Historical note.Papermaking is an ancient craft. The Chinese are credited with a capability to produce a writing paper from mulberry fibers at the beginning of the Christian era. One of the many skills which Marco Polo became familiar with in China was Chinese ability to produce handmade paper. It is believed that he brought back to Europe some of the Chinese knowledge of paper production. Through the first 18 centuries all paper was produced by hand and throughout this long period of time many skillful techniques produced very fine papers. Present-day papers are nearly all produced on a Fourdrinier machine, which was first developed in England and produced paper in the early 19th century.
Ingredients.Paper is composed of a fibrous interlace (usually wood cellulose), sizing (rosin or starch), filler (clay, carbonate, or titanium), and coatings of china clay, carbonate, or titanium fixed to a base sheet by casein, protein, or latex.
There are many varieties of paper but all can be classified into two broad groupscoated and uncoated papers. Coated papers are essentially an uncoated paper base to which is applied one of several types of coating material. The coating material provides a highly smooth glossy or dull surface for production of high-quality halftone and color work.
Manufacturing.The basis of manufactured paper is a cellulose or other fiber. The sources of fiber are rags and, principally, wood. All of these raw materials must be reduced to a clean mixture of water, fiber, and other ingredients so that they can be formed into a thin layer of wet slurry and dried into a sheet of paper. This process requires large amounts of relatively pure water for washing and purifying of raw materials. It also forms the final slurry which produces the web of paper.
Paper production is a continuous machine process. The treated pulp, fibers, and, in most instances, size, are placed in a beater or macerating machine which thoroughly mixes the ingredients with water to the desired consistency. After the ingredients are mixed carefully to the desired consistency and content, this slurry is poured into a huge vat at the wet end of the Fourdrinier machine. This allows the correct rate of feeding of slurry onto an endless bronze screen which permits the water to drain as the slurry gradually forms a thin web of semidry paper. The traveling web is passed over a succession of suction boxes and between rollers which further remove much of the water from the traveling web. Toward the drying end of the Fourdrinier machine, the web of paper is transferred to a felt blanket and carried through a succession of steam-heated drying cylinders to bring the final moisture in the paper from about 8o percent at the beater to about 10 percent after drying.
Coated papers are produced from uncoated stock by passing the base web through coating rollers and then through a drying unit. Cast coated papers are dried against a highly polished chromium roller which imparts a very smooth finish to the coated base stock. This paper is the most ideal surface for printing.
Lithography is more versatile than the letterpress or gravure processes because a wider variety of kinds and surface finishes of papers can be used for production of fine-screen halftone images. To a less extent, line images and solids are processed on a wide range of stocks more easily by lithography. The variety of kinds of paper which are adaptable to the lithographic process are: newsprint, bond, machine finish, supercalendered, english finish, vellum, parchment, machine and cast coated, coated and uncoated board, laminated metal foils, plastic sheets, and pyroxylin-filled or other binding cloths. The variety of capability indicated does not mean that a stock of poor quality can be used successfully for offset lithography. There are several factors in litho-graphic production which require specific properties of any sheet to produce good work and to function properly on the lithographic press.
Paper manufacturers recognize the particular needs of lithography and have produced the great diversity of papers and surface finishes used in the printing and publishing industry. Special requirements for paper are not an exclusive property of lithographic production. All of the major processes have specific requirements which have been determined and satisfied by paper manufacturers. Some of the factors which are required considerations for lithography are (1) uniform pressure is applied over the entire surface of a sheet of paper and no allowance for unevenness is present; (2) the blanket is in full contact across the entire length of a sheet of paper and offset inks generally have more tack than other inks; this factor necessitates greater strength built into the sheet; (3) the press plate is a single sheet of metal and individual illustrations on pages cannot be moved to adjust register; therefore, an offset sheet must be stable in its construction, and (4) moisture is present across the nonprinting areas of the plate and this can affect stability and register if the paper is too moisture absorbent.
DESIRABLE QUALITIES OF AN OFFSET PAPER
Flatness.Because the entire printing surface of the offset plate is flat, an uneven or wavy sheet will be subject to wrinkling during the impression cycle. Moisture is the principal cause of uneven sheets, and paper must be made, stored, and processed under ideal and uniform moisture conditions at all times. Either wavy edges from moisture gain or tight edges and curl from moisture loss will cause register problems in production. Most paper is surrounded and sealed with asphalt-lined paper wrapping to avoid moisture loss or gain during shipment. This seal should not be broken unless the area is conditioned to a relative humidity of about 50 percent. Open skids of paper should never be placed in an area of widely fluctuating humidity conditions.
Grain direction.The grain of the paper should be in the direction of the long dimension of the sheet. Paper is affected by moisture across the grain of the sheet more than with the grain. This is so because the fibers swell across their girth more than across their length. This means that a sheet of paper on the press will have good dimensional stability across the cylinder. The greater variation will be around the circumference of the cylinder. It is possible to lengthen or shorten the print to accommodate a register requirement around the cylinder by adjusting the amount of packing on the plate or blanket cylinders. It is virtually impossible to lengthen or shorten the dimension across the length of the cylinder. Therefore, it is highly desirable to have all paper with the grain across the cylinder.
Pick resistance.Offset inks are generally tackier and therefore require a sheet of paper which is more pick resistant. Picking is the lifting of fibers or other material from the surface of the paper when printed. Elimination requires a more tightly bound construction of the paper and is completely the responsibility of the manufacturer. A poor pick-resistant paper causes fibers to be pulled from the sheet and causes contamination and hickies in the inking system in addition to a poor quality product.
Moisture absorbence and resistance.The offset stock must absorb immediately those minute quantities of water which have been de-posited on the inked image so that the surface of the paper can accept and bond with the ink on the image. The sheet of paper also must be sufficiently moisture resistant to prevent fiber separation from the sheet when the fibers come in contact with moisture during printing. The surface characteristics of offset papers require a very precise balance between absorption and resistance with respect to moisture and tightly bonded surface fibers to insure maximum press performance.
Ink setting and drying.Ink can be formulated to print and dry on a wide variety of stocks. The importance of the manufacture of paper in regard to ink drying is that the stock must act uniformly for a given set of printing conditions and a uniformity must be present from lot to lot and from successive mill runs of the same kind of paper. Lack of uniform absorption and drying cause delay and difference in appearance of printed work.
Chemical inertness.Lithography is a chemical process. Fountain solutions and water contain many different compounds. Ink contains many different compounds. Each revolution of the press forces more or less contact with all these chemicals and water. Each component including paper must have a resistance to reaction from other elements in the process. If paper pH is not correct, it can cause a condition of bleeding of ink into the fountain and then to the printed sheet. Such a condition is called tinting and is usually associated with coated stocks. Proper chemical inertness is an important factor in paper manufacture if difficulties in production are to be avoided.