Secret Message Revealed After Wet

Does anybody know of a way to have a paper with a secret message on it (pretend the message says, "It Rained"), and once the paper has been soaked with liquid (water, or maybe a mild acid or base), the secret message is revealed permanently?

So like, if you had a little paper sign out in your yard with the secret message, and it rained, and the paper got wet, when you come home, you would see the dried out paper but with the message revealed.

(note, i don't actually care about a "it rained" indicator, just an example)

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blorgggg (author) 6 years ago
Here's the results. Print a message (in this case a QR code), coat it with a silver Gouache (gives it a nice lotto scratcher ticket feel), and then it rinses off quite easily. Note: a tiny bit of directed water force, or a longer amount soaking is necessary to remove it. Thus a sprinkle of rain might just smear the silver a bit, but a decent rain would get rid of it. For my cases this is precisely what i want. Thanks for everyone's help and suggestions.
blorgggg (author)  blorgggg6 years ago
This rinsed off in about 3-4 seconds under the sink at medium flow.
blorgggg (author) 6 years ago
I got some Gouache (opaque watercolor) from an art store.
Maybe i can write a permanent message on the paper, then apply the Goache all over, and then (hopefully) the Goache will be easily rinsed off.

I'll report my findings!

This is an interesting conundrum! Your suggestions are awesome!
blorgggg (author)  blorgggg6 years ago
The Gouache works quite well. It will not disappear from just being wet, you need to hose it off a little bit, but for my purposes this is exactly what i wanted. I got a silver colored Gouache and it feels like a lottery scratcher ticket that you can "scratch off" with a water gun.

Ill post pictures later
blorgggg (author) 6 years ago
More suggestions I have had from friends for those who are interested:

Water sensitive paper:

And here is a link to those color wonder papers people were talking about:
I'm sure this is something a quick google would reveal - IIRC, lemon juice will show up under a blacklight, for example.
blorgggg (author)  Lithium Rain6 years ago
yeah, I want the same effect, except with water as the (permanent) activator instead of UV or heat
Wait - you want the indicator to be water specifically, or just any liquid? If it can be another liquid, that's not hard, and it is trivial to find different recipes through search.

The issue with having water as the indicator has to do with the way invisible and disappearing inks which depend on chemical reaction (as opposed to heat or light activation) work. In most cases, it seems, this type of invisible ink is made visible (and sometimes, invisible again) through acid-base reactions. Water, being neither an acid nor a base, doesn't lend itself very well to that. I'm not saying that a way doesn't exist, because I've only cursorily examined the question, but it isn't a very popular or well known solution (ha!), if it's out there.
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