by Tom Adair
Invisible inks have been around for centuries; used by good and bad forces alike. Spies use them to report home and prisoners use them to pass messages to the outside. All along, CSI (types) have endeavored to uncover these secret writings so that the truth will out. Secret messages have great value to the writer and the reader. Sometimes even more to the unintended reader. Such was the case with British spy Benjamin Thompson in 1775 Boston and such is the case today with other unmentionables. I bring this up because using invisible inks might come in handy for your protagonist or the bad guys they are chasing. But in order to use invisible inks you have to know a little about them.
For our purposes let's assume that the best "inks" are the ones readily available and don't leave significant staining or odor once dried. These are most commonly organic in nature. Some common types include diluted urine, vinegar, sugar water, lemon juice, and milk. You're probably conjuring up images at this moment of how a character might use these inks. Maybe they are in a public bathroom with a killer outside. Maybe they are in a restaurant staring at the man that wants to abduct them. You may even imagine a prisoner sending a letter out through his attorney or mother.
The clever writer chooses a location that enhances the "invisibility" of the message. It might be on the back or in the margins of a letter written in colored ink. It may be on the inside of an envelope. It could even be under several layers of rolled toilet paper. CSIs learn to search for these messages without actually "seeing" them. There are two basic types of methods used to detect simple organic invisible inks. The application of heat or the use of a chemical reagent. The application of heat is one of the oldest methods. Heat will generally darken the writing making it visible. Some inks like urine can also be seen with an ultraviolet light without applying heat. To apply heat the most common methods are the use of an oven (low setting) or steam iron (without the steam). Documents can even be left in the sunlight but the development process may take hours or days depending on the temperature.
The use of chemical reagents obviously is a bit more involved in that obtaining the reagents requires more effort. Lemon juice can be developed with iodine and very diluted blood can be developed with Phenolphthalein or Ninhydrin. Some of the chemical reagents are hazardous and may require special handling procedures which is why CSIs typically use them more than others. Acquiring these reagents often leaves a "trail" that police or others can follow.
So if your character needs to pass a secret message I advise following the K.I.S.S. model. Keep it simple and use materials and methods that any third grader has access to. Don't be afraid to get creative in the hiding place either. Sometimes the most effective location is in plain sight. There may even be a "system" for delivering messages such as the third napkin in the dispenser on a given day. Some of that depends on the sophistication and circumstances of your character but as long as it makes sense to them the reader will follow along.