People have always wanted to communicate privately. There once was a time when regular ink was secret enough, because not much of the world knew how to read. Then literacy ruined it for everyone, and drastic steps had to be taken. Enter invisible ink, the way to keep what was written secret, sometimes by unsavory means.
Here's how invisible ink was invented, and all the forms it has taken right up into the present day.
Photo of invisible ink letter via Letters of Note
The first invisible ink in recorded history was made by Pliny the Elder. He was a Roman general, author, and scientist who was perhaps most famous for saying 'Fortune favors the brave' directly before proving himself wrong by sailing to Mount Vesuvius as it was erupting and subsequently dying of smoke inhalation. His more cautious endeavors proved more fruitful. He let people know that the milk from the thithymallus plant could be used to make marks on paper which were invisible to the eye, but which could subsequently be treated to show legible inscriptions. The fact that he was a military man in his time was not a coincidence. Most invisible ink advances seem to have come during times of war, when secrecy was most crucial to all sides.
There are generally three kinds of invisible ink: those developed by heat, those developed by light, and those developed by chemical reactions. Surprisingly, the ones developed by chemical reaction were among the first to be used to a widespread degree.
The American Revolution
Chemically developed invisible inks turn colors in the presence of certain chemicals. Many are now little more than the pH labs we did as kids. Vinegar turns colors when exposed to red cabbage water - and is tough to sneak by anyone with a nose. Ammonia has the same problem.
The ink that everyone was taught to use as a kid is lemon juice, which turns paper light blue when sprayed with iodine. (It also turns paper brown when exposed to heat, which is why it's so commonly used by kids and so rarely used by actual spies. Too many indicators.) Simple salt is developed by silver nitrate.
During the American Revolution, both sides used chemically developed invisible ink, but they needed it to be bit more advanced than something that would show up as soon as someone waved a cabbage at it. Spies on both sides used iron sulfate dissolved in water to pass secret messages. This would only show up when the paper was brushed with bicarbonate of soda - baking soda - so to decipher these messages the other side would at least need baked goods.
Using Whatever's At Hand
As chemical testing agents became more abundant, and the circumstances of the letter-writers became more constrained, heat developed inks were used. The 'ink' subtly damaged the paper, causing the inked parts to turn dark first when the paper was heated. If the entire paper were slightly moistened, then minor heating wouldn't show the marks and it might slip through suspicious hands to its intended target.
Documents that have only recently been declassified reveal the secret recipe to "German Invisible Ink," which was simply an ounce of alum - a kind of salt - and an ounce of white garlic juice, stirred together and applied carefully to paper. Later ironing would bring the message to light. The name of the game was hiding things, not well, but in plain sight. Starched handkerchiefs could be moistened and the starchy solution used as ink. Bodily fluids, like saliva or urine, could be used as well. One young MI6 agent patriotically reported that the best bodily fluid to use as invisible ink was semen, since when he tested it, it did not become visible when sprayed with iodine, the way some other fluids did. It was also, in his words, readily available. (Well. To some.) Although the discovery was treated as very important by the head of the division, the discoverer had to transfer from the department when people would not stop making jokes about him.
Food like honey, milk, sugar solutions, and clear colas were also developed by heat, and could be used by civilians. Even soapy water worked, in a pinch. Possibly the most valuable discovery of the time was disappearing and reappearing ink. Cobalt chloride is a blue solution that appears when heated, and then disappears again after a while. It could be passed around and heated as necessary.
The Second World War
By World War II, the Geneva Conventions, provisions which outlined humanitarian treatment of prisoners, guaranteed the right of prisoners of war to write home and limited the use of heat treatments as a means of detecting invisible inks. Since officials were prohibited from using heat, they turned back to chemistry.
Treating the letters after they had been written was prohibited, so the only way to keep the letters secure was to treat them before they were touched. The Allies developed two kinds of paper, Sensicoat and Anilith, meant to reduce the number of secret messages sent. Sensicoat was heavy and expensive, but it was coated with a powder that reacted to all liquids, and to acids, by turning bright green. Anilith was less expensive, and was not coated but still chemically treated to show acids and liquids. Prisoners could send public messages, but no private ones.
Axis powers responded by developing 'dry ink' and hiding packets of it in care packages. Upon discovery of the packets with another round of creative chemistry to develop new papers with sensitivity to fluid and dry inks. There would probably have been another few rounds, if World War II hadn't ended.
One of the primary current uses for invisible ink is private security. People keep a lot of valuable objects in their homes that may be difficult to pick out of a line-up or spot by a dealer on their own merits, but are too pretty to visibly mark under normal light. Ultraviolet light, though, which isn't generally used in homes, and so is a good work-around. Certain materials glow under ultraviolet light, and they are used to make light developed invisible inks.
Yet again, the go-to material is bodily fluids. Spit, semen, and blood serum all fluoresce under UV light. They might have been used extensively, except for the fact that most people don't like getting them on expensive objects. Soaps and detergents do the same, with less of an unpleasantness factor. Other invisible UV inks include quinine and vitamin B-12 when dissolved in vinegar. So for those out there who want to mark their laptops - there are plenty of options.