Frottage is taking a rubbing using paper and usually a graphite medium to show the textural surface of an object such as a tile, leaf, etc.
Frottage was used to stimulate the imagination, acting as a jump off point for an image to express the subconscious imagery of the artist.
In Surrealism it was a form of automatism (chance rather than automatic) where the unconscious mind produced an initial frottage outcome. It should be noted that once the frottage had been produced, Surrealists used it consciously within a final image.
It was first introduced by Max Ernst (1891-1976) in 1925 when he was inspired by rubbings of floorboards which he said intensified his visionary powers (Oxford Art Online, 2013) His frottage drawings were exhibited and published in 1926 in Histoire Naturelle, a collection of 34 frottage drawings, now part of the Museum of Modern Art (MOMA) collection. He combined various frottage to create fantasy landscapes, plants and animals. An example from the MOMA collection is L’origine de la pendule (The Origin of the Clock) from Histoire Naturelle (Natural History) in which a fantasy woodpecker-like bird and the tree trunk has been built-up from various pieces of frottage from different rubbings. See here.
Ernst went on to adapt frottage into grattage where layers of paint are both rubbed on and scraped off to reveal unexpected images in the under-layers, which are then incorporated into the final image with additional overpainting.
Ernst was a member of the Surrealist movement (having formerly been part of the Dada movement) and he produced art that challenged the Western Academic Art aesthetic code of the time and went against both Christian doctrine and conventional morality (Oxford Art Online; 2013).
Other artists who used frottage include:
Saburo Husegawa (1906-1957) in Rhapsody: At the Fishing Village (1952). See here.
Simon Hantoi (1922-2008) and Henri Michaux (1899-1980)
Robert Rauschenberg (1925-2008) Preparatory drawing from In Memory of My Feelings (1967). See here.
Oxford Art Online (2013) Ernst, Max Biography [online] At: http://www.oxfordartonline.com/subscriber/article/grove/art/T026563 (Accessed on 15 November 2016)