Time Delay Ink?

I know whenever I purchase an item that has a warranty on it, the ink on the receipt disappears before the warranty runs out.

Apparently they've started printing receipts with ink that is both heat and light sensitive.

I'd google for invisible inks. When I was a kid (yes, a baby goat in a prior life ;-), I played with invisible inks. Some were invisible and made permanently visible with heat (like lemon juice). At least one was invisible and only made temporarily visible somehow--I'm not sure it was heat. I think the ink was an off the shelf chemical like from a chemistry set--maybe copper sulfate or something like that.

I suspect these won't meet your 30 second time frame initially, but maybe they can be modified.

Following up, I guess cobalt chloride was what I used, from Wikipedia.org: Invisible ink

"Cobalt chloride, which turns blue when heated and becomes invisible again after a while (if not overly heated)"

Also, what about any of the invisible inks that are visible under ultraviolet light? (I can't recall, but I don't think they're visible under sunlight, even though sunlight includes ultraviolet.)

I have a thought Tonymech:

Is thermochromic paper a possible solution? After the label has served it's purpose, expose to heat and the label will completely darken.

This does not really meet the description you outline, but may be a solution.

Lemon juice as ink, heat as activator, burn the evidence ;)

Remembering back to school chemistry, Silver nitrate solution is clear, but when dry and exposed to light turns indellible black.

In one science class we managed to destroy a number of white blouses by spraying the girls with "water" and hours later they had black stains wherever it was wet.

As the royals would say "We are NOT amused." was the response from the girls. Never di get feedback from the teacher.

Sounds like you want a 'disappearing ink' which is in the Wiki link previously provided.

Another idea is use this then expose it to this

Thank you all for your contributions, I'm rapidly coming to the conclusion that chemical timers are simply not accurate enough and that I may have to direct them towards a time stamp type of solution.

I have concerns that even using a disappearing ink that the mechanism for starting the reaction will be complicated (expensive) and the time delay will be affected by heat.

Tony

So you want to use these inks on election ballot papers..

the preprinted ink mark in your favour will appear after two days and the one used by voter will disappear in 30 minutes. hehehe

I was a guest at a semiconductor plant a few years ago. the guest badge had back ground ink that disappeared in 24 hours. They have a bunch of these badges that they exposed to something, just before being given out, that started the ink to disappear. Most of the color was gone in 8 hours.

Also There are heat specific chemical that change color with heat. Keep your labels in a fridge and then after use the ink will change color depending on how well insulated the cover for the label is.

Thank you silvCrow, unfortunately my application needs to disappear rapidly when the time has expired.

Tony

I don't have a link handy, but I saw LED and/or liquid crystal 'clothing fabric' a while back.

For your 'clubbers' I guess.

I.e if you want a finite accurate period - then 'gone' - either above and a charged capacitor may do it.

If not - have a look at the Polaroid 'masks' in that film developing system.