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Author: Subject: Making invisible markings on paper (needs to be not commonly known or easily seen)
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[*] posted on 20-5-2018 at 20:35
Making invisible markings on paper (needs to be not commonly known or easily seen)

So I've heard that in the past people used things like lemon juice and urine to write on paper which could then be uncovered by heat or some other solution. I'm looking for something similar to this or something that can be incorporated into the paper when it is made and I'd like to be able to incorporate a design or some text if it is incorporated into the paper. I'm looking for something that is as rare as possible so that it won't be uncovered.

If it is incorperated in the paper, I'm fine with having some solution that is brushed/sprayed on the paper to bring it out even if it has to be put under a special light.

Does anyone know od anything like this? I know alot of papers have water marks but those are too noticeable and wouldnt' work. I need something invisible until a very special procedure uncovers it.

Any ideas?
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[*] posted on 20-5-2018 at 20:41

If you have a UV light, bleach makes great invisible ink, as it absorbs UV, making it look like black ink.

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[*] posted on 20-5-2018 at 22:01

Here is my recent effort on this one:

I spent a bit of time playing with ideas but not too much and I am sure that more dramatic inks are possible.
If you don't have iodate then you could use iodide and an oxidant with much the same effect.
I did play with adding starch as well for a deeper colour but decided it was not worth it. You might need it for the iodide variant.

I was toying with the idea of having a competition to design and demonstrate an invisible ink. With a bit of clever redox chemistry or perhaps some acid-base indicators you could get some nice colours.

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[*] posted on 21-5-2018 at 00:43

Thanks for the suggestions! I think I should have been a little more clear as to what I want to do and what I need.

Imagine someone making counterfeit currency but they want to mark it as fake but have it hidden either in the paper or in a printing process. Obviously if the hidden text were revealed the printer would be in BIG trouble, so it needs to be something extremely rare and that very few people know about.

The printing process will most likely be off-set printing or maybe something a little more advanced than the 150+ year old tech. I don't think the process can take advantage of laser printing but possibly inkjet if the resolution can get down to a certain level - so I'm not certain what type of printing process would be used.

So there needs to be some data incorporated into the paper/ink of about 25 characters that can not be seen unless revealed by magical chemicals.

IIRC ammonium thiosulfate was used for a process like this either as the printing ink or as the substance that reveals the hidden printing.

Just to make this clear, this has nothing to do with currency forging, I just used that as an example of how important hiding the data/characters is and hiding the data in this case is just as important if not more. Think about hiding bank account numbers in a will so that attorney's can't steal the money from (young children) - beneficiaries not of age. This is something very similar in importance.

So any ideas would be greatly appreciated even if the chemicals are very expensive, rare or toxic, that is of little concern as this requires very small amounts.

[Edited on 5-21-2018 by RogueRose]
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[*] posted on 21-5-2018 at 00:58

Would you go as far as radio isotopes? That could be revealed by photographic paper placed adjacent long enough for the radiation to leave an imprint. Or is that just a bit too dangerous.

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[*] posted on 21-5-2018 at 18:41

OK, Ive been reading this forum for ages, and learning so much - Thanks to everyone. But I've finally made an account because I can help here.

There are lots of processes out there that may help. Depending on what you want to do.
Steganography - Hidden Embedding data into images and text onto the page
Clickable paper - Online verification and weblinks, based on the structure of images on the page

There is a new process for verifying paper ( tested at law) based on the minute variations in fiber density caused by vibrations in the papermaking machines. Think the analogue grooves in an old Vinyl album. these are like fingerprints, and can track paper down to the ream.
Someone was prosecuted for falsifying documents supposedly printed on opposite sides of the US. they showed that the paper was from the same ream.

Every sheet of paper that is printed ( digitally) is embedded with the serial number of the printing device. this is to prevent fraud and government leaks. Authorities can physically locate a device within hours of getting a counterfeit sample.
This has been in place since early 2000's

Microfonts - printing at pointsizes .1 and less.

UV sensitive inks - think banknotes (only reacts to 365nm)
White inks - Printed in the b/ ground
MICR inks - used extensively by banks

1 pixel ( ie 1 / 2400 inch) patterns in the background ( ie non reproducible), Binary code for a bitcoin wallet perhaps?

Actually this may help, and lists some techniques

But basically - more info on what you want to achieve would help.

but more in the vein of this board - how about a minimal threshold product that is luminol sensitive. could be printed or silk screened.

the product that is used in thermal fax paper, that is sensitive to sharpie solvent and turns Black ( hold a sharpie tip next to old fax paper roll)

some form of chemical that "develops" when exposed to an unusual gas - I've seen some wonderful coloured copper salts?

there are many things that could be done to paper. ask away. :-)

Experience : 30+ years in printing industry.

So glad to be able to finally share and help
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[*] posted on 21-5-2018 at 23:58

A very simple and reliable form of "invisible" ink is a solution of potassium hexacyanoferrate(II), a.k.a. yellow prussiate of potash.

Dissolve some of this in water (e.g. 1 gram in 10 ml of water, but the concentration is not critical at all). You get a very pale yellow, nearly colorless solution.

With this solution you can write or paint on a piece of paper. Allow it to dry. You will not see anything at all.

In order to make it visible, dissolve some ferric chloride in 0.5% acetic acid (mix vinegar with 10 times its volume of water). A good solution is 10 grams of ferric chloride in half a liter of such dilute acetic acid. You can also use ferric nitrate or ferric sulfate.

Put the solution in a flower sprayer and with this you can make very fine droplets of this liquid. Spray the liquid thinly over the written or painted message and it will appear in bright blue, clearly visible. Do not use too much spray, otherwise liquid will run over the paper and this will ruin the message.

Cleaning of the flower sprayer can be done with 1% acetic acid, spraying this for some time and then spraying clean tap water. This removes the last traces of iron. This cleaning is important, because otherwise you may get pale brown/yellow rust stains on items which are sprayed with the flower sprayer.

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[*] posted on 22-5-2018 at 02:05

@ausnewbie very interesting info, thanks for sharing!

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