Understanding Lithography
1 | 2
An understanding of the principle of lithography can be difficult but may be facilitated by comparison of lithography with other printmaking techniques. For instance in both relief printing and in intaglio, the image to be printed is formed by physical manipulation of a surface such that in say, linocut all those areas not to print are cut away; whilst in intaglio the drawing will be scored, cut or etched into the surface of the metal plate. As a planographic process however, in lithography where the drawing is made on the totally flat surface of the stone, selective inking and printing of the image is only possible following chemical manipulation. This chemical manipulation or processing, also known confusingly as etching is completed using nitric acid and Gum Arabic mixed together in varying proportions as etches.
During etching two simultaneous chemical reactions occur on the stone. Being hydrophilic Gum Arabic causes the negative areas of the image to become water-attracting and hence grease and ink repellent. Meanwhile acid in the etches causes saponification or the formation of soap within the structure of the stone and directly beneath the original drawing. Being oleophilic it is this substance representing a complete copy of the drawn image that inevitably attracts ink during printing. As a consequence two quite distinct and opposing surfaces are formed on the stone, one which will attract water and will repel ink; the other being diametrically opposed will attract ink and will repel water.
During printing it is most important therefore that the stone or plate is kept damp by using a sponge to distribute an even film of water over the entire surface. Providing that the moisture on the surface of the stone is maintained it is then possible to ink the image using a roller charged with lithographic printing ink. If however the surface of the stone dries out, ink will then be deposited over the entire surface and it will become very dark, causing the image to fill in and inevitably loss of the entire drawing.
Once fully inked, printing occurs when the stone together with the paper is run through the press under extreme pressure causing the inked image to transfer from stone to the paper placed on top. When printed on a direct press, the resulting printed image always appears as a mirror image of the original; only when an offset press is used will the image be printed in the same orientation as that drawn on the stone. |
 |
|
Inking up a stone on the press |
Sponging the stone |
|
 |
 |
|
Printing the stone |
Pulling the proof |