Common Use Dates: 1960s-present
Alternate Names: Offset litho, Photo-offset lithography, Offset printing, Litho-offset, Photo-lithography, Photo-offset, Lithography
Mistaken For: Letterpress halftone, Inkjet, Electrophotography
Process Family(s): Photomechanical, Ink, Paper Support, Planographic, Print
Offset Lithography is a planographic photomechanical printing technique that combines lithography, photography, and offset printing. It is based on the repulsion of oil and water. During this printing process an inked plate bearing an image or text is printed onto a rubber blanket before being transferred (or offset) to its final substrate. The flexible rubber blanket allows for printing on a variety of surfaces including paper, wood, metal, cloth, and leather. It can be used both as a reproductive process and to print original work. Prints can include text, line work, large areas of ink, and halftone images. Offset lithography can also reproduce images originating from other processes such as relief and intaglio. There is no visual distinction between an offset lithograph and a transfer lithograph aside from date or context.
The development of modern offset lithography came about by accident following three important innovations – lithography, the offset printing press, and photography.
Lithography, invented by Alois Senefelder at the end of the 18th century, is a planographic printing method based on the immiscibility of oil and water. Senefelder’s process involved drawing on a stone matrix with fatty ink then chemically etching the stone with a mixture of acid and gum arabic. This left the image areas in slight relief. An application of water was then applied to the matrix, which would only be retained in the porous, etched areas. When the stone was subsequently inked, the oil-based ink would not adhere to the moistened areas, sticking only to the original illustration. Multiple prints could then be produced in a flatbed press with the matrix being re-inked between each pull.
In 1875 Robert Barclay developed the first offset lithography printing press for transferring images and text to tin cans. His press combined transfer technologies from the mid-19th century with the rotary printing press. Instead of printing directly on metal, a specially treated sheet of cardboard was wrapped around a cylinder and used to transfer the image from the printing matrix to the tin surface. Barclay’s offset printing press was only used by tin makers until the early 20th century when a lithographic pressman, Ira Rubel, discovered it’s potential.
Around 1905, Rubel was operating a lithographic press that printed an image directly from plate to paper. A sheet of paper failed to feed into the press and the plate printed the image onto the impression cylinder instead. The next paper sheet was printed normally but also received an offset image from the impression cylinder on its verso. Rubel noticed the offset impression was crisper than the image printed directly from the plate and decided to put his discovery into practical use by building the first lithographic press that employed an “offset” rubber blanket cylinder.
The modern day offset lithographic press is based on the same principle that Barclay and Rubel employed – an image moves from plate to blanket to paper. The press is composed of several main parts: the inking system, the dampening system, the plate cylinder, the offset or blanket cylinder, and the impression cylinder. During the printing process, ink and water are transferred onto the printing plate, which is wrapped around the plate cylinder. The plate cylinder transfers the inked image to the blanket cylinder. During the paper feeding process the paper is pressed against the blanket cylinder by the impression cylinder, which provides the pressure necessary to offset the ink onto the paper surface to create the final printed image. Presses can either be fed with single sheets or web-fed, where a roll of paper moves through continuously.
The offset lithography printing plate can be produced with a variety of methods. The most common type of plate used in modern offset lithography is a surface plate, which consists of a metal (typically aluminum) that has been grained to make it water receptive and coated with an oleophilic, light-sensitive solution. The two basic categories of printing plates, negative-working and positive-working, are determined by how they are exposed. Negative-working plates are produced when light exposes a sensitized plate through a negative film of the image to be printed. The light shines through the image areas and hardens the plate coating in direct proportion to how much light it receives, producing an insoluble image on the plate. The unaffected coating is then dissolved away, exposing the grained, hygroscopic metal beneath. A solution of gum arabic is applied to the non-image areas to render them even more water-receptive. Positive-working plates are produced in a similar manner with the substitution of a positive film. During this process the plate is exposed through the non-image areas and unexposed in the image areas. The different chemistry of the positive-working light-sensitive coating allows the areas exposed to light to be rendered soluble so they may be dissolved away in the same manner as the negative-working plate. Both negative- and positive-working processes result in a plate that is oleophilic (oil-receptive) in the image areas and hygroscopic (water-receptive) in the non-image areas. After any final corrections are made the printing plate is attached to the printing cylinder and is ready to print.
Other common printing plate types used today make use of electrostatic printing for electrophotographic plates, electroplating for bimetal plates, heat for thermal plates, and digital print making techniques for ablation. Ablation, where laser diodes burn small holes in a coating applied to the surface of the plate, is a computer-to-plate technology that is being used with increasing frequency.
While text and line work can be transferred as is to the printing plate, if the piece to be printed includes a photograph or other full-toned image it must first be broken up into halftones. The original way to do this was to re-photograph the original image through a halftone screen in a process camera. The halftone screen traditionally consists of an arrangement of parallel or intersecting lines suspended on a transparent support. These intersecting lines create a network of evenly spaced apertures of equal size through which light can pass in a camera, splitting the image being photographed into a series of ‘dots.’ The screen design dictates what the shape of the halftone dots will be. The most common dot shapes are round, elliptical, or square, although special effect screens can be used to break the image into unique shapes for artistic effect. By the 1970s the physical screen was replaced with a technology called digital halftoning, which removed the need for a process camera by using a computer program to break-up the image into halftones. This practice is still standard today.