Hi Boffins and Boffettes,

 

I need to create an ink that after application will remain invisible to the naked eye for a period of 4 days, (or thereabouts). It must reveal itself without a secondary agent (just time and normal environmental conditions), and needs to be made from non-hazardous, readily available (though not necessarily household) chemicals.

 

Really hoping someone can help me. It's a bit of an esoteric request, but it's actually quite important.

 

Thank you.

you`r not really going to find something that will do that in 4 days and make a sudden change without some sort of catalyst, you could probably get silver chloride to do this, but it will appear slowly over these 4 days, and really fast if exposed to direct sunlight.

 

perhaps some sort of agent that will change color on air (oxygen) exposure bound in some sort of lipid that will slowly evaporate to expose it?

In principle you might get something like this

http://en.wikipedia.org/wiki/Iodine_clock_reaction

to work but 4 days would be a challenge.