Something that could be visible for long enough to be read, but then completely disappear, with no trace? Like writing in the steam of a shower on the mirror. But at normal temperature and humidity.
You may find this interesting:
New Type of Disappearing Ink
Nanoparticle inks that fade away in hours could be ideal for secure communications.
http://www.technologyreview.com/communications/23316/
There’s also this: http://www.thinkgeek.com/gadgets/security/bf21/
Do you want there to be no trace of the ink itself, or just no trace of the message? If the latter, then you could use something that quickly decays into something else and coat the entire paper with that something else, but you’d still be able to detect whatever the decay product is and know that there was a message on it at one time.
I could cause a lot of trouble with one of those pens…
On the Psych detective show last night, the Shawn Spencer character found out what had been written on a postit removed from a desktop. He just huffed on the vacant area and the text appeared in his breath. Only I haven’t been able to do it myself. I guess the principle has something to do with felt markers leaking solvent through the paper to the desk top? I wish I’d recorded the episode.
Something like that for fading images has existed for a long time, but it just fades to black once. It’s blueprint paper that hasn’t been “fixed”. Also regular photographic paper. Although the fading is usually unintentional.
There used to be some that fits the criteria. It would only last a few seconds, like 10 seconds, give or take. It could be found in novelty shops (do they even have novelty shops any more?). Look online at something like Johnson Smith Co., or whatever.
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