Color lithography uses multiple stones, one per color.
Alois Senefelder invented a method of printing called lithography in Germany in 1798. Lithography is used to print magazines, maps, newspapers, posters and other kinds of mass-produced, printed items of text and graphics. Lithographic printing firms increased 3000 percent between 1900 and 1970. Today, most books are printed using offset lithography.
Invention
The inventor of lithography, Alois Senefelder, was a playwright frustrated by how expensive it was to make copies of his work. Since copperplate engraving was so difficult, he decided to try engraving on slabs of Bavarian limestone. To correct his mistakes, he used a fluid made of lamp-black, rainwater, soap and wax. Senefelder discovered that his correction fluid, being greasy, repelled water and absorbed ink, so if he drew on the limestone with the fluid, wet the stone and then rolled on ink, he could then use the stone to ink his image onto paper.
Transfer Process
Senefelder discovered that images made with special ink on special paper could be transferred to the lithographic stone to become the printing image. This meant the image could be created the way it would appear when printed, instead of mirrored. It could be transferred more than once to the same stone if necessary, which increased productivity. This discovery made lithography more popular for making copies, since printers could transfer existing images to lithographic stones for reproduction.
Photolithography
Alphonse Louis Poitevin discovered in 1855 that if a solution of potassium bichromate and albumin was dried on the lithographic stone and exposed under a photographic negative, the parts exposed to light became insoluble and ink only adhered to those parts. In 1885, Frederick Ives invented a halftone screen made of two exposed glass negatives scored with equidistant lines crossing at right angles. Rephotographing an original photograph with the halftone screen before the new film would create a new halftone negative. Light areas on the original reflected more light to the film, creating large dots; darker areas created smaller dots. In 1890, Max Levy developed a manufacturing process for the screens. Newspapers used photolithography to reproduce pictures of newsworthy events.
Offset Press
Direct contact between the stone and the paper is abrasive, which makes the image wear off the plate. Offset printing adds a rubber blanket surface as an intermediate step to preserve the image. Paper manufacturer Ira Rubel first used offset printing of paper for bank deposit slips in 1904 or 1905 in Nutley, New Jersey. Soon afterwards, Charles Harris of the Harris Automatic Press Company designed an offset press similar to the much faster rotary letterpress machine.
Lithographic Plates
The original limestone plates were heavy, costly, hard to store and couldn't bend to make a cylinder in a rotary press, so metal plates were introduced. In 1951, the American company 3M developed the first practical pre-coated aluminum plate. In the late 1970s, 3M also developed "Aqualith" plates which didn't need special developing agents after exposure; the unexposed areas could be removed with water. In the 1990s, Japanese firm Toray introduced a waterless lithographic plate which repels ink from the areas that would previously have attracted water.
