Wednesday, March 14, 2012

Lithography vs. Offset lithograpy

 The following is an explanation of the differences between  stone lithography and  offset lithography.
Offset lithograph
LITHOGRAPHY works because of the  mutual repulsion of oil and water. The image is drawn on the surface of the print plate with a fat or oil-based medium such as a wax crayon, which may be pigmented to make the drawing visible. After the drawing of the image, a solution of gum arabic, weakly acidified with nitric acid is applied to the stone. The function of this solution is to create a  layer of calcium nitrate salt, and gum arabic on all non-image surfaces. The gum solution penetrates into the pores of the stone, completely surrounding the original image with a 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 arabic and water, but ready to accept the 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 which applies even pressure over the surface, transferring the ink to the paper and off the stone.

High-volume lithography is used presently to produce posters, maps, books, newspapers, and packaging—just about any smooth, mass-produced item with print and graphics on it. Most books, indeed all types of high-volume text, are now printed using offset lithography
For offset lithographyhotographic processes, flexible aluminum, polyester, mylar or paper printing plates are used instead of stone tablets. Modern printing plates have a brushed or roughened texture and are covered with a photosensitive emulsionographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to ultraviolet light. After development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image.