Corinth explored every print technique except aquatint; he favored drypoint and lithography. Occasionally the two techniques of aquatint and mezzotint are combined. The principal printing technique was copperplate etching, but engraving and aquatint were also used. Several scholars have suggested that mezzotint and aquatint were also used. They were later exhibited in a mixed technique of engraving, etching, and aquatint in 1881. Goya used mainly the techniques of etching and aquatint in this series. Hale experimented with a variety of etching techniques, including hard-ground, soft-ground, aquatint, and color inking. The first 14, in etching and aquatint, are dated 8 January 1937. By the 1830s, the aquatint process was largely superseded by lithography. Goya used aquatint for most of his prints. Aquatint uses acid-resistant resin to achieve tonal effects. In the "Blue Prints" series, 1999, Kiki Smith experimented with the aquatint process. Aquatint is a variation giving only tone rather than lines when printed. The original aquatint by Audubon is owned by the Brooklyn Museum. An aquatint requires a metal plate, an acid, and something to resist the acid.