what is involved in the technique known as frottage
Frottage (art) In art, frottage (from French frotter, to rub) is a surrealist and automatic method of creative production developed by Max Ernst. Details[edit] In frottage, the artist takes a pastel or pencil or other drawing tool and makes a rubbing over an uneven surface.
In 1925, Ernst began using the technique of frottage, which involves laying a sheet of paper over a textured surface and rubbing it with charcoal or graphite (the same process can be used to create a rubbing of a coin or an engraved surface).