https://www.cia.gov/news-information/press-releases-statements/press-release-2011/cia-declassifies-oldest-documents-in-u.s.-government-collection.htmlsoaking a handkerchief or collar in a mixture of nitrate, soda and starch, then drying the fabric. The chemicals come out when the cloth is placed in water. The liquid becomes invisible ink, that can be loaded into a pen and used to write a message. The recipient develops the writing by applying a second chemical, iodate of potassium.