Printing is the large-scale reproducing of texts and images in black and white or color by applying pressure against an image carrier – such as a sheet of paper, cloth or other material. It is an essential part of the production of books, newspapers, maps, posters, etc. In the past, printing dominated the dissemination of information but it has been challenged by audiovisual media such as radio, television, film and tape recording.
The first printed materials were written on wood, but the invention of movable type revolutionised the process. In Song China (1127 – 1279 AD), each Chinese character was carved in relief on a block of baked clay and then mounted on an iron plate so it could be easily separated, moved, and reused. This allowed a great increase in the number of items that could be printed, from dictionaries to mathematical text. Eventually printed books became commonplace in society and a substantial collection of them was considered a status symbol.
Several techniques of printing are now in use and they are grouped together under the general term ‘offset printing’, a term which describes all processes where an image is transferred from a plate to paper or other substrate under pressure. The five major printing processes are distinguished by the method of image transfer and the type of image carrier used.
Those that are direct transfer processes include gravure, flexographic and screen printing, whilst those that involve an indirect transfer are offset printing, hot metal (letterpress), silkscreen and lithography. The image carrier in lithography is usually a stone or now more often a metal plate – zinc or aluminium – that has been treated with a mixture of gum arabic and weak acid to make the blank areas receptive to water. The etched areas on the plate pick up ink when it is inked, but reject ink when the plate is uninked.