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I would like to make some invisible ink that apears by itself after some time, preferably at least a couple of days. I want to write a dedication in a book with it, so it would probably be fine if the colour apears after half an hour of air exposure. Is this even possible using easy to get ingredients?

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  • $\begingroup$ Some organic molecule with colored oxidation product and low oxidation rate, dissolved on solvent, but cant think none in particular. $\endgroup$ – Orr22 Aug 23 '15 at 0:21
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You might experiment with fresh-ground apples, which contain polyphenol oxidase and catechol oxidase, enzymes that turn brown in air (a tiny amount of copper or ferrous salt is also present, acting as a catalyst).

For that matter, dilute ferrous salts are often almost colorless (pale green) but darken to brown on exposure to air. (Digging in a podzol, soil of a pine forest, shows that color change.) So, you could extract a bit of liquid from greenish podzool and load your pen or marker with that.

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