Editing Quickstartguide talk:Litho (section)

=== Processing the stone ===

The way in which the stone is processed depends entirely on the materials used to make the drawing, and the amount of grease in the stone.

The stone is processed using gum arabic, sometimes with a very small amount of nitric acid added. The purpose of the gum is to chemically separate the image and the non-image areas, so that the greasy image areas become water repellent or ‘hydrophobic’ and the non-image areas become water receptive or ‘hydrophilic’, so that when printed, only the image areas receive the ink and print. This chemical change happens in the very top layer of the and creates the ‘gum adsorb layer’ which is less then 1mm thick. Nitric acid is sometimes added to the gum in order to stop the grease in the stone from spreading. When nitric is used in the gum it referred to as an ‘etch’.

It is usually necessary to ‘etch’ the stone twice. The first etch is carefully applied to the stone with either a clean paintbrush or sponge. Once the whole stone is covered
with gum, a clean sponge and then a clean rag are used to buff the gum down to a thin,
even layer. Heat is then applied to dry the gum. The stone should then be covered and,
importantly, left overnight to allow time for the gum to chemically change the stone and
establish the image within the stone.