Invisible ink
Substance used for writing which is invisible and can later be made visible
Invisible ink
Invisible ink, also known as security ink or sympathetic 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, such as heat or ultraviolet light. Invisible ink is one form of steganography.
Read on Wikipedia · Image Benedict Arnold; Peggy Shippen Arnold, Public domain
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