Colored lithographs utilize multiple plates for different colors.
Lithography is a group of processes used to transfer images, such as pictures or text, onto a substrate, such as paper or cloth. The first lithographic process was an invention of need. Playwright Alois Senefelder pioneered the technique in 1798 to publish his plays at a lower expense, according to the University of Houston. Most types of lithography achieves results due to the tendency of oil and water not to mix. Today, lithography is used in both fine arts and commercial printing.
Stone Lithography
In stone lithography, the artist or printer starts with a limestone plate and draws a reversed image onto it with a greasy substance, such as a grease pencil. Water or a water-based solution is applied to the stone. The grease in the image repels water, while the portion of the stone without the drawing retains water. The printer applies ink to the plate, typically with a roller, according to Monoprints.com. The ink adheres to the greasy image but not to the dampened areas of stone plate. The printer covers the plate with paper, and a press transfers the inked image to paper.
Color Stone Lithography
Early lithographs were black-and-white images. Colored lithography, or chromolithography, evolved in the 1800s. The process requires that the artist draw on multiple plates -- one plate for each color required by the image. In some cases, reports the University of Delaware, the artist employs a tint-plate to give the entire background a specific color. The lithographic work of Henri de Toulouse-Lautrec in the 1890s remains a potent example of the range of options available to an artist employing chromolithography. Monoprints.com notes that zinc sheets have largely replaced stone plates in contemporary artistic lithography, though the other features of the process remain similar.
Offset Lithography
Offset printing is used in commercial printing of relatively large print runs -- up to about 1,000,000 impressions, according to the Printer's National Environmental Assistance Center. While offset lithography makes use of the same principles as stone lithography, it combines that process with a rotary printing process. A plate, chemically treated to make the image area retain ink, is affixed to a roller. As the roller spins, fountains apply a dampening solution and then ink to the plate. The plate transfers the image to a rubber-encased roller that applies the image to the paper.
Other Types of Lithography
A number of other processes receive the general name of lithography, such as X-ray, electron-beam and ultraviolet lithography. These processes are used, for example, to produce computer chips by laying patterns onto silicon wafers. These processes do not make use of the oil-and-water mix of classic lithography, casting doubt on whether or not they are true lithographic processes. In these cases, the use of the term "lithography" may be nothing more than convenient shorthand.
