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I'd like to ask, how to make permanent ink, that will be invisible when used, but after like 30 minutes it will appear (preferably dark color)? Thanks.

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Depending on what you want to do (what do you want to do?) you might be looking for gall ink. Briefly, the chemistry works like this: the recipe consists of soluble polyphenols (typically gallic acid) and a Fe(II) salt. The preparation must be stored in an air-tight container and even then decomposes over time. As soon as it is applied to paper and exposed to air, the Fe(II) is oxidized to Fe(III), which forms insoluble deeply coloured complexes with your polyphenol. The permanent colour takes about a day to develop. That's why often a non-permanent dye is added so people can see what they are writing.

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You are looking for a Vat dye. Speed of color appearance may vary, so further research according to your requirement is needed.

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I do not think there is a precise answer to your question, but I would use some form of pigment/dye in a precursor state which is able to react (slowly) with air, either oxygen or water vapour, to produce the coloured form of the pigment/dye. Given the type of reaction, especially if it is a form of oxidation, this could be irreversible, therefore making the ink permanent.

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  • $\begingroup$ Thanks, but I'm a total beginner, could anyone please make a bit more detailed "tutorial" for me? (Yes, I tried googling, but I didn't find anything) $\endgroup$ – Jake S. Nov 26 '14 at 15:53
  • $\begingroup$ I'm sorry but I do not think that's possible. It's not that easy. It'd require at least some research to find candidates, experiments to assess their reactivity and suitability, etc. $\endgroup$ – entropid Nov 26 '14 at 15:55
  • $\begingroup$ Well, It is possible, because friend of mine uses this kind of ink, that appears after about 30 minutes and has dark brown color, but he doesn't want to tell me the recipe... $\endgroup$ – Jake S. Nov 26 '14 at 21:14
  • $\begingroup$ Please re-read my answer. I never said it's not possible. I just said it's not that straightforward going from a “request” to an actual formulation. Unless something already exists, as noted above. $\endgroup$ – entropid Nov 26 '14 at 21:15
  • $\begingroup$ Sure, sorry.. Also I have just found this experiment, where you mix methylene blue with something so it loses it's color and after exposure to oxygen, it regains it. Could that be used for this purpose? $\endgroup$ – Jake S. Nov 26 '14 at 21:17

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