History and methods of marking manuscripts are described here. Since marking is for security, and to show ownership, the marks need to be permanent (but, hopefully, not damaging). Current marking methods include embossing, punching, perforating, and inking (visible and invisible). The Library of Congress provides, free, bottles of a secret formula, non-irradicable ink, which may solve marking problems for some archives, libraries, and individuals. The ink's characteristics are explained. Fourth in the Library of Congress Preservation Leaflet series.