Invisible Ink Pen

This is cool. It writes like a normal pen, but if you run a hair dryer over the written words they disappear. And if you put the paper in the freezer the words reappear. Fantastic.

EDITED TO ADD (5/20): This is the same technology as the widely available Pilot Frixion pen. Here's a temperature sensitivity test, and a freezer test.

Posted on May 19, 2009 at 6:49 AM • 45 Comments


SeanNMay 19, 2009 7:39 AM

Useful, unless you live in the Sahara or North pole where the climate may give the game away....


RichMay 19, 2009 7:45 AM


I had the same thought. The default is to turn invisible in the heat so that works fine in Israel, but carry the letter outside in a Minnesota winter and the contents will be revealed.

Cristian BertoldiMay 19, 2009 7:48 AM

All ball pens leave an engraved traces on paper, depending how much pressure made with the pen, what was the thickness of the sheet, and if there was something soft behind it. You'll see these traces even if you make the ink disappearing. Just use the old technique of coloring the surface with a soft pencil.

PhillipMay 19, 2009 8:01 AM

Maybe they knew about this 300 years ago and it'll be featured in the next "National Treasure" movie?

Kiaser ZohsayMay 19, 2009 8:02 AM

This sound like the same technology used in my kids Hot Wheels Color Shifters. Put the blue van in hot tap water and it turns white, then back to ice water and it turns blue again.


SeanNMay 19, 2009 8:03 AM


Most inkjet printers heat the ink to a very high heat to ensure the ink dries quickly and is less likely to run (bleed). This may destroy the special properties of the ink.


JonathanMay 19, 2009 8:13 AM

Reminds me of how the Comintern used invisible ink in the 1920s. Their ink re-appeared with heat, and when communicating with agents in India, their letters were read in Peshawar with ease and interest. Nice idea as this pen is, I'd like to know just how cold it has to be before writing becomes visible again; most of us have cooler climates than Israel!

MFHPHMay 19, 2009 8:16 AM

@Cristian Bertoldi:

Seems they at least made an effort to avoid this exploit:

"The Mossad Pen utilizes the latest gel pen technology to provide smooth, quality writing performance. It's designed to require very little downward pressure on the paper when writing, so telltale indentations on the paper can be avoided."

Rodney RichardsonMay 19, 2009 9:11 AM

"Note: this ink has been engineered to work on white paper only. "

Does the ink turn transparent? Or merely white?

Refills?May 19, 2009 9:13 AM

With credit to Steven Wright: "how do you know when your invisible ink cartridge is empty?".

BillFMay 19, 2009 9:37 AM

Security by Obscurity is not a very good security mechanism. Although this is "neat", I wouldn't rely on it as one of my protection mechanisms for anything that is highly sensitive or classified.

JudyMay 19, 2009 9:53 AM

" amazing breakthrough in covert ink technology."
When I was a kid, we called it "lemon juice."
"Write" on a paper with lemon juice and let it dry. It was invisible unless you put too much on and the paper wrinkled. To read it, just hold a lighter under the paper and presto!bingo! Secret revealed.
High tech!

AnonymousMay 19, 2009 10:00 AM

Clear tool to secure hand written documents from being exposed. Is there a cartridge available for printers. As these days it is hardly handwritten.

vedaalMay 19, 2009 10:01 AM

as a complement to 'invisible ink' ;-)

i developed an 'invisible font' ;-)
if anyone is interested

it was primarily to embed invisble pgp messages in pdf documents, and can do so on any background

it is retrievable by selecting 'all' text,
and then copying and pasting into notepad or any editor

it 'could' achieve some measure of security if the security in pdf docments were 'real'

(pdf documents can be opened by the default ubuntu pdf reader,[as well as many others], no matter what security restrictions were selected when the pdf document was generated (Adobe Acrobat Writer)

this is because, by pdf standards, the key is included within the document,
and not well protected ;-(( )

ClausiusMay 19, 2009 10:04 AM

According to Kahn any wetting of paper is detectable - so no invisible ink is much good. (There are enough David Kahns on google to keep the federation in sequels for a long time.)

Davi OttenheimerMay 19, 2009 10:07 AM

"Even a law enforcement-level search is unlikely to divulge documents written with the Mossad Pen."

Unless they put all your documents in a freezer for a few minutes, which seems like a low-risk and fairly convenient test.

"The possibility of interception is remote, and can be decreased even further by writing in places that wouldn't even be looked at (for example, on the back of printed sheets of paper)."

That was the first place I thought of to look...printed sheets would surely be kept as evidence anyway so the chance of interception is higher. Writing on blank sheets in a feeder tray, or stored in boxes that are sealed to look unused might be smarter. Wonder if it writes well on toiletpaper. That probably has the lowest chance of interception.

Although for disposable writing the KGB pen looks like the better deal.

kyser soseMay 19, 2009 10:31 AM

@BillF "Security by Obscurity is not a very good security mechanism."

The greatest trick the Devil ever pulled was convincing the world he didn't exist.

dragonfrogMay 19, 2009 11:03 AM

I wonder if there's the potential for DNA-based computing here - an ink that is coded to turn visible only in response to a specific DNA sequence.

A nice touch would be if it turned visible only in response to your intended recipient's saliva - they could turn it visible by licking the back of the letter, but no one else can...

periMay 19, 2009 11:14 AM

If parts of a computer get hot enough to make the ink disappear then it would make a great pen for covert password lists. Just write you password on the glass of a framed photo or art piece, place on the computer until the ink disappears, move the frame somewhere else and finally bring out frozen peas when you need a reminder.

Tangerine BlueMay 19, 2009 11:20 AM

> The greatest trick the Devil ever pulled
> was convincing the world he didn't exist.

I don't think we'd call that "security by obscurity," because his alleged handiwork is anything but obscure.

Mark RMay 19, 2009 11:23 AM

@vedaal - (sorry, a bit OT)

This is true if you're setting a password to "restrict editing and printing", but not requiring one to open the document. If you set a password to open the document, there is no easy way to retrieve the key; you have to brute-force it or break the crypto (AES in newer versions).

But I suppose this is irrelevant to the scenario you're talking about (hiding info in plain sight with invisible fonts).

AnonymousMay 19, 2009 11:51 AM

I had a old mini-pci card that would not work correctly, but after a month in the freezer, it worked, and still does. System had been compromised.

Would be neat if some electronics had some dual state switch, that requires a cycle of cold.

Sure saves some nic cards that are hard to find. Although I do not trust the card anymore, it is interesting.

nomellamesMay 19, 2009 1:02 PM

This is not new, and you can get it cheaper here

frixion pens use the same technology, they just mark it differently. You "erase" the ink by friction heat. I discovered that you could retrive the erased stuff but, yeah, freezing the paper ;) and erase using a lighter. Cool trick for getting chicks....

bobMay 19, 2009 1:38 PM

Despite never having been a Boy Scout per se( "be prepared") I have carried a pen a/o pencil (as well as a "7-level" screwdriver, thereby dating me as older than the IBM PS/2 and its modular case) with me pretty much all my life. Consequently by the time someone hands me a credit card receipt I usually have my pen out and ready to sign (which I suspect decreases my exposure to other peoples germs by 2% or so as well; but thats not important right now).

I am always surprised when people let me do this as I could easily have some sort of disappearing ink in the pen. And I doubt the people who collect them would notice or realize until 2-3 months later when I would (hypothetically) contest the bill, long after they would probably have reduced it to some form of facsimile and destroyed the original piece of paper. Of course the last time I contested a bill that I know for a fact had no signature anywhere the credit card company said they did not care, I would have to pay anyway, so it would probably not matter.

AnonymousMay 19, 2009 1:44 PM

Surely ownership of a SHOMER-TEC pen would be a red flag. Call me when the pens are indistinguishable from your average bic pen.

JohnMay 19, 2009 4:46 PM

Invisible vanishing ink. I use a fountain pen and Noodler's Bullet-proof Black.

Even if you used this in a fountain pen (no pressure, no etching), the ink would slightly discolor the surface. Hold the paper at an angle and you'd see a water mark; the fibers shift, and anything not evaporated out (i.e. not water) would surface bind and look shiny (it'd be smoother than paper face). On a perfectly smooth surface, it'd create a surface imperfection (raised ridge) that can be seen by naked eye.

Try again.

TSMay 19, 2009 9:41 PM

Of course, if there's a blank page in an envelope, it would be a little suspicious. On the other hand, if you've already established some communication between the two parties, and send an innocuous printout of a web page, and use the pen on the *back* of one of the pages, then it would be much less likely to be noticed.

Of course, there's no point in relying on obscurity alone, might as well encrypt the message.

RonKMay 20, 2009 1:38 AM

Uh, Bruce, maybe you could help some of us who are more sarcasm-detection-challenged when you post?

Right now I'm not quite sure if some restaurateur didn't drug your food in an attempt to get a better review, and it didn't wear off before you got back home to the computer...

2Easy2Worry2MUCHMay 20, 2009 10:35 AM

@RonK, lighten up!

Bruce said "This is cool..." He didn't say it was secure or smart or a big deal ;-)

Maybe he was simply referring to the fact that a freezer is used.

The thing is certainly worthy of a passing mention on a security blog though.

periMay 20, 2009 3:33 PM

I bought a 3 pack of Pilot Frixion pens for $6 yesterday.

Eraser hides it with some heavy rubbing.
Light bulb hides it.
Not with PC PSU exhaust.
Not with water heater outlet pipe.

Freezer: very quickly.
Fridge: slowly.

Erased ink comes back faded.
Ink doesn't jump out under blacklight.
Ink remains visible on glass.
Hidden ink is, at arms length, somewhat Visible via gloss and marred paper.
Pen is serviceable but not great.

The interesting behavior of erased ink made me wonder whether Frixion is a modified version of the formula. If so, the Frixion eraser probably will erase Shomer's ink. If not, it might be better at hiding writing and thus justify being 18 times more expensive. I would be grateful if anyone who has both Shomer and Frixion pens to tell me whether they use identical formulas.

DustinJune 15, 2009 11:45 AM

There is a sight in China where an individual sells inks that are UV/Magnetic/IR reactive and he also sells ink SPECIFICALLY FOR inkjet style printers. I haven't used or know anyone personally who has used these so I can neither condone nor reject his claims (which are in bad Engrish anyways)

If anybody does try this I would be interested to know the results.

ZvicaJune 16, 2009 10:29 AM

Writing with urine will generally be invisable. To subsequently make the writing appear, iron the sheet with a pressing iron. Kid's stuff.

TomAugust 30, 2009 10:56 PM

I am amazed that so many people have posted comments about the pen. How did you arrive at Bruce's site? Did you Google "invisible ink" as I did or are you frequent visitors? Has anyone stopped to wonder why Bruce chose to describe this outdated and unsecure technology as cool? Perhaps he was hoping that someone would draw a parallel between older methods of communicating secretly and newer ones? Bruce is a smart man; I admire his insights into security issues and I know he would see an outdated security system for what it is...

randomApril 13, 2010 1:43 PM

hey um is this mossad pen better then the frixion for disapearing or no can u see any of the two pens more then the other in "disapeared" state or are they both gone completely and witch requiers more down force in writing? ty

angJanuary 20, 2011 6:08 PM

this is kinda on a different page but is there anyway to retrieve stuff regular pencil writing that has been erased

Clive RobinsonJanuary 21, 2011 12:19 AM

@ ang,

"but is there anyway to retrieve stuff regular pencil writing that has been erased"

The simple answer is any and all writing irrespective of the choice of stylus can be detected by the damage it does to the fibers of the paper both by dragging and indentation (some tests such as ESDA can reveal the difference between the two typpes of damage) irrespective of the "ink" or "lead" used by the stylus so shows all secret writing.

Sometimes all that is required to see it "is to tip the paper through the light" that is a light source at an angle nearly incident to the paper will cast a shadow into the impression left by the stylus.

Another technique that has been used is iodine vapour it shows the different absorbancy of the fibers which show up either the stylus damage causing increased absorbtion, or any residual ink that changes the level of absorbtion by any fiber with respect to the other fibers in the paper.

Then there are more modern tests using electrostatics and fine carbon or "toner" dust etc

It is important to remember that one of the priciples of forensics is to "not damage evidence" so not all possible tests are used in forensics.

One highly sensitive test that is was developed by the South West London forensics laboratory of the UK Home Office back in the mid 70's known as the ESDA test, you can read up on it,

Like a similar test for latent finger prints using super glue vapour it is something you can "knock up in your garden shed" at home BUT it is dangerous.

In the case of the ESD test with the bits from a photo copier (which is what the original prototype used) or laser printer and "shrink wrap" / "cling film". However it is dangerous the corona wire power supply can kill you if you don't take care, and most "toner" powder will cause cancer if breathed in....

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