures for use in advertising leaflets, endpapers, etc. They are also sized for offset printing. Cover papers are used for heavier papers, and are chiefly used for covers for pamphlets, journals, and paperback books. Newsprint is made for printing newspapers, advertising catalogs, inexpensive paperbacks, and other items that will probably only be read once if at all, and then thrown away. It is made from machine pulp, usually with some chemical pulp added for strength. Bond is made mainly for office use and ranges in quality from top-grade papers made from 100% rag pulp to low-grade stocks consisting largely of machine pulp.

Paper weights

Paper varies in thickness and weight. Both measurements are used to calibrate stock. At the paper mill, the thickness of a sheet is measured in thousands of an inch (mils). For the purposes of bookmaking, this number is converted into pages per inch. Book papers may vary from 200 to nearly 1,000 pages per inch, but the commonly used 50-lb (23-kg) machine-finished papers generally run about 500-550 pages per inch, each leaf counting as two pages. That corresponds to an average thickness of about 0.004 in (4 mm) for the thickness of one sheet of 50 lb (23 kg) machine-finished paper.

Paper is sold by weight. Different grades of the same type of paper are distinguished by the weight of some standard quantity of that paper. For most of the world, the standard quantity is one sheet of paper per one square meter in area. In the United States, the system of basis weights is used to compare the weights of papers. The standard quantity is one ream, or 500 sheets, but the standard sheet size varies from one category of paper to another. For book papers, the standard sheet measures 25 x 38 in (64 x 97 cm). For cover stocks, the standard size is only 20 x 26 in (51 x 66 cm), so a 50 lb (23 kg) cover paper is nearly twice as heavy as a 50 lb (23 kg) book paper. For bond papers, the standard size is 17 x 22 in (43 x 56 cm), so a 20 lb (9 kg) bond is approximately equal to a 50 lb (23 kg) book paper.

In Europe, the metric A series of stock sizes is based on a standard sheet of paper, rectangular in shape (841 mm x 1189 mm) and one meter in area. This is called size A0. Cutting this sheet in half produces size A1; cutting the A1 sheet in half produces size A2, and so on down to size A5, which is 1/32 the area of A0. In this sizing system, all of the sheets have the same shape: the ratio of the short side to the long side is identical throughout the series of sizes.

Resources

books

The Chicago Manual of Style. Chicago: The University of Chicago Press, 1982.

Hunter, Dard. Papermaking: The History and Technique of an Ancient Craft. New York: Dover Publications, Inc., 1974.

Lee, Marshall. Bookmaking. New York: R. R. Bowker Company, 1979.

Shannon, Faith. The Art and Craft of Paper. San Francisco: Chronicle Books, 1994.

Randall Frost

KEY TERMS

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Calendar rolls

—Highly polished metal rollers used to compact paper after it has dried.

Fourdrinier machine

—The machine that forms paper from pulp, named after the English family that financed its development in the early 1800s.

Furnish

—Specific combination of pulp and other ingredients used to make a particular kind of paper.

Kraft process

—A process in which sodium sulfate is reduced by heating with carbonaceous matter in a furnace to form sodium sulfide, which is then used in a water solution with sodium hydroxide as a cooking liquor. The wood pulp is then cooked under pressure and at high temperatures. The kraft process, also known as the sulfate process, has a less corrosive influence on iron and steel than the sulfite process.

Sulfite process

—A process in which sulfur dioxide is passed through calcium carbonate to form calcium bisulfite in an excess of sulphurous acid as the cooking liquor. The wood pulp is then cooked under pressure at high temperatures.