Lithography is a technique invented in 1798 by Alois Senefelder and based on the chemical repulsion of oil and water. A porous surface, normally limestone, is used; the image is drawn on the limestone with a greasy medium. Acid is applied, transferring the grease-protected design to the limestone, leaving the image 'burned' into the surface. Gum arabic, a water-soluble substance, is then applied, sealing the surface of the stone not covered with the drawing medium. The stone is wetted, with water staying only on the surface not covered in grease-based residue of the drawing; the stone is then 'rolled up', meaning oil ink is applied with a roller covering the entire surface; since water repels the oil in the ink, the ink adheres only to the greasy parts, perfectly inking the image. A sheet of dry paper is placed on the surface, and the image is transferred to the paper by the pressure of the printing press. Lithography is known for its ability to capture fine gradations in shading and very small detail.