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can somebody tell me the scientific reason behind that? Answered

i saw  when i wrote with onion juice on a paper, it could not be seen.but after a while, when i put a slight heat on the paper or ironed it, i could see a faint  appearance of the letters i wrote.how did it happen? 

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Yes a water mark is put in the paper when it is made - for hand made paper it is woven into the wire grid the paper is formed on. It slightly reduces the density of the paper fibres at that point and results in a visible pattern when viewed against the light.

As far as I recall the wet writing method is not visible when held up to the light so remains secret until the paper is wet again.

Rick: I believe the wet writing method *is* visible when lit from behind. Easy enough to test, of course. Might or might not depend on the paper in question, though I'd be a bit surprised.

Ok have tried this wet writing - Not a water mark - when dry writing is invisible but shows dark when wet. isn't science wonderful :-)

Surprising, and interesting. Hmmm. Need to figure out why.

I will give it a try and get back - Long time since I did it.

Same with milk

Same with sugar dissolved in water

Cobalt chloride is soluble in water and when wet turns blue but is a pale pink when dry

Wet a bit of paper - Put a dry sheet over the top Write message. Allow bottom sheet to dry. Write anything you like on it. message appears when you re wet the paper.

BTW, that last is a watermark.

If you websearch "secret writing" and/or "invisible ink" I'm sure you'll find a few thousand more.

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Burf

6 years ago

The heat carbonizes some of the organic esters and sugar making their traces visible.

I wish I could talk like that!