The technology of printing has undergone dramatic changes over the past five centuries. The first commercial printers in Europe were limited to lead type, hand-made paper and inks, and slow, wooden presses to transfer an image to paper. Today, with electronic transmission and laser technology, it is possible to "print" material simply by converting electronic impulses into words or images on a page.
Printing involves more than producing books, newspapers, and magazines, however. The process also transfers words and images to textiles, packages, billboards, wallpaper, shopping bags, postage stamps, labels on canned goods, paper money, and any other surface that carries text or images. Even the circuits for electronic boards used in computers and other high-tech equipment are printed--not with ink but with lasers that score the pattern into each board.
Before any material can be printed, the text must be set in type and checked for accuracy, and illustrations must be prepared for reproduction. When a manuscript is delivered to a printer, the first step is to choose a type for the text and typeset the material. Until recently hot-metal typesetting was used, so called because the type was cast from a molten lead alloy. Today most typesetting is done by a procedure called photocomposition, also known as cold-type composition.
In the early days of photocomposition, an operator typed the text into the composing machine, following instructions on type size and style, the length of each text line, and the space between lines. With the advent of computers, the process has become more automated. In many systems, operators can typeset text directly from an author's floppy disk or through telecommunication lines that transmit the text from the author's computer to the composing machine.
When typesetting is complete, a preliminary copy of the text, in the form of galley proofs or page proofs, is sent to a proof-reader. The proof-reader checks these proofs against the original material and marks misspelled words, typographical mistakes, lines dropped or repeated, and other errors. The corrected proofs are returned to the typesetter, who makes the necessary changes.
Because typesetting can produce only letters, numbers and symbols, and punctuation marks, illustrations are prepared separately from the text. A simple line drawing, without grey tones, can be printed as a line reproduction without any special handling. More complicated black-and-white illustrations, particularly photographs, must undergo a halftone process. Each illustration is photographed through a screen that breaks down the tones and shading into tiny dots of varying sizes. Larger and more closely packed dots create the impression of darker shades, while smaller, more widely spaced dots convey lighter shading. By a special process, colour illustrations and photographs are analysed into four basic colours--red, blue, yellow, and black. Each colour is then applied separately in the printing process to re-create the four-colour image.
The text, line drawings, and other illustrations are arranged as they will appear on the final printed pages. All but the halftones are pasted down on a board or stiff paper; space is left where the halftones will be placed later. The completed page is called a mechanical, or camera-ready copy. In the 1980s computerized, or electronic, publishing (also known in some instances as desktop publishing) had begun to replace these methods.
The mechanicals are photographed, and the negatives are processed. Halftones are prepared on separate negatives and then stripped in, or aligned, in the blank spaces left for them during paste-up. The resulting composite negative is then used to make what is known as a printing plate. Usually this is a metal plate that has been coated with a photosensitive material and subsequently exposed to the composite negative. The texts and illustrations are finally burned, or chemically etched, into the plate, which is then ready for printing.
The German printer Johannes Gutenberg is generally credited as being the first European to bring together in about 1450, the two main concepts of modern printing: movable pieces of metal type that could be reused, and a printing press for producing sharp impressions on paper over and over (see Gutenberg). Until the advent of computerized printing, this fundamental process remained virtually unchanged for five centuries. Modern commercial printing generally uses three basic techniques
Letterpress, or relief, printing is one of the oldest processes. The type is raised from the background plate so that when the type is inked and pressed against paper, only the raised portions transmit the image to the paper's surface. Originally type was cast piece by piece in small, hand-held casters. By the 19th century, type could be cast and set into text in a single operation, either by Monotype machines, which set single characters, or Linotype machines, which set entire lines in a single slug. The soft metal casts tended to wear down quickly in the printing process, however, and had to be replaced with fresh type.
To solve this problem, a means of making more durable plates was invented. Two of the most common methods are electrotyping and stereotyping. Electrotyping uses an electroplating process to create printing plates. Type is set and a cast made, usually out of wax, which is then coated in graphite and placed in an electroplating bath. A copper shell, or casting, is built up in the shape of the original type. When backed with metal, it makes a durable letterpress plate for high-volume printing. Stereotyping begins by making a mould of set type using a heat-resistant papier-mâché. Molten metal is then poured into the mould to create the cast plate. In more recent years, lighter and less expensive plates of rubber and plastic have been developed.
Letterpress printing uses three designs of printing press: sheet fed platen, sheet fed flatbed, and rotary presses, which can be sheet fed or web fed. In sheet fed presses, individual sheets of paper are fed into the press. In web fed presses, a continuous roll of paper is fed through the press.
Platen presses are the oldest type and are used for small printing jobs. They consist of a flat bed on which the images (type or plates) are laid and another flat surface, the platen, which is connected to a screw or lever. Paper is laid on the bed and pressed by the platen to transfer the image to the surface. Printing on early presses was a slow process, since the paper had to be fed one sheet at a time into the press. Improvements made in the 1830s and 1840s featured automatic inking and mechanical power, which increased printing speed from a few dozen sheets an hour to more than a thousand.
Sheet fed flatbed presses feature a cylinder, on which the paper is attached, that rolls over the bed of inked type. This design overcame the problem of uneven impression on the paper surface that often occurred in platen presses.
In rotary web fed presses a continuous roll of paper is fed between two cylinders, the plate cylinder and the impression cylinder. The plate cylinder, either electrotype or stereotype, is automatically inked and pressed against the paper as it unrolls. After the paper is printed, it can be cut into individual sheets and folded. Flatbed and rotary presses can be designed to print on both sides of the paper simultaneously and to reproduce colour illustrations. These machines were responsible for the enormous growth of newspapers during the 19th century.
In intaglio printing, the text or image to be reproduced is not raised above the plate's surface, as in the letterpress method, but is engraved or etched into the plate. Ink applied to the surface fills these depressions, then plate and paper are pressed together. The paper is actually forced into the ink-filled depressions to transfer an image to the paper.
Intaglio printing often uses highly polished copper plates on which a design has been engraved with sharp steel or diamond-tipped tools. These methods date back to about 1440 in Germany and Italy and were used by Albrecht Durer and other artists of the 1500s. In some instances, the copper plates can be coated with an acid-resistant substance and a design traced on the surface with a fine-pointed tool that exposes the copper. The plate is then placed in an acid bath so that the lines of exposed copper are chemically etched by the acid into the plate's surface. When the acid-resistant substance is cleaned off, the plate can be used to print etchings. Since copper wears quickly when subjected to great pressure, in some cases steel plates are used to make a large number of prints.
Traditional intaglio plate making had to be done by a skilled artist or draftsman and was a slow process. In the early 1880s, a photographic process was invented that came to be known as photogravure. The image to be printed is photographed through a screen, breaking the image into a series of small dots. The screened image is transferred to a special carbon-coated tissue that is mounted on a coated plate. When the paper is peeled off, the carbon dots remain on the plate, allowing the image to be burned into the plate in an acid bath. The deeper the dots are etched, the more ink they hold, and the darker the image.
Photogravure can be used on platen, flatbed, or rotary presses. When rotary web fed presses are used, the process is called rotogravure. The advantage of this process is that it allows a greater density of pigment per unit area than any other method of printing. Multiunit rotogravure presses can produce one- to four-colour images at high speed and today are used mainly for high-volume magazine production and by the packaging industry.
In the past few decades, offset printing (also called offset lithography) has replaced letterpress and intaglio methods almost entirely for commercial work. The name offset refers to the fact that the printing plates do not come into direct contact with the paper. Instead, the inked printing plates (which are attached to a cylinder) transfer, or offset, the image to a rubber blanket covering another cylinder. As the inked blanket cylinder rotates, it deposits the image onto the paper, which is fed from another set of rollers.
The offset technique was made possible at the beginning of the 20th century after the development of certain photographic processes and the rotary web fed press. Offset printing plates are usually made of steel, aluminium, or a chrome-copper alloy.
Unlike letterpress or intaglio printing, offset does not depend on raised or etched surfaces to transfer images. Instead it relies on the fact that grease and water do not mix. As the plate cylinder rotates, the plate passes first under water-soaked damping rollers and then under inking rollers that carry a grease-based ink. The offset plate is chemically treated so that the area to which the ink is transferred retains the greasy ink and repels water. The rest of the plate retains water and repels ink. As the cylinder continues to rotate, the plate presses against the rubber blanket, which accepts ink from the plate and transfers it to the paper. Since there is no type to wear out, an offset plate can make a large number of impressions.
Offset presses can be designed to print both sides of the paper at once and to reproduce images with one or more colours. Because web fed presses are usually used for this method, material can be printed at high speed. Offset plate making and storage costs are much lower than the costs of letterpress methods.
These methods include serigraphy, collotype, flexography and various mechanical processes used by people who are not trained as printers. Serigraphy, also known as silk-screen printing, is essentially a stencilling process that transfers an image onto a surface. The operator stretches a fine-mesh screen over a frame, outlines the design or text to be printed, and covers the rest of the screen with gum. A squeegee is used to force ink through the uncovered screen; the ink forms small droplets that are deposited evenly on the surface of the printed material. Serigraphy is commonly used to print on cloth, particularly cotton T-shirts.
Collotype printing combines lithography and photography methods to reproduce text or illustrations. A glass plate coated with a gelatine containing dichromate is exposed to a photographic negative, causing the plate to accept the image. When further exposure to light hardens the gelatine surface, the glass can be used as a lithographic plate. The continuous-tone quality of collotype allows for fine detail in printing, but the fragile gelatine surface and glass plate limit the number of impressions that can be made. Recently aluminium plates have replaced glass, permitting the plates to be used longer on web fed presses.
Flexography also uses modern plates to improve on a traditional printing process. Flexible rubber instead of glass or metal plates is used on web presses for relief printing.
Organizations, businesses, and schools often use printing methods that are inexpensive and easy to operate. These include mimeographing, which uses a stencil for a plate; spirit duplication (a lithographic process); and photocopying (an offset process).
The newest printing method in the industry is computerized, or electronic, printing. This process uses a computer, typesetting and page-design software, optical scanners, and computer printers to produce a variety of materials ranging from newsletters to reports to books. The operator can input the text through a keyboard or set it directly either from an author's floppy disk or via telecommunication lines. Typesetting software is used to select type style and create page designs. Illustrations can be read by an optical scanner and entered into the computer in the form of electronic signals.
In a process known as an electronic page makeup system, the operator can rearrange text and artwork for each page on the computer screen without the use of boards or paste. The pages are then printed on a LaserJet or other type of letter-quality printer without the need for presses. Electronic signals manipulate an electromagnetic field to create letters, figures, and illustrations on photosensitive paper. As the technology for electronic publishing becomes more sophisticated and inexpensive, this method is likely to replace offset printing for a wide range of printing jobs.
In the early years of printing, illustrations were coloured by hand--a slow and expensive procedure. In order to improve the process, printers developed what is called flat-colour printing. In this technique, the illustration is reproduced on a number of blocks--one block for each colour.
A six-colour illustration of a parrot sitting on a tree branch, for example, would require six different blocks. One would print only the brown of the branch, while others would provide the green leaves, red and yellow plumage, blue for the sky and markings on the bird's head, and a black block for details and outlines to provide sharper definition of the figures. It requires great skill to match the blocks precisely so that each colour is perfectly placed in the final print. Although an illustration with numerous colours and shades can be printed using a flat-colour process, the number of blocks required and the difficulty of aligning them make it a tedious, expensive method.
A more efficient process known as colour separation (four-colour process printing) was eventually developed. This method is based on the fact that all colours are created by some combination of the three subtractive primary colours: yellow, cyan (a shade of blue), and magenta (a shade of red). Black is added to reproduce detail. Colour separation is generally used in offset printing. The original illustration is photographed four times with four different filters, one for each of the primary colours, with a fourth plate for black. The resulting negatives, which are used as printing plates, show only the different intensities of each primary colour.
The four-colour press has a colour plate cylinder for each negative. As the paper passes through the press, each plate deposits its own primary colour onto its blanket roller, which transfers the colour to the paper's surface. In effect, the proper colours are mixed on the paper itself. Thus, this process can create an infinite variety of colours with only four plates.
In recent years the use of electronic scanning machines has rendered the process of colour separation more accurate than was possible by using photographic filters. A computer-driven scanner "reads" the colours of an original illustration and breaks them down with great precision into the three primary colours plus black. These machines can either produce photographic negatives or prepare printing plates.
Although printing has been used in Western countries for more than 500 years, the creation of reproductions by mechanical means has a much longer history. Relief printing using stamps to impress designs into soft clay or wax has been known for thousands of years in the Middle East and in other parts of Asia. Designs or inscriptions were also carved into stone or cast into metal to make seals.
China was the first country to print with paper, ink, and carved wooden blocks, a process called xylography. The invention of paper in China in the 8th century AD provided a smooth, flexible surface on which to reproduce an image. In this process, a single carved wooden block of text was used to print impressions on whole pages. By the 11th century, the Chinese had cut the blocks into individual characters, creating the world's first movable type.
Xylography was also the first printing method used in Europe in the early 1400s. By 1450, Gutenberg's combination of movable metal type and the printing press had produced Europe's first typeset book--the Gutenberg Bible. Gutenberg's process spread quickly to other European nations. Over time, the literacy rate gradually rose among the population of Europe. Literature and scientific and religious texts, once read only by scholars, nobility, and the educated priesthood, were now available to an ever-widening audience.
As the demand for printed books steadily increased, printers had to improve their methods and equipment. They developed sturdy metal presses to replace the common wooden press, created stereotype and electrotype plates to make greater numbers of copies, and designed mechanically driven and automatically inked presses to increase printing speed and quality.
Not all advances in printing technology came from printers or designers and manufacturers. In 1796 German author Aloysius Senefelder, in his search for an inexpensive means of publishing his own plays, developed the techniques of lithography. Joseph-Nicephore Niepce, a French landowner and inventor, discovered in the 1820s that certain chemical compounds were sensitive to light. His work marked the origins of photogravure and eventually led to the invention of photography and the use of photographic processes to reproduce images.
Beginning with the invention of the offset technique in the United States, a series of 20th-century innovations made mass production, high speed, and economy in printing possible. Automated composition, first developed after the 1920s, gave way to programmed composition in the 1950s. Many of today's computerized typesetting machines can set 1,000 characters (individual letters or symbols) per second. Phototypesetting equipment of the future could conceivably reach speeds of nearly 3,000 characters per second, or about 10,000,000 characters per hour.
Inventors also created pressure less printing, which eliminated the need for a printing press. In 1948, two Americans conceived of a type of electrostatic printing in which the colouring agent is not ink but a powder that is sensitive to the pull of an electric charge induced on a plate. This technique gave birth to xerography and the now-familiar copying machines. The various processes developed to duplicate and reproduce documents have been grouped under the name reprography.
Warren Chappell - A Short History of the Printed Word (Godine, 1980).
David Diringer - The Book Before Printing (Peter Smith, 1983).
John Gough - Into Print (David & Charles, 1979).
Howard Greenfeld - Books: From Writer to Reader (Crown, 1978).
Tony Hart - Printing and Patterns (David & Charles, 1984).
Herbert Simon - Introduction to Printing: The Craft of Letterpress (Faber, 1980).