William Poundstone blogs about Goya's miniature paintings on ivory, two of which are now on view at the Getty Museum. 

Poundstone notes that "To create these pictures, Goya coated a small slip of ivory with lamp black and dropped water on it. This produced random splotches not unlike those prized by Japan’s contemporary Rimpa school. Goya, however, took the splotches as a starting point. His younger contemporary Antonio de Brugada reported, 'Goya took advantage of these traces and always turned them into something original and unexpected.'… In basic concept, this was similar to the surrealists' technique of frottage."