Jonathon

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.