Invisible ink
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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,
A coded letter from Benedict Arnold, originally written in invisible ink. Handwriting by Peggy Shippen Arnold is interspersed with coded communication in Arnold's hand.
A common task in escape rooms is assemble an eye-safe blacklight torch to reveal messages in invisible ink
A US$20 bill showing a visible stripe under an ultraviolet light
Cobalt(II) chloride
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Cobalt chloride refers to inorganic compounds with the formula CoCl2·n. These compounds include anhydrous CoCl2 as well as several hydrates. The anhydrous form is a blue solid; the dihydrate is purple
Invisible ink
…Wine, or vinegar Cobalt chloride, which turns blue when heated and becomes invisible again after a while (if not overly heated) The writing is rendered visible by heating the paper…
Cobaltous chloride
Cobaltous chloride anhydrous
The deep blue colour of this moisture indicating silica gel is due to cobalt chloride. When hydrated the colour changes to a light pink/purple.