Lithography is a printing method that utilizes the mutual repulsion of oil and water. The artist uses an oil based medium such as wax crayon to draw an image on a slate of limestone, which is then covered with a layer of gum arabic and acid. The chemical component sensitizes the drawing so that the darkened areas absorb ink while the non-image areas don't. The method was invented around 1798 by a Bavarian playwright called Alois Senefelder who discovered he could easily replicate his scripts this way. The method was relatively unknown for the next twenty years during which Senefelder refined the techniques and published a manual in 1818 titled A Complete Course of Lithography. Though at first only possible in monochrome, the second half of the 19th century saw an emergence of colored lithographs, each hue requiring a separate stone.
Lithography became a popular means to mass produce writing and posters, eventually developing into the printing method widely used today, called off-set lithography. Yet, the medium was also enthusiastically embraced among now-famous Impressionist artists such as Eugene Delacroix and Francisco Goya. Unlike other printing methods such as etching and woodblock printing, lithography was natural for artists since the process was no different from painting with a brush. It also allowed for greater gradation effects, allowing the artists to express finer tonal differences of landscape such as shifting light over the ocean or in the sky. Similarly in the 20th century, Alexander Calder, Marc Chagall, Henri Matisse, Joan Miro and Pablo Picasso produced many of their prints using lithography.