Letterpress printing is the oldest printing technique, in which a raised surface is inked and then pressed against a smooth substance to obtain an image in reverse.
Early Chinese wood-blocks used characters or images carved in relief, and this form of image printing was known in Europe in the 13th century. In the 1400s, Johann Gutenberg is credited with the invention of printing from individually-cast, reusable letters set together in a forme. He used a wooden press where the type surface was inked and paper laid carefully on top by hand, then slid under a padded surface and pressure applied from above by a huge threaded screw. The amount of pressure per square inch or "squeeze" is greater on some highlight dots than it is on larger shadow dots. Expensive, time consuming adjustments must be made throughout the press run to make sure the impression pressure is just right. Later metal presses used a knuckle and lever arrangement instead of the screw, but the principle was the same.
A letterpress is a machine that automates the process of inking a raised surface and pressing it against sheets of paper. While there are many different kinds of presses, some common types are:
With the advent of industrial mechanisation, the inking was carried out by rollers which would pass over the face of the type and move out of the way onto a separate ink-bed where they would pick up a fresh film of ink for the following sheet. Meanwhile a sheet of paper was slid against a hinged platen which was then rapidly pressed onto the type and swung back again to have the sheet removed and the next sheet inserted. In a fully-automated 20th century press, the paper was fed and removed by vacuum sucker grips.
Further information and links:Introduction To Letterpress Printing