Printing plate

Offset printing is a commonly used printing technique in which the inked image is transferred (or "offset") from a plate to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a water-based film (called "fountain solution"), keeping the non-printing areas ink-free. The modern "web" process feeds a large reel of paper through a large press machine in several parts, typically for several metres, which then prints continuously as the paper is fed through.

Development of the offset press came in two versions: in 1875 by Robert Barclay of England for printing on tin, and in 1904 by Ira Washington Rubel of the United States for printing on paper.

Lithography was initially created to be an inexpensive method of reproducing artwork. This printing process was limited to use on flat, porous surfaces because the printing plates were produced from limestone. In fact, the word "lithograph" historically means "an image from stone" or "printed from stone". Tin cans were popular packaging materials in the 19th century, but transfer technologies were required before the lithographic process could be used to print on the tin.

The first rotary offset lithographic printing press was created in England and patented in 1875 by Robert Barclay. This development combined mid-19th century transfer printing technologies and Richard March Hoe's 1843 rotary printing press—a press that used a metal cylinder instead of a flat stone. The offset cylinder was covered with specially treated cardboard that transferred the printed image from the stone to the surface of the metal. Later, the cardboard covering of the offset cylinder was changed to rubber, which is still the most commonly used material.

As the 19th century closed and photography became popular, many lithographic firms went out of business.Photoengraving, a process that used halftone technology instead of illustration, became the primary aesthetic of the era. Many printers, including Ira Washington Rubel of New Jersey, were using the low-cost lithograph process to produce copies of photographs and books. Rubel discovered in 1901—by forgetting to load a sheet—that when printing from the rubber roller, instead of the metal, the printed page was clearer and sharper. After further refinement, the Potter Press printing Company in New York produced a press in 1903. By 1907 the Rubel offset press was in use in San Francisco.

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