LITHOGRAHY
Letterpress is a relief process; gravure is a recess process and lithography is a planographic process, where the printing surface is essentially flat.
Originally, that surface was a smooth chunk of limestone, which has two properties: porosity and an affinity for grease.
When grease is applied to limestone, it forms a new substance: oleo-margarate of lime. This is unaffected by water or spirits – and is also extremely durable. Images in grease on limestone cannot easily be rubbed out.
Grease and water do not mix. And by treating the stone with nitric acid, gum Arabic, then finally a layer of water, you could apply ink to the whole of the surface of the stone, but it would be chased off the non-image areas by water. Lay a sheet of paper on the top and you have your print.
Lithography was discovered by Aloys Senefelder of Bavaria
