Lithography

Lithography is a printing technique that relies on the natural repulsion between oil and water. It is a planographic process, meaning the printing surface is flat. The image is printed from a smooth stone, typically lithographic limestone, or a metal plate.

German author and actor Alois Senefelder invented the method in 1796, and it was first used mainly for printing musical scores and maps. The process can reproduce text or images on paper and other materials. The term "lithograph" is reserved for fine art prints and certain older printed items, not for products of modern commercial lithography.

In the traditional method, a greasy substance like oil, fat, or wax is used to draw the image onto a smooth, flat limestone plate. The stone is then treated with a weak acid and gum arabic solution, which makes the unprotected areas of the stone hydrophilic, or water-attracting. Before printing, the stone is moistened; water clings only to the etched, hydrophilic parts, making them even more oil-repellent.

An oil-based ink is then applied, sticking solely to the original greasy drawing. Finally, the ink is transferred to a blank sheet of paper, creating the print. This traditional technique remains in use for fine art printmaking.

Modern lithographic process

Modern commercial lithography uses a different approach. The image is transferred or created as a patterned polymer coating on a flexible plastic or metal plate. These plates can be made using a photographic process, sometimes called "photolithography," though that term more commonly refers to a similar process in microelectronics manufacturing.

Offset printing, or offset lithography, is a refinement where the ink is transferred from the plate to the paper indirectly via a rubber cylinder, rather than by direct contact. This keeps the paper dry and enables fully automated, high-speed operation. Since the 1960s, offset lithography has largely replaced traditional lithography for medium- and high-volume printing; most books and magazines, especially color ones, are now printed this way using photographically created metal plates.

Principle

Lithography differs from other printing technologies. Intaglio printing, or gravure, involves engraving or etching cavities into a plate to hold ink. Woodblock printing and letterpress printing apply ink to the raised surfaces of letters or images.

The principle behind lithography is simple chemical interaction. The positive part of the image is made of a water-repelling, or hydrophobic, substance, while the negative image is water-retaining, or hydrophilic. When the plate is exposed to a compatible printing ink and water mixture, the ink adheres to the positive image, and the water cleans the negative image. This allows a flat print plate to be used, enabling longer and more detailed print runs than older physical methods like intaglio or letterpress.

Senefelder invented lithography in the Electorate of Bavaria in 1796. The name comes from the Ancient Greek word for "stone," reflecting the early use of smooth limestone. After the oil-based image was applied, a solution of gum arabic in water was used, with the gum sticking only to the non-oily surface. During printing, water adhered to the gum arabic areas and was repelled by the oily parts, while the oily printing ink did the opposite.

Lithography on limestone

On limestone, the image is drawn with a fat- or oil-based medium, such as a wax crayon, which may be pigmented for visibility. The durability of the image depends on the lipid content of the material and its ability to withstand water and acid. After drawing, an aqueous solution of gum arabic, weakly acidified with nitric acid, is applied.

This creates a hydrophilic layer of calcium nitrate salt and gum arabic on all non-image surfaces. The gum solution penetrates the stone's pores, surrounding the original image with a hydrophilic layer that rejects printing ink. The printer then removes excess greasy drawing material with lithographic turpentine, but a hydrophobic molecular film remains, rejecting gum arabic and water but accepting oily ink.

During printing, the stone is kept wet. Water is naturally attracted to the gum and salt layer from the acid wash.

Printing ink, based on drying oils like linseed oil and varnish with pigment, is rolled over the surface. Water repels the greasy ink, but the hydrophobic areas left by the original drawing accept it. When the hydrophobic image is loaded with ink, the stone and paper are run through a press that applies even pressure, transferring the ink to the paper.

Senefelder experimented with multicolor lithography in the early 19th century. In his 1819 book, he predicted the process would eventually be perfected to reproduce paintings. Multi-color printing was later introduced through a new process developed by G.