Commonly regarded as the most difficult printmaking method to learn and master, lithography was invented in Germany in 1796. The technique involves drawing directly on flat stone with an oil-based implement, then coating the stone with a water-based liquid. The applied marks repel the liquid, so that when oil-based ink is applied to the stone it fills in the image, allowing it to be transferred onto a sheet. Since the 19th century, lithography has been widely used by commercial and fine artists as a means to mass-produce images, maps, and texts. It was explored early on by Eugène Delacroix and Honoré Daumier, but it was artists in the latter half of the 19th century who unleashed the medium with bold experimentation. Henri de Toulouse-Lautrec and Édouard Manet took to the stone with a marked freedom, depicting motion and fleeting effects of light, while Pierre Bonnard and Édouard Vuillard infused the rich, natural colors of Impressionist pastels and oil paintings into their lithograph prints. Creative collaborations between artists and master printers have yielded innovative work by artists like Pablo Picasso, Jasper Johns, Robert Motherwell, and Vija Celmins.