Properties of An "ideal" Invisible Ink
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.
The World War II SOE training manual identified the following properties of an "ideal" invisible ink:
- mixes with water.
- Non-volatile, i.e. no pronounced smell.
- Not depositing crystals on paper, i.e. not easily seen in glancing light.
- Invisible under ultraviolet light.
- Does not decompose or discolour the paper e.g. not silver nitrate.
- Unreactive with iodine, or with any of the other usual developers.
- Potential developers for the ink should be as few as possible.
- Should not develop under heat.
- Easily obtainable and has at least one plausible innocent use by the holder.
- Not a compound of several chemicals, as this would violate #7.
From practical experience "6" and "9" were usually incompatible. SOE agents were trained not to risk their lives through reliance on insecure inks, most of which were of World War I vintage. In general, SOE used invisible inks as a back-up method of communication, for when more secure communication techniques were unavailable. The agency was known to supply special inks to its field agents, rather than have them depend upon improvisation from obtainable everyday chemicals. When agents were forced to improvise, they were to dilute their invisible ink as much as possible to reduce chances of detection.
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