The general principals in offset lithography are identical to traditional metal plate lithography; the difference between them is in the manner by which they are printed. Various offset printing presses may have different roller systems but all share three major components; a plate cylinder that holds the printing plate, a blanket cylinder wrapped in rubber that carries the image to be transferred, and the impression cylinder which applies the pressure to print the image. A gear train connects all three cylinders together so they are in perfect synchrony with one another. Printing begins when a processed litho-plate containing an image is mounted on a cylinder, mechanically dampened with a wetting agent, and then rolled with ink. The oily ink is repelled from the damp areas and is attracted to the dry image areas. A blanket cylinder is then rolled over it, picking up the inky image onto its soft rubber surface. Paper then passed between this blanket cylinder and the hard impression cylinder, which presses all three surfaces together, transferring the image to the paper. Previously the printing plate and paper needed to make perfect contact, but here the soft blanket can pick up and deposit ink much better than a hard surface, creating impressions on almost any material with greater fidelity. Even though the first offset press was built in 1904, the method only became popular after 1951 when an easy to use, storable, photosensitive aluminum litho-plate was developed. Offset lithography quickly became the standard in photochrome postcard production. Nearly all postcards produced today are made through process printing on offset lithography presses, though they have largely been adapted to digital technology.
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