The principle
Lithography is a printing process that uses chemical processes to create an image. For instance, the positive part of an image would be a hydrophobic chemical, while the negative image would be water. Thus, when the plate is introduced to a compatible ink and water mixture, the ink will adhere to the positive image and the water will clean the negative image. This allows for a relatively flat print plate which allows for much longer runs than the older physical methods of imaging (e.g., embossing or engraving).

The early process
Lithography was invented by Alois Senefelder in Bohemia in 1796. In the early days of lithography, a smooth piece of limestone was used (hence the name “lithography”—”lithos” (?????) is the ancient Greek word for stone). After the oil-based image was put on the surface, acid burned the image onto the surface; gum arabic, a water soluble solution, was then applied, sticking only to the non-oily surface and sealing it. During printing, water adhered to the gum arabic surfaces and avoided the oily parts, while the oily ink used for printing did the opposite.

The chemical process
Lithography works because of the repulsion of oil and water. The image is drawn on the surface of the print plate with an oil-based medium (hydrophobic). The range of oil-based mediums is endless, but the dexterity of the image relies on the lipid content of the material being used–its ability to withstand water and acid. Following the placement of the image is the application of an acid emulsified with gum arabic. The function of this emulsion is to create a salt layer directly around the image area. The salt layer seeps into the pores of the stone, completely enveloping the original image. This process is called etching. Using lithographic turpentine, the printer then removes the greasy drawing material, leaving only the salt layer; it is this salt layer which holds the skeleton of the image’s original form. When printing, the stone or plate is kept wet with water. Naturally the water is attracted to the layer of salt created by the acid wash. Ink that bears a high lipid content is then rolled over the surface. The water repels the grease in the ink and the only place for it to go is the cavity left by the original drawing material. When the cavity is sufficiently full, the stone and paper are run through a press which applies even pressure over the surface, transferring the ink to the paper and off the stone.

Refinements
Senefelder had experimented in the early 1800s with multicolor lithography; in his 1819 book, he predicted that the process would eventually be perfected and used to reproduce paintings.[1] Multi-color printing was introduced through a new process developed by Godefroy Engelmann (France) in 1837 known as Chromolithography.[1] A separate stone was used for each colour, and a print went through the press separately for each stone. The main challenge was of course to keep the images aligned (in register). This method lent itself to images consisting of large areas of flat color, and led to the characteristic poster designs of this period.