What is Lithography?

This article gives a really good description of Offset Lithography

Offset lithography is a printing technique which is widely used around the world. Most books, newspapers, and magazines are printed using offset lithography, and this printing technique is widely regarded as the workhorse of printing, because it is fast, efficient, cheap, and relatively easy. The “offset” in the name refers to the fact that the ink is transferred to a separate surface before being applied to the paper.

The first step in offset lithography is making a plate with the image to be printed. If the image is in black and white, only a single plate is required, because the plate can simply be inked with black ink. Color images are produced using a four-color separation process, in which four different plates are made for the cyan, magenta, yellow, and key (black) inks; when the plates are printed, the colors blend together visually, creating a color image.

Plates in offset lithography are entirely flat, in contrast with the textured surfaces of engraved plates and movable type. They are made by creating a film negative of the image, placing it over a photo-sensitive plate, exposing it, and then developing it. Once the plate is made, it can be mounted in a press, and the real fun begins.

This printing technique takes advantage of the fact that oil and water do not mix. The plate is brushed with rollers coated in water, and then with rollers covered in ink. The ink is attracted to the parts of the plate which were exposed earlier, while the water keeps the unexposed portions clear so that they do not smear or transfer ink. Then, the plate transfers the ink to a rubber roller known as a “blanket,” and the blanket rolls across the paper; typically the paper is fed between the blanket and another roller to ensure that the image stays crisp.

An offset press can run continuously, which makes it extremely fast. Depending on the job, the press may be sheet fed, which means that individual pieces are pulled from a stack by the press and run through, or web-fed, in which case the paper is on huge rollers. In both cases, the paper is typically run through an oven after printing so that it dries quickly, preventing smears, and then it can be cut, bound, folded, and prepared for distribution.

The first press for offset lithography was developed in 1903, and the concept quickly caught on in the printing community. Offset presses vary in size from the massive presses used at commercial publishers, which can be bigger than a house, to smaller models around the size of dump trucks used at smaller printing companies.