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 messsage 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.
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.
- 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.
- In Akagami no Shirayukihime 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 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).
- In Naruto, the Uchiha tablet on first glance gives the basic informations regarding secrets of the Uchihas. But use your Mangekyou Sharingan or Rinnegan and you 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.
- 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 Speed Racer Pops has put the blueprints to the Mach 5 in invisible ink on the windshield of the car.
- 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".
- 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.
- National Treasure: A secret message was written on the Declaration of Independence with invisible ink that was revealed with heat. Further clues were embedded in the Declaration of Independence, visible only through a pair of glasses with switchable chromatic filters.
- 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.
- Constantine: 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.
- 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.
- In Pacific Rim, Newton tracks down Chau's kaiju-parts dealer by spotting hidden symbols on signposts with a UV penlight.
- 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 whichever building the holder is in, 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 Literature/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.
- Parodied in the book 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.
- Also, the Unseen University faculty includes the Reader in Invisible Writings.
- 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.)
- 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.
- One episode has the names of Starling City's most corrupt criminals written in a handbook owned by Oliver's father. He only realized it wasn't blank when he put it near a flame and the names came up. The ink can also be revealed by way of a special pair of glasses.
- The team finds some notes written the same way in Sara's boot 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 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 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.
- 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 second season of Gravity Falls, Dipper discovers that the Journals have additional writing in invisible ink made visible through a black light.
- 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.
- 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.
- 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.