Believe it or not, lithographic printing – lithography – was invented as far back as 1796 by Alois Senefelder in Bavaria. He was an author and actor and needed a cheap way to reproduce plays and books. The word lithography is derived from two Greek words meaning “to write” and “stone”.
Lithography was originally used to draw an image in wax, oil, or fat on to a smooth piece of limestone. This was then treated with a mix of gum arabic and acid which etched the parts of the stone not protected by the wax. Then the stone would be dampened with water which the etched areas retained. After that an ink which was oil based would be applied which would be rejected by the wet areas, as oil and water do not mix, the ink adhering only to the image. Finally, the inked area would be transferred to a sheet of paper thus ending up with a printed page. This method is still used to day for some types of fine art printing.
Offset lithography today uses photographic processes, and plates of aluminium, polyester or mylar are used instead of stone. The plate is covered with an emulsion and a photographic negative of the required image is placed on it and exposed to ultra violet light. After it has been developed the emulsion displays a reverse of the negative image which is, of course, the same as the original positive image.
The plate is then affixed to a cylinder on a printing press where water is applied by dampening rollers, the emulsion areas rejecting the water, and an ink is then applied by the inking rollers, which adheres only to the image area.
If the image was then transferred to paper by the rollers the paper would become wet, and the image would be a mirror image rather than the positive image desired. What happens instead is that the plate rolls against a cylinder which is covered by a rubber blanket. This squeezes out the water and picks up the ink which is then transferred to paper.
This is called offset lithography or offset printing because the image is offset to the rubber blanket before being transferred to the paper.