The basic principle involves writing with a substance that is transparent or near-transparent on paper, and that reveals itself when the paper is subjected to some specific treatment: heat, chemical reagents, or ultraviolet light. Common formulations have included diluted acids or bases (lemon juice, milk, and urine are historical examples that become visible when the paper is heated), chemical compounds that react with specific developer solutions, and fluorescent compounds that become visible under ultraviolet light.
By the Second World War, the intelligence services on both sides had developed sophisticated formulations of invisible ink that were more difficult to detect than simple heat-activated substances. The British Special Operations Executive and the American Office of Strategic Services both maintained technical sections devoted to covert communication technologies, including invisible ink. German security services were correspondingly sophisticated in their detection methods, running mail and documents through various analytical processes when they had reason to suspect covert communication.
The intelligence written on Baker’s sheet music pages was presumably a formulation that would not be easily detected by routine inspection — not simply a heat-activated substance of the kind that any competent security official would know to test for, but something more sophisticated. The specific formulation is not documented in the publicly available sources consulted for this article.
The choice to use sheet music as the carrier — rather than, say, the newspaper or the clothing itself — reflects a specific judgment about which of her legitimate possessions was most likely to be examined and least likely to be subjected to intensive analysis. Sheet music has a distinctive visual appearance: staves, notes, markings. A person looking at sheet music pages sees music, and unless they have been specifically instructed to test for invisible ink on those pages, they are unlikely to subject them to the analysis that would reveal the hidden text. The music was real music; the legitimacy of the carrier was real; the concealment was embedded in the legitimacy.