An offset lithograph is a print made by the process of offset lithography. The term offset indicates that the print is not made directly from printing plates, instead the image is transferred to another surface and from that surface to the paper. This process is most often used for commercial printing, seldom for fine art. High-volume printing such as newspapers, magazines and advertising are most often offset lithographs.
History
The first known offset printing was an accidental discovery. In 1903, Ira Washington Rubel noticed that the impression he got when an image accidentally transferred from the stone printing cylinder to the rubber impression plate was clearer than the original image. The soft rubber gave a cleaner, clearer image than the hard stone plate when it was transferred to paper. He used this information to invent a new type of printing press. About the same time, Charles and Albert Harris made a similar discovery and produced their own version of the offset press. The press made by the Harris Automatic Press Co. had a metal plate and a "blanket cylinder." This arrangement allowed for the use of paper from large rolls. By the 1950s offset lithography was the most common form of printing and still is, with the addition of some digital pre-press innovations.
Advantages
Offset lithographs have a consistently high-image quality. Because the rubber "blanket" is slightly flexible, it conforms to the texture of the surface being printed on and therefore produces sharp, clean images. It is also quicker and easier to produce the printing plates, and the plates last longer because they only come in contact with the printing blanket, which is softer and less abrasive than paper. Offset lithography is also the cheapest way to produce high-quality images in large quantities.
Disadvantages
The process is not practical for small quantities because of the cost of producing the plates. The image quality, while excellent for commercial purposes, is not as good as that of rotogravure or photogravure printing. Those are more likely to be used for fine art images. The anodized aluminum used in the plates can become chemically sensitive over time and produce ghost images in non-printed areas.
Process
First an image is created or transferred photographically or digitally onto a thin printing plate made of metal or plastic. In offset lithography the image on the printing plate is not raised or recessed as it is in letterpress or engraving. Oil-based ink is applied to the printing plate and only adheres to the image areas of the surface. The inked image is then transferred by pressure to the rubber blanket cylinder and from there it is printed on the paper.
Lithographs in Art
Art lithographs are a different and multi-step process. They also use oil-based inks, but each color is applied separately, by hand, and so an individual lithograph print takes months to produce. Offset lithographs of art are printed reproductions and thus are not original works of art. To see the difference between original hand-drawn lithographs and offset lithographs, use a magnifying glass. In an offset lithograph image you will see that the color is made up of lots of dots. On original lithographs, you will see a solid color.
