Throughout the modern period artists have learned their craft by copying other artists, and Johnson was no exception. Using both drawing implements and paint (see Themes 4.0 and 5.0 for his European painted copies), Johnson chose artworks that indicate his admiration for the Renaissance masters, Rembrandt, and his famous contemporaries. —PH
MacGibeny, 2022: Drawing from casts was a basic method for learning to draw at the Düsseldorf Academy, where Johnson was studying art in the winter of 1849–1850. According to the Lexikon der Düsseldorfer Malerschule [Lexicon of the Düsseldorf School of Painting], 1998, provided by Kathrin DuBois, Acting Head of the Gallery of Paintings at Kunstpalast Düsseldorf, he enrolled formally in only one class at the Academy: Anatomy and Proportion with Professor Heinrich Karl Anton Mücke. See the linked image of the plaster cast of Lorenzo de'Medici, Duke of Urbino at the Victoria and Albert Museum, which suggests what Johnson's drawing of the head of Michelangelo's sculpture may have looked like.
Baur 1940, p. 80: "Drawing from a cast."