Invisible Writing

"It wouldn't do me any good to sign your petition, Mr. Pfister. You see, I'm a spy and my fountain pen is filled with invisible ink."

— Maxwell Smart, Get Smart

Describe Invisible Writing here.

Something that looks innocuous and featureless actually has some writing on it. You aren't able to see it with the naked eye; you'll need some way to reveal it.

A simple example is writing a message with citrus juice. Once it dries, the print becomes invisible. One must warm the paper to reveal the words. Other times, you will need specialized equipment such as a fluoroscope or a chromatic filter to view the writing. In fantasy worlds, magic or Green Rocks are needed to make the writing visible. Possibly, the writing can only be read by a specific person, or at a specific time frame or location. Otherwise, the writing is invisible.

As with any secret message, this is done so the message can't be read by just anyone, and can easily be overlooked during a search.

May lead to Fridge Logic: If one uses an "invisible ink" to write, wouldn't the writer have a hard time writing it? (It may be handwaved by turning the text invisible after it was written, in some way.)

Truth in Television: This was a valid secret communication technique right until the 20th century, when UV lamps (a/k/a "Wood's Light", "black light") rendered it obsolete.

A frequent Spy Fiction trope. Condensation Clue and Writing Indentation Clue are subtropes. See also See the Invisible on ways to make things visible. For examples where invisible ink gives something else invisibility, see Invisibility Ink.

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Examples:

Advertising

• A few breakfast cereals during The '70s would print trite messages on the boxes, disguised as random dopplings of color. A piece of tinted cellulose, usually red, was used to screen out the "noise" colors, leaving the remainder to form legible print.

Anime & Manga

• In Gugure Kokkuri San, Kokkuri-san is transformed into a woman by a cursed box whose sole contents is a piece of paper reading "I told you not to open it," and immediately burns the paper. Kohina reveals a little later that words would appear on the paper if it's held up to the light (probably some form of watermark).
• A major subplot in Lone Wolf and Cub details how the Yagyu clan has been using invisible writing as part of a plot to usurp control over the Shogunate. Itto discovering how to reveal the invisible writing and thereby threatening to reveal the Yagyu's grip on the Shogunate is treated by Retsudo as a far bigger problem than the fact that Itto is a One-Man Army out to slaughter the Yagyu clan down to the last.
• In Naruto, the Uchiha tablet on first glance gives the basic informations regarding secrets of the Uchihas. But someone using their Mangekyou Sharingan or Rinnegan will see even more details regarding the history of the ninja world. Such as the history of Senju and Uchiha war and the origin of the Ten-tails.
• Subverted in One Piece: The Movie: Nami thinks there's invisible ink on Woonan's treasure map, and tries to heat it up with a match to reveal it. She just ends up burning the map to ashes.
• In Ouran High School Host Club Tamaki's father sends him messages in invisible ink (made with lemon juice). The club believes that the messages come from the person sending them threatening letters and hilarity ensues.
• Sgt. Frog has a chapter where he receives a message from his superiors that had to be revealed by rubbing an eraser on it. In the anime, the message also gets partially revealed due to entry through the atmosphere.
• In Snow White with the Red Hair Shirayuki and Ryuu are searching for an old book that describes a rare quickly spreading illness that has started appearing in Lyrias. When they finally track it down most of the writing has faded to the point of being illegible so they work with a botanist and another researcher to come up with a way to get the writing to show again.
• In Speed Racer Pops has put the blueprints to the Mach 5 in invisible ink on the windshield of the car.

Comic Books

• Wonder Woman Vol 1: Zara uses invisible ink to write on her victims without their knowledge which is then activated later in dramatic fashion to better convince them that her cult is legitimate.

Films — Animation

• Cale Tucker, The Hero of Don Bluth's Titan A.E., receives a ring from Captain Korso. The ring has been coded to Cale's genetics so that it reveals a star map on Cale's hand. As a further safeguard, the map's pointer shifts direction while Cale is standing on a platform on Sessarim.
• Curious George (2006): While stowing away on a boat, George and Ted notice that the small idol George took earlier has a map when shown by sunlight. They use the map to find the real idol.

Films — Live-Action

• In Arabesque, Pollock comes across this at one point, and lights a match to expose it, which Yasmin is puzzled by:
  Pollock: Weren't you ever in the Boy Scouts?
  Yasmin: I flunked the physical.
  Pollock: Should've tried my troop. We'd have waived it.
• One Charlie Chan mystery Charlie Chan in Shanghai has some writing on a piece of linen. The glyphs appear to be Chinese, but Chan declares them as gibberish. He then points out that, despite plenty of writing paper available, the marks were made on cloth. Chan rinses the linen in a bowl of water, which washes away some of the ink, but leaves the true message in broken Roman letters intact.
• Constantine (2005): Angela and her sister Isabela left hidden messages on windows and mirrors when they were kids, visible only when fogged by their breath. Isabela left her sister one last message before she was Driven to Suicide.
• Gone in 60 Seconds (2000): Memphis (Nicolas Cage) and his crew use this to hide a list of target cars, from Detective Castlebeck. The writing is only visible under "black light".
• Knives Out: Harlen writes messages to his adult daughter Linda in invisible ink, simply because it amuses both of them. Including the letter where he tells her that her husband is cheating on her, which said husband had dismissed as a bluff after seeing a blank sheet.
• National Treasure: The film's treasure map is the Declaration of Independence, its clues written in invisible ink. It has two messages: one revealed with heat and acid that leads to a special pair of glasses with switchable chromatic filters; and one revealed with said glasses that leads to the treasure room beneath Trinity Church.
• In Now You See Me, the Horsemen's New Orleans show seems to use some form of Invisible Ink which shows up under a flashlight.
• In Pacific Rim, Newton tracks down Chau's kaiju-parts dealer by spotting hidden symbols on signposts with a UV penlight.
• Rendezvous: The German spy ring is in the habit of using this, like when they send Olivia a message in invisible ink written on a seemingly innocuous advertisement. William and his counterintelligence team can sometimes tell that there is an invisible ink message, but still not be able to read it since they don't have the reagent to make it show.
• Who Framed Roger Rabbit: Mr. Acme was the inventor of the 'disappearing-reappearing ink' which would become invisible almost immediately, but become visible again through time. Mr. Acme also wrote his will with it, leading it to be believed lost.

Literature

• Two short stories by Agatha Christie (one featuring Hercule Poirot and one Miss Marple), involve rich eccentrics writing their will in Invisible Ink, and challenging the heirs to find it.
• Encyclopedia Brown had a story which used lemon juice as the ink of choice.
• In Edgar Allan Poe's "The Gold Bug", the whole plot is triggered by a piece of parchment being accidentally held near a fire long enough to bring out a message in invisible ink.
• In Tolkien's Legendarium, there are several kinds of magical invisible writing. First is called "moon letters" and is written with silver pens and unknown ink that is only visible in moonlight. In The Hobbit, Thorin's map to Erebor has such invisible script. Second (not quite an ink but similar) is ithildin, a metal alloy that only reflects starlight and moonlight when certain magical spells are recited, and is normally transparent, used to decorate the Gates of Moria which blend seamlessly with rock. Revealing ithildin is the only way to see these gates.
  • Bilbo Baggins's magic ring seemed ordinary enough, with nothing apparent on its inner surface. However, once Gandalf the Grey threw that ring into the hearth fire and later retrieved it, it's fateful inscription appeared: "One Ring to rule them all ..."
• Harry Potter:
  • In Harry Potter and the Chamber of Secrets, the team initially thinks that Tom Riddle's diary was written with invisible ink, with Hermione trying spells and a magicked eraser called a "revealer" to try and make something visible. Subverted: The diary contains Tom Riddle's memories in a very literal sense, and is capable of responding to written messages.
  • In Harry Potter and the Prisoner of Azkaban, Fred and George introduced the Marauder's Map, which doesn't just show the map of Hogwarts, but also shows the footsteps of people. It looks like just an empty sheet of old paper... unless you know the chant for activating it.
• James Bond in On Her Majesty's Secret Service uses his urine ("the oldest [secret ink] in the world") to write down the names of the allergy treatment patients in Piz Gloria, before he tries to make his escape from the place.
• Briefly appears in one of the Deltora Quest books, Isle of the Dead. Tom the shopkeeper sends Lief, Barda and Jasmine a gift of firebeads and sweets. Since Tom is notorious for his No Hero Discount attitude they are wondering why he did so, when they notice that the wrapping they had left by the fire they just made now has writing on it. The wrapping is a note from Tom advising them on the next stage of their quest... with a postscript asking them, when convenient, to pay for the items he has sent.
• In In the King's Service, Alyce de Corwyn gets a letter from her late father that appears to have a banal message about a bequest of family jewels, yet he left a second message revealing that Vera Howard is really Alyce's twin sister that only becomes visible when Alyce activates her father's spell by kissing the wax seal.
• Mortally wounded curator Jacques Sauniere leaves an important clue scrawled on the Louvre's parquet floor, that can only be seen under ultraviolet light, in The Da Vinci Code. "O Draconian Devil / Oh Lame Saint." It was made not to implicate his killer, but rather to indicate his killer's objective.
• One of the Greyfriars stories had a main character receive a letter from a missing relative, asking whether he remembered a particular party trick. He doesn't, but the letter is left out in the hot sun for a while, which reveals the real message saying that the relative has been kidnapped.
• In Aunt Dimity and the Lost Prince, young neighbour Bree Pym shows Lori's young twins how to write invisibly with lemon juice and reveal the messages with the heat of a lightbulb. It's part of her unexpected arsenal of talents for entertaining kids, a bag of tricks that includes juggling, acrobatics, yoga and sleight-of-hand magic.
• The Count of Monte Cristo: The key to the colossal fortune was found in an old piece of paper used as a bookmark. When Faria tried to use it to light a candle, he noticed words forming on it and quickly extinguished it, but was arrested before he could find it.
• Discworld:
  • Parodied in Witches Abroad. The witches come upon a dwarf mine, identified by invisible runes. Magrat states she can't see them, which Nanny Ogg points out is how you know you got your money's worth with invisible runes.
  • The Unseen University faculty includes the Reader in Invisible Writings, although this is more to do with using L-Space theory to find books as yet unwritten.
  • In A Hat Full of Sky, it turns out all Discworld medicinal herbs have their use, side-effects and dosage written clearly on them. Yoiu just have to figure out what combination of coloured lights and lenses allows you to see it.
• The bracelet given to apprentice white witch Susan, in Alan Garner's fantasy novel The Moon of Gomrath, is apparently of plain silver with nondescript black inlay. But in times of great need, it manifests readable writing, which when spoken by her becomes a spell of great potency and confounds the creations of evil.
• Nick Velvet: In "The Theft of the Used Teabag", Nick's client is certain the teabags are being used to pass something along. After cutting the bag open, searching it for contraband and finding nothing, Nick points out the pasteboard tag on the bag could be used to pass on a message. There is the name of a meeting place written in invisible ink that is revealed by dunking the tag in hot tea.
• The Thinking Machine: In "The Broken Bracelet", Van Dusen is asked to uncover the secret of a blank scrap of paper found inside the eponymous bracelet. The secret is a cryptic set of directions written in invisible ink. When Hatch questions the feasibility of doing this in prison, Van Dusen points out lemon juice or milk would serve the purpose. (Presumably delicacy prevented him from mentioning the possibility of urine.)
• The Mermaid Chronicles: In Quest for Atlantis, the protagonists find a map that looks like a blank piece of paper. Maya realizes that it's actually a form of invisible writing that can be revealed using the ink of a particular cephalopod. Unfortunately, that species went extinct long ago, but there's still enough fossilized ink available to reveal the whole map.
• Wings of Fire: Darkstalker outsources all of his magical powers onto a scroll that lets him write whatever he wants to use his powers to do. He writes spells that do kind favors to other dragons in normal ink, but when he wants to write a spell he knows his friends will reproach him for for its cruelty (like killing an IceWing dragon every month), he writes it in invisible ink.

Live-Action TV

• 3-2-1 Contact's "Bloodhound Gang" has a story called "The Case of the Secret Message" in which they find an old lady's stolen purse and finds that it only has a seemingly blank piece of paper. The Gang suspects invisible ink, but have to go through various methods to find out like heating it to expose lemon juice writing, which doesn't work. Eventually, the way to expose the writing is it to rub pencil graphite on the paper to expose the secret writing that was written in salt water.
• Arrow:
  • The List is a handbook owned by Oliver's father containing the names of Starling City's most corrupt businessmen and criminals. Oliver only realized it wasn't blank when he was about to burn it as kindling while marooned on the island and the names appeared from the heat. The ink can also be revealed by way of a special pair of glasses.
  • The team finds some notes written the same way in "The Magician" episode. Nyssa calls it "Ghost Ink" and says it is one of the standard methods that the League of Assassins employ for concealing secrets.
  • The MacGuffin in "The Thanatos Guild" is a map that is printed with bacteria that is only revealed when exposed to the enzymes of a particular person's DNA.
• Blindspot: Jane Doe's body is already covered in tattoos that contain hidden meanings but in the "Bone May Rot" episode, it is discovered that she also has tattoos that are only appear under the exact frequency of UV light that used for decontamination at the CDC.
• In the Bones Season 7 finale a man in a mental institution leaves a secret message written on the walls in his own saliva. It works like invisible ink; it only shows up under special light.
  • On the Back Door Pilot for The Finder on Bones, the treasure map has writing that can only be revealed with blood.
• Criminal Minds: One episode features an UnSub covered in tattoos. During the autopsy (he'd comitted suicide at the start of the episode), Redi discovers that the UnSub had a second set of tattoos done in invisible ink that only appear under UV light.
• CSI: NY:
  • The victim in the B-case of "Jamalot" is found at autopsy to have words written all over his body in invisible ink. Later, more words are found on the walls of the room in which he was murdered, also in invisible ink. Both are discovered with the use of an ALS, or Alternate Light Source.
  • In "Child's Play," a joke shop owner signs a contract in disappearing ink. When the other party is questioned in a murder case both men are connected to, he produces the (apparently un-signed) contract as evidence. Stella uses a hand held ultra-violet light to reveal the jokester's signature.
• Doctor Who: In "Rosa", the Doctor has a marker with special ink that can be sonicked to make it invisible or visible as required, making it perfect for brainstorming on the walls of a motel without technically vandalizing the place.
• Elementary: When Sherlock and Joan retrieve the Body of the Week of the "Art in the Blood" episode for an autopsy, they are more than a little surprised to find that his apparently ordinary arms had been stolen and nothing else. After some investigation they find out that his ex-wife is a tattoo artist and he had her cover his arms with tattoos that only show up under UV light.
  • In Solve For X, Sherlock detects an unusual smell in one room of a victim's home, notes the blacklight bulb in a lamp in that room, and turns it on to reveal the walls are covered in formulae and notes written in UV ink. When asked what it is, he replies "I'm going to take a very literal stab in the dark here and say, 'Maths'..." It's quickly identified by his Maths expert as being an attempt to solve P versus NP, a notoriously difficult and still unsolved problem in computer science.
• An episode of Hetty Feather has a new Foundling (Eliza) arrive at the hospital and it turns out she stayed with the same foster family as Hetty. Hetty learns from Eliza that her foster brother mentioned the secret squirrel house they used to play at and that her foster brother did not mention Hetty to Eliza at all, leading Hetty to think that he has forgotten about her. When Hetty's friends learn of this, they have Eliza write a letter home, with the line "I hope you are reading this by your lovely warm fire" and on Eliza's letter, they write a message using lemon juice, which won't show up on the paper unless it is held up to a fire, to tell the foster brother what has happened to Hetty.
• In House of Anubis, Sibuna find an old dictionary but when they open it, nothing is written in it. Later it turns out the text was in invisible ink, and the heat from the flashlight made the words appear.
• Monk: In "Mr. Monk Goes to the Asylum," his physician starts gaslighting him when Monk begins drawing too close to exposing a murder several years ago. On the outside, Sharona finds evidence to vindicate him and sneaks a message to him by writing it on a picture of Trudy, using ink that's invisible but glows in the dark.
• In one episode of The Office (US), Dwight attempts to pass a secret message to the other salespeople by handing them letters with a second message written beneath the main text in invisible ink, hinting to them to expose the letters to heat to make the secret message visible. Of course, none of them bothers to do so. Dwight also cheerfully admits that the "invisible ink" in question is in fact his own urine.
• Person of Interest. Finch goes to investigate a lockup rented by a terrorist group. It appears to be empty, but when an infra-red light is shined on the wall, a message has been written there.
• In Raumschiff GameStar, the employee contract clause that forces all crew members (except The Captain and the medical officer) to wear bright orange overalls is printed in ink that is only visible if you hold the paper against the light of a full moon on a certain date and day of the week.
• One of the things Abraham Woodhull does as part of his spy work for George Washington in Turn is transmit messages via invisible ink that requires the application of a coat of "reagent" for the message to be visible.

Tabletop Games

• Dungeons & Dragons
  • The Book of Marvelous Magic has Inkwell of Concealment, which is an Everfull Inkwell, but after a hour everything written with its ink vanishes and can be read only via effects allowing to see invisible objects.
  • Tolkien's version above was thrown in when sourcebook Player's Options: Spells & Magic introduced 'Moon Rune' spell that displays letters when its triggering condition is met, including moonlight in the proper phase or presence of creatures (set by species).
  • The spell Arcane Mark can functionally be an invisible ink. The campaign setting Eberron notices this, and uses it on documents to increase the difficulty of forging.
• Forgotten Realms lore has Thayan "cobrascale ink", normally invisible and bright when heated.

Theatre

• Spider's Web: An apparently innocent paper found in a secret compartment in an antique desk turns out to have the address list of a drug ring written on it in invisible ink.

Theme Parks

• At Universal Studios' The Wizarding World of Harry Potter, the Scribbulus store has a scroll with invisible ink on it, which can only be revealed via a spell.

Video Games

• Alone in the Dark 3: At one point, while exploring the town hall, you find an apparently blank book. Another tips you of its true nature and what to do with it: reading in while standing near a light candle will reveal it's actually a book written by Pregtz, the Big Bad from the first game.
• In the Assassin's Creed series, many instances exist of characters using Eagle Vision in order to see hidden messages.
• The dilapidated studio of Bendy and the Ink Machine has messages all over its walls. Some of them can't be seen until Henry gets a special tool from Allison.
• In Dark Fall The Journal, the picture of a pigeon can be heated over the gas stove upstairs, causing the names of four Lyrics to appear.
• In Final Fantasy VII, there's an invisible choice that can be highlighted in the notes of Shinra Mansion, detailing of giving clues on how to unlock the secure safe.
• In Final Fantasy VIII, when you slowly make your way towards the blue pillar holding Bahamut, who tells you to turn back... If one manages to successfully make it to said glowing pillar, you have to fight two Red Dragons and give the proper answers after defeating each one before he/she angrily asks why they're fighting against him/her. The true answer to the third and final question is completely invisible, the resultant answer shown is truly soul-chilling to first-timers, to say the least...
  Damned imbeciles. Why do you wish to fight?
  For the sake of protecting something
  None of your business
  (It's our nature...)
  (There is no real reason...)
  (Maybe we were born... only to fight.)
  Beat
  I see... interesting...

• In the Nancy Drew game The Haunted Carousal; she helps out the bookkeeper with some puzzles that would reconnect her childhood (long story). One puzzle involved a piece of lemon scented paper that showed a message when ironed.
• In Przygody Reksia, Reksio finds a bottle with a white piece of paper after being stranded on an island. It turns out that it's a map of the Treasure Island that leads to a treasure, but its content is written with sympathetic ink. Reksio has to use a fire to make the content visible.
• Resident Evil (the original, and its remake) has a puzzle where placing a wooden board above a fireplace reveals a map of part of the mansion.
• In the video game The Riddle Of The Sphinx, holding a sheet of paper over the candle in the tent lets you see a secret message in it.
• Uncharted 2: Among Thieves has several maps, inscriptions, etc. showing the way to the legendary Cintamani Stone that are only visible in the blue light created by burning a special resin. The resin is the congealed sap of a tree that grants great physical prowess and immortality, the true form of the Stone.

Webcomics

• In Girl Genius, the librarians at the Incorruptable Library shine a UV light at Van Rijn's notebook (using the same wavelength as the Storm King's lightning) to reveal an entirely new work that Agatha had missed completely. He apparently used multiple methods to conceal his work including "phasing ink, momentum writing, four-dimensional cryptograms, and lemon juice". Agatha later gets a bit over-enthusiastic about finding the hidden writings, and discovers that if you study the vellum (calfskin) pages really closely, they contain instructions for building a cow.

Western Animation

• My Little Pony: Friendship Is Magic. As a practical joke, Pinkie Pie and Rainbow Dash replaces Twilight Sparkle's ink with invisible ink. Twilight eventually did the same thing to Princess Celestia.
• In Avatar: The Last Airbender episode "The Avatar and The Fire Lord", Zuko received a note with a secret message written in invisible ink which was revealed with heat.
• In the Gravity Falls episode "Scary-oke", Dipper panickedly flips through Journal 3 while standing near a UV light from the party that was being held, and discovers that the Journals have additional writing in invisible ink made visible through a black light. Throughout the remainder of the series, these invisible ink messages are shown to include some cipher sequences used to encode cryptograms found throughout the rest of the journal, a correction to an entry about giant vampire bats clarifying that they are actually fruit bats, the location of a hidden bunker and a guide for getting past its security system, information on how to defeat certain monsters, and several warnings.
• In an early episode of Rocky and Bullwinkle, Natasha reveals a message that was in invisible ink by holding the paper in front of a candle: "Keel Moose." Boris drops a safe to land on Bullwinkle's head. After the Commercial Break Cliffhanger Natasha says that two more words have appeared: "Do Not." Boris races the safe to save Bullwinkle.
• This was also used in the Garfield and Friends episode "The Legend of Long Jon". Garfield and Odie were reading from the journal of Jon's pirate ancestor Long Jon Arbuckle, when Odie started drooling on an apparently blank page, which causes writing to appear on the page. It turned out that Long Jon Arbuckle had written on the blank pages in invisible ink, telling about how he was able arrest some bandits that were carrying phony documents.
• Similar to the Gone in 60 Seconds (2000) example above, an episode of Fillmore! involving a bike thief ring that used the same 'special chalk' trick, albeit replacing blacklight exposure with special lenses. This ends up helping to catch the secret leader of thief ring, who had gotten some of said chalk on their shirt.

Real Life

• One poor unfortunate decided to rob a bank and his "disguise" was lemon juice on his face, thinking that, per its use as invisible ink, it would keep the security cameras from recording him. He was wrong.
• The idea of using citrus oils as in National Treasure was a common means of sending covert messages through the end of the Cold War. An ordinary incandescent light bulb is usually enough to reveal such messages.
• A really Squicky example: Fresh semen may be used as an invisible ink much as lemon juice may be applied. Apparently, all men can access a stock even if in a place where no lemons are available.
  • Saliva does the job, too, so you don't have to be male to improvise Invisible Writing.
  • In fact, any organic liquid works, even soda ("organic" here meaning organic chemistry, not pricey fancyness).
• Also, if you're not above the pink color, phenolphtalein solution can be used to write messages. Then you make it visible with an alkaline solution. It will disappear again after time, because the solution will absorb carbon dioxide from the air and neutralise itself.
• OR use ordinary table salt and make it visible with silver nitrate solution. This needs time to develop and doesn't disappear. It does turn the entire sheet black after some time, though.

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