What is an Offset Lithograph?
The most commonly used commercial printing process is offset lithography. This is a method of printing from a metal or stone surface on which the printing areas are made ink-receptive. Rather than pressing inked images onto paper using plates with raised or etched plates, an image is transferred to the paper using oil-based inks, chemically-treated plates made from photographic negatives, and an offset cylinder.
The offset lithography process works by first transferring an image photographically to thin metal, paper, or plastic printing plates. Unlike other forms of printing, the image on the printing plate is not recessed or raised. Rollers apply oil-based ink and water to the plates. Since oil and water don’t mix, the oil-based ink won’t adhere to the non-image areas. Only the inked image portion is then transferred to a rubber blanket (cylinder) that then transfers the image onto the paper as it passes between it and another cylinder beneath the paper. The term offset refers to the fact that the image isn’t printed directly to the paper from the plates, but is offset or transferred to another surface that then makes contact with the paper.
