Art, artifacts, collectibles and other valuables are often security marked in case of theft, loss, dipsute or other later need to identify the item and/or owners. The markers range from overt holograms and serial numbered stickers to invisible tags, and allow the marked items, and often the rightful owners, to be identified.
An interesting covert marking system uses microdots. Microcodots are miroscopically small metal discs that have identifying information microetched on them. The information can be read under a microscope. The dots can be the size of standard printed perdiod (.). The etched information can be a unique serial number that identifies the object’s owner. Applied via clear adhesive to a valuable painting or sculpture, the dots will go completely unnoticed by the average thief, but can be used to trace the item back to the rightful owner.
Microdots and related covert ‘shrinking down text’ is an old time application. German spies used microdots to covertly pass information during WWII. Even as early as the 1870 Franco-Prussian War, carrier pigeon messages were photographically shrunk so the bird could carry more text.