Robert Indiana kept a series of illustrated journals during the late 1950s and 1960s, in which he discusses the development of his work as well as his daily life on Coenties Slip.

In his journal entry for February 25, 1961, Indiana records that the day began late, and was quiet and uneventful except for a visit from Lenore Tawney, who came over with a check for J. (his partner, fashion designer John Kloss) and cookies for both of them. She reported on the exhibitions she had seen, Ediwin Dickinson, Alexander Calder/Juan Miró, and Robert Mallary, "being very struck by the latter." He also notes that Kloss made popcorn, and that they saw ninety minutes of Leonard Bernstein "conducting and elucidating" Stravinsky's Oedipus Rex.

Indiana records giving The Sweet Mystery a coat of lamp black, the matte of mars black having been unsuccessful. A small sketch of the work's two doubled ginkgo forms appears at the top of the journal page.