I posted already on the use of an USB microscope to identify the oldest manual printing methods, relief and intaglio.
I now look with my USB microscope at some modern monochrome printing methods to highlights some of their characterizing features. As usual, I don’t discuss the actual printing methods but just how to identify them.
Planographic is a large category which encompasses the numerous variant of lithography whose first process was discovered in 1798. There are wonderful videos on the internet that illustrate how this method is done; just search for “lithography”. Briefly, with the invention of lithography, the artist was freed from any curving and engraving; the artist could just draw as usual but on a prepared stone using specific greasy pencils for lithography. Then the surface is covered with a film of water and then one of ink. This last one will stick only to the greasy drawing and the stone can be etched.
- Flatness. The general characteristic of lithographic printing is both ink and paper ﬂatness.
Also another large category. The term “process” means that the printing block image was not created by a craftsman, but by means of some mechanical, chemical or photochemical process. There were a great number of ingenuous methods conceived, starting from the 1800, in order to speed up the production of prints.
- In an area where different shades of grey merge, the dots are all of much the same size but of different intensity, usually square in shape and separated by a grid of straight white lines. The ink in the dark areas will submerge the white –line grid entirely, becoming solid grey or black.
- Gravure differs from relief printing and offset lithography in that it is capable, like any intaglio process, of depositing varying depths of ink.
A b/w photograph is reduced to pure blacks dots with white spaces by using a screen between the photograph and a light-sensitized surface which will become the actual printing block. The method was fast but low in quality so it was used for newspapers and economic books from 1880s until 1960s, then replaced by offset lithography.
Halftone printing is identified by:
- Squeeze-out. This feature is the result of more ink being deposited at the edges of individual dots than at the center.
- Edge’s rim. In an area where different shades of grey merge, the dots shows clean edge with ink rim
Light pass through a photograph negative and hit a lithographic plate coated with light-sensitized gelatin. The gelatin stroken by light hardens and will take the ink while the soft gelatin is then dissolved by water. The term “offset” means that the ink is not transferred directly from the plate to the paper but is set off from the plate onto an intermediate rubber roller and from that is transferred to the paper.
- In an area where different shades of grey merge, the dots are of differing size and have a blurred edge, but the ink will lie evenly within the dots.
Xerography (electrostatic printing) is the process used in photocopy machines and in laser printers. The black lines of a xerox image are made up of thousands of tiny dust-like grains of pigment. A few specks of pigment are not inline and is these deviants which give the method away.
In inkjet printers lines and letters will have a much softer edge than in an original print.
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