Offset Printing
In 1840, a British inventor, M. Ponton, invented the albumin process.
In 1868, a German, J. Albert, invented collotype.
In 1869, a British scientist, D. Hauron, invented trichromatic color lithography using subtractive color mixing.
In 1876 (Qing Dynasty, 2nd year of Guangxu), a Frenchman, Father Vénard, and a Chinese man, Qiu Ziangong, established the Tushehuan Lithographic Press in Shanghai for printing church magazines.
Offset printing is based on the principle of water and oil repulsion. The printing process proceeds as follows:
Firstly, the inked image portion is formed on the offset plate. The image can be drawn directly on the plate with an oily pencil or created using a photographic method.
Secondly, the plate is dampened with water. Because water and oil repel each other, water covers the non-image areas of the plate, which are rejected by the image.
Thirdly, the entire plate is covered with ink. Since water and ink repel each other, the ink is rejected by the dampened areas of the plate, adhering only to the oily image areas.
Finally, the paper is pressed onto the surface of the plate, transferring the inked image onto the paper.
Offset printing is also commonly referred to as lithography. Unlike letterpress and gravure printing, where the printing image is raised or recessed on the plate, in offset printing, the printing image is merely flat on the plate surface. During the printing process, as the paper rubs against the image, the image quickly wears off, especially when the plate is used in high-speed printing presses. To reduce wear on the image areas of the plate, the image is first transferred onto a rubber blanket.
According to available records, offset printing was invented by the German Alois Senefelder in 1798, who directly printed oily patterns on paper from a heavy limestone block. Various types of offset plates are used for high-speed and small offset printing presses, which produce more output than any other printing method.
Offset printing, invented by Alois Senefelder of Bohemia in 1798, was the first new printing process since the invention of letterpress in the 15th century. In the early days of offset printing, smooth limestone was used, hence the English name "lithography" or "lithos," derived from the ancient Greek word for "stone." After oil-based images were applied to the stone, acid etched beneath the stone's surface, and then a gum arabic solution was applied, which adhered only to the non-oily areas and sealed. During printing, water adhered to the gum arabic surface but not to the oily parts, while the oily ink used during printing adhered to the oily image areas.
In the years following the invention of offset printing, this printing process was used for multi-color images, known in the 19th century as chromolithography. Many exquisite chromolithographic prints and publications from the 19th century are still preserved in museums across the United States and Europe. Each color used a separate stone plate. The main challenge was accurate registration of the images.
Today, aluminum plates are used in offset printing. The plates are brushed with a grain texture, or "graining," and then coated with a smooth layer of photosensitive emulsion. The photographic negative of the desired image is placed on the plate and exposed, creating a positive image on the emulsion. After chemical processing, the unexposed emulsion is removed. The plate is mounted on a roller on the printing press, and a dampening roller passes over the plate, applying water to the rough areas or non-image areas of the plate. Then an ink roller passes over the plate, applying ink only to the smooth areas or image areas of the plate.
If this image were directly transferred to paper, it would produce a reversed image, but the paper would become wet. The method used is to pass a roller covered with rubber cloth over the plate surface, which squeezes out the water and transfers the ink. (In reality, the rubber cloth is elastic and better at transferring the image, while still transferring water, although in smaller amounts, which can be ignored.)
The roller passes over the paper, transferring the ink onto the paper. Because the image is first transferred to the rubber cloth roller, this process is also called "offset printing." This term originates from the offset transfer of the image from the plate to the rubber roller and then onto the paper.
Over the years, this process has seen many innovations and technological improvements, including multi-color printing presses that can print several colors on a single pass, Dahlgren ink systems that eliminate the need for damping (incorporating damping into the ink delivery process).
At the input end, advances in desktop publishing have enabled almost anyone to produce professional-quality layouts, and the development of typesetting machines has allowed printing houses to bypass the intermediate step of phototypesetting; typesetting machines can directly produce film from computer-generated images. Since the 21st century, direct plate-making machines have eliminated the need for film and can directly image onto the printing plate.