[kchap (OP)]

I am trying to find a chemical or ink that will show up or disappear after 2 hours.  I would like this to activate with exposure to oxygen or, possibly, through a chemical reaction.  The key is to have a non-toxic way to indicate that 2 hours has passed and to be able to display this on paper or other material.  Any thoughts are appreciated.

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I am trying to find a chemical or ink that will show up or disappear after 2 hours.  I would like this to activate with exposure to oxygen or, possibly, through a chemical reaction.  The key is to have a non-toxic way to indicate that 2 hours has passed and to be able to display this on paper or other material.  Any thoughts are appreciated.

http://en.wikipedia.org/wiki/Invisible_ink
<<Disappearing inks
Inks that are visible for a period of time without the intention of being made visible again are called disappearing inks. Disappearing inks typically rely on the chemical reaction between thymolphthalein and a basic substance such as sodium hydroxide. Thymolphthalein, which is normally colorless, turns blue in solution with the base. As the base reacts with carbon dioxide (always present in the air), the pH drops below 10.5 and the color disappears.[3] Pens are now also available that can be erased simply by swiping a special pen over the original text. Disappearing inks have been used in gag squirtguns, for limited-time secret messages, for security reasons on non-reusable passes, and for fraudulent purposes>>.

About the time, you have to make some trials with different concentrations.

Of course there are many others indicators which could be used as well.