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) 4 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)  blorgggg4 years ago
This rinsed off in about 3-4 seconds under the sink at medium flow.
blorgggg (author) 4 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)  blorgggg4 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) 4 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 Rain4 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.
blorgggg (author)  Lithium Rain4 years ago
You nailed the problem. It pretty much has to be water. nice chemi-pun!
Edit: I am not saying that it isn't technically possible - after all, they have those kids's "painting books" where you take a wet paintbrush and the colors appear - just that I'm not sure how you DIY it.

http://www.freepatentsonline.com/4212393.html might be useful; it describes the kind of children's toy I'm talking about.
lemonie4 years ago

At what temperature does copper sulphate dehydrate to "white"...?

Kiteman lemonie4 years ago
The problem with copper sulphate is that it will rehydrate from moisture in the air, even on a relatively dry day.
lemonie Kiteman4 years ago

I wondered whether a bit of PVA might seal it enough..?

caitlinsdad4 years ago
Maybe a chemical wetness indicator? http://www.newton.dep.anl.gov/askasci/chem03/chem03260.htm
blorgggg (author) 4 years ago
This sort of thing might work :http://www.thebestkidsbooksite.com/craftdetails3.cfm?CraftID=2111
, but im not sure how much pressure is needed to actually wash the ink off (like would a light rain that soaks the paper, still wash off the paint appropriately).

blorgggg (author) 4 years ago
Also i have access to a laser cutter
Kiteman4 years ago
Alternatively, paint or dust the sign with something that would wash off?
Kiteman Kiteman4 years ago
(That is, it washes off and reveals the message previously painted beneath.)
blorgggg (author)  Kiteman4 years ago
Hmm, good idea. I could coat the paper the wash-off able substance, then stencil a layer of spray on glue into the design i want (on the front and the back maybe). Then when it gets soaked, the pigment will stay on the area that is glued.

The paper will be ideally about business card sized, biodegradeable, and coated with a pigment that you can keep in your pocket without getting the pigment all over.

any ideas for an easily wash-off able substance?
Something soluble. Whitewash?
blorgggg (author) 4 years ago
I tried playing with those crayola color changing markers to no avail