Lithography uses simple chemical processes to create an image. For instance, the positive part of an image is a water-repelling substance, while the negative image would be water-retaining. Thus, when the plate is introduced to a compatible printing ink and water mixture, the ink will adhere to the positive image and the water will clean the negative image. This allows a flat print plate to be used, enabling much longer and more detailed print runs than the older physical methods of printing. Lithography was invented by Alois Senefelder in the Kingdom of Bavaria in 1796. In the early days of lithography, a smooth piece of limestone was used (hence the name "lithography": "lithos" (λιθος) is the ancient Greek word for stone). After the oil-based image was put on the surface, a solution of gum arabicin water was applied, the gum sticking only to the non-oily surface. During printing, water adhered to the gum arabic surfaces and was repelled by the oily parts, while the oily ink used for printing did the opposite. Our collection "Illustrated Encyclopedia Plants" is made with this technique. The prints can therefore easily be mistaken for being "pixelated". They are not. The slightly coarse and smeared expression is precisely what makes these posters absolutely wonderful. Photocrom: The process was invented in the 1880s by Hans Jakob Schmid (1856–1924), an employee of the Swiss company Orell Gessner Füssli. From the mid 1890s the process was licensed by other companies. In the 1890s it was the leading method in coloring black and white images. The method was used until the Second World War and the last photochromic printer worked until 1970. A tablet of lithographic limestone called a "litho stone" was coated with a light-sensitive surface composed of a thin layer of purified bitumen dissolved in benzene. A reversed halftone negative was then pressed against the coating and exposed to daylight (ten to thirty minutes in summer, up to several hours in winter), causing the bitumen to harden in proportion to the amount of light passing through each portion of the negative. Then a solvent such as turpentine was applied to remove the unhardened bitumen and retouch the tonal scale, strengthening or softening tones as required. Thus the image became imprinted on the stone in bitumen. Each tint was applied using a separate stone that bore the appropriate retouched image. The finished print was produced using at least six, but more commonly ten to fifteen, tint stones. Our collection Photocrome collection are prints from this technique.