Lithography is mainly used by commercial printers and printing companies who print thousands of copies of the same item in one production run. Lithography machines can print on both sides of paper and card, and they rely on four basic colours; yellow, cyan, magenta and black. This is also known as the CYMK process. A printing plate with a relief image is dampened with water and then coated with ink; the ink only sticks to the parts of the plate that aren’t wet with water. The printing plate is fixed to a roller and the image is transferred onto paper, fed under the roller.
In offset-lithography, the paper doesn’t come into direct contact with the printing plate. Instead, the image is transferred to a rubber roller. Lithography is used for medium and long print runs of products such as magazines, posters, packaging and books.
Lithography On Limestone
Lithography works because of the mutual repulsion of oil and water. The image is drawn onto the surface of the print plate with a fat or oil-based medium, such as wax crayon, which may be pigmented to make the drawing visible. A wide range of oil-based media is available, but the durability of the image on the stone depends on the lipid content of the material being used, and it’s ability to withstand both water and acid. After the drawing of the image, an aqueous solution of acacia gum, weakly acidified with nitric acid is applied to the limestone. The function of this solution is to create a hydrophilic layer of calcium nitrate salt, on all non-image surfaces. The gum solution penetrates into the pores of the stone, completely surrounding the original image with a hydrophilic layer that will not accept the printing ink.
Using lithographic turpentine, the printer then removes any excess of the greasy drawing material, but a hydrophobic molecular film of it remains tightly bonded to the surface of the stone, rejecting the gum and the water, but ready to accept the very oily ink. When printing, the stone is kept wet with water. Naturally the water is attracted to the layer of gum and salt created by the acid wash. Printing ink based on drying oils such as linseed oil and varnish loaded with pigment is then rolled over the surface. The water repels the greasy ink but the hydrophobic areas left by the original drawing material accept it. When the hydrophobic image is loaded with ink, the stone and paper are run through a press that applies even pressure over the surface, transferring the ink to the paper and off the stone.