invisible ink to make marked bicycle cards
Invisible ink, also known as security ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means. Invisible ink is one form of steganography.
Invisible ink can be applied to a writing surface with a specialty purpose stylus, stamp, fountain pen, toothpick, calligraphy pen, or even a finger dipped in the liquid. Once dry, the written surface looks as if it were blank, with a similar texture and reflectivity as the surrounding surface.
What an "ideal" invisible ink is depends on its intended use. For example, property marking should ideally be done with ink easily read under ultraviolet light, whereas in espionage such an ink would be considered too easily detectable since a large number of letters may be screened relatively quickly using UV light.
Invisible inks are inherently "insecure" against a determined and well-equipped inspector, which must be balanced against the logistical difficulty in carrying out mass-screening of posted mail. It is easier to perform large-scale undetected screening of millions of electronic communications, than to mass-screen even a small fraction of conventional mail. Apart from in dictatorships where large numbers of personnel are employed to spy on fellow nationals, screening of posted mail is only feasible in particular situations, such as letters to and from a particular suspect or facility.
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.
For practical reasons, the inks are listed here according to their method of development. It must be understood however that some inks - particularly those of organic origin or those consisting of a mixture of several chemicals - may be made visible by several methods. For example, invisible writing with soap water may be made visible either by heat, reaction with phenolphthalein, viewing under ultraviolet light, or by placing the page inside an iodine fume cupboard.
Who can use invisible ink to make Marked Bicycle Cards?