How to make permanent ink that appears after some time?

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. 




Depending on what you want to do (what do you want to do?) you might be looking for [gall ink](http://en.wikipedia.org/wiki/Iron_gall_ink). Briefly, the chemistry works like this: the recipe consists of soluble polyphenols (typically [gallic acid](http://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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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