Here is a nice round-up of some simple sneaky inks. The old favorites like lemon juice and vinegar are there, but also some more sophisticated systems using stuff like cerium oxalate and iodine fuming.
From the pages of MAKE:
Mike Golembewksi shows us how to make an invisible inkjet printer in MAKE 16 - Spy Tech.
Void your warranty, violate a user agreement, fry a circuit, blow a fuse, poke an eye out. Make: The risk-takers, the doers, the makers of things... Welcome to Make: Online!
More contributors: Mark Frauenfelder (Editor-in-Chief, MAKE magazine), Kipp Bradford (Technical Consultant/Writer), Chris Connors (Education), Diana Eng (Guest Author), Peter Horvath (Intern), Brian Jepson (O'Reilly Media), Robert Bruce Thompson (Science Room)
Behind the Scenes at MAKE and CRAFT
In January, many of the remote MAKE/CRAFT team members (myself included) convened at the Maker Media headquarters at O'Reilly Media in Sebastopol, California. Take a look behind the scenes of your favorite DIY publications as Goli Mohammadi gives us...
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You can also do the following simple procedure:
Wet a sheet of paper - Put a dry sheet on top and write message with a ball point pen or pencil.
Destroy top sheet, (to keep the secret), when dry the lower paper seems blank so write something innocent on it.
To develop the writing wet the sheet again and the secret message is visible when you hold up to the light.
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Good one! Thanks. I'm thinking some more secret ink content may be in order.
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