Invisible Ink - Screening Letters For Secret Messages

Screening Letters For Secret Messages

Any invisible ink can be made visible by someone who is sufficiently determined, but the limitation is generally time available and the fact that one cannot apply hours of effort to every single piece of paper. Thus successful use of invisible ink depends on not arousing suspicion that invisible ink may be present.

Telltale signs of invisible ink, such as pen scratches from a sharp pen, roughness, or changed reflectivity of the paper (either more dull or more shiny, usually from using undiluted ink), can be obvious to a careful observer who simply makes use of strong light, a magnifying glass and his or her nose. Also, key words in the visible letter, such as "red cabbage" or "heat", in an odd context may alert a censor to the presence of invisible ink. Invisible ink should not be used with glossy or very smooth paper types, since the sizing of these papers prevents ink from being absorbed deep into the paper and it is easily visible, especially if the paper is examined under glancing light. There are, however, commercially available inks for non-porous surfaces that are only visible under ultraviolet light and are otherwise virtually invisible on such surfaces.

Using either ultraviolet light or an iodine fume cupboard, messages can be quickly screened for invisible ink and also read without first permanently developing the invisible ink. Thus, if a censor uses this method to intercept messages, he or she may then let the letter be sent to the intended recipient who will be unaware that the secret message has already been intercepted by a third party.

A "screening station" could theoretically involve visual and olfactory inspection, an examination under ultraviolet light and then the heating of all objects in an oven before finally trying exposure to iodine fumes. In theory, some invisible inks may even show up using a camera sensitive to the infrared light spectrum.