Like etching, aquatint is a  printmaking technique that uses acid to “bite” into the printing plate.  Powdered, acid-resistant resin is sprinkled onto the plate and heated so that it adheres. The fine, textured patterns created by the acid and resin produce graduated tonal effects in the print.  Aquatint is usually used in combination with other techniques, such as etching or engraving. Web resource here.

Francisco de Goya. Estan Calientes (They Are Hot) (det.). Aquatint and etching.  1799.  Spencer Museum of Art, Lawrence, Kansas.