In frottage the artist takes a pencil or other drawing tool and makes a rubbing over a textured surface. The drawing can be left as it is or used as the basis for further refinement. While superficially similar to brass rubbing and other forms of rubbing intended to reproduce an existing subject, and in fact sometimes being used as an alternate term for it, frottage differs in being aleatoric and random in nature.
It was developed by Ernst in 1925. Ernst was inspired by an ancient wooden floor where the grain of the planks had been accentuated by many years of scrubbing. The patterns of the graining suggested strange images to him. He captured these by laying sheets of paper on the floor and then rubbing over them with a soft pencil.