Concocting recipes for invisible ink is the stuff of scientists. Putting it to use is generally the work of spies and others in pursuit of nefarious deeds. It's the work of the sleuth to figure it all out, at least that's the way most good mystery novels work! But invisible ink has also had its place in history.
Artifacts from World War II turn up quite frequently, but a certain postcard turned up a few years ago which caught everyone's interest. The postcard had been sent out of Poland during the occupation. There wasn't anything of interest to the Germans on the postcard, or so they thought. Beneath a one sentence letter on the postcard was a message written in invisible ink! The message makes an urgent request for supplies, including invisible ink. Fragmented sentences read like a nightmare, describing a Nazi death camp with gas chambers. It told a story about a person we might not have even known about otherwise. Invisible ink had made this possible, but what is invisible ink? And how does it work?
There are actually quite a few things which will function as invisible ink. The most basic are liquids which are heat sensitive. Lemon and onion juice work great, milk or vinegar work ok, but not as well. The idea is simply to write the message using the liquid (by way of a toothpick or old ink pen) and then letting it dry thoroughly. When you want to reveal the message, you simply hold the paper over a candle or light bulb. A hot iron will also work. When the paper warms up, the juice will darken and the words will appear.
Science comes into the picture when you want special writing, such as a certain colour. Mix some cornstarch and water over a burner for a bit and then let it cool and write with it. After it dries, to reveal the message sponge the paper with a solution of iodine and water. You will get dark blue writing on a light blue paper. Substitute lemon juice for the cornstarch and you'll get white text on a light blue background.
But, suppose it is Valentine's day and you want to send your sweetheart a love message in red. That will be a bit harder, but it can be done with varying success. The element you will need is called Phenolphthalein. That likely doesn't sound like anything you've ever heard of, but it is found in most over the counter laxative pills! You'll need a box, and be sure it does have Phenolphthalein in it. If the pills have gotten have a coloured coating, you'll have to scrape that off. Take five or six tablets and crush them using a spoon. Then add a few tablespoons of water and mix. This is your ink and you may write your message.
When you are ready to reveal your message (after it has dried.) you will need to mix four tablespoons of hot water with four teaspoons of washing soda (you could also use ammonia,) . Get a plate and pour some water in it, lay the paper on the plate. You want the paper to be really saturated with water. Next, carefully dribble the washing soda solution onto the paper, allowing the drops to spread over the entire surface. Be careful to not touch the paper while it is wet or it will smear your message. The writing should appear in reddish pink.
The basic science behind most invisible ink reactions (except for the ones involving heat) is related to acid/base chemistry and requires pH indicators. The ink is a colorless liquid with either acidic or basic pH. After the colorless liquid dries on the paper is becomes invisible. However, even though you cannot see the ink the acidic or basic properties remain on the paper. The next step in the process is to introduce a pH indicator to elucidate the ink. The indicator changes color when it comes in contact with either the acid or base.
Copyright © 2001 Kathy A. Miles and Charles F. Peters II