Re: What does one use to make invisible ink, which, when heated, turns red?

Dear Curious Father,

There are quite a variety of materials that one can use for invisible inks. The heat sensitive invisible inks are usually simple organic liquids that appear invisible once they have dried on a sheet of paper, but will darken when held over a heat source. Lemon juice or onion juice work best, with milk or vinegar lagging close behind. they are applied with toothpick or pen , and allowed to air dry. The dried paper is held some distance over a candle, or pressed with a hot iron.

An alternative is to use a cornstarch and water mixture, cook it a bit, let it cool, and write your message. Once dry, sponge it with a solution of iodine and water and your writing will appear in a dark blue; the paper will be a lighter blue. If you substitute lemon juice for the cornstarch solution, you will end up with white text on a light blue background.

But, Curious Father, you stated you were looking for a red color for the writing. The only one that comes out red is a method that uses phenolphthalein. This one is a little tricky to do, and does not always turn out completely invisible. The reacting chemical , Phenolphthalein which is a component of some laxatives will give the pink red color you are looking for, however , it does not use heat. This is what you will need:

                Laxative pills. They must contain phenolphthalein. 
                Washing soda (or another safe alkali) 
                Paper 
                Water 
                Jar for mixing 
                Plate 

Allow to dry. To make it visible, mix four teaspoons of washing soda with four tablespoons of hot water. Lay the paper over a plate where a bit of water has been poured. You want the paper to soak up the water and saturate. Carefully dribble the washing soda solution onto the paper, allowing the drops to spread over the entire surface. Don't touch the paper where the message is while it is wet, as this will smear it. The writing should appear reddish pink.

While doing a search for this information I stumbled across a fascinating account of a message hidden beneath a one sentence love letter on a 1943 postcard from Poland, a message in invisible ink makes an urgent request for supplies: flare pistols for nighttime escapes, a camera , invisible ink. In fragmented sentences that read like a horrific poem, the writer makes a wrenching cry for help, vividly describing a Nazi death camp with works like "killing by gas" and "agonizing hell" For the complete story on this postcard in what appears to be an eyewitness account of a concentration camp from the inside go to the website

http://www.athensnewspapers.com/1997/103197/1031.a4hiddenmsg.html

This website also has a picture of the actual postcard that was sent to a Jacob Rosenblum of Bucharest.

I hope the above information is of interest to you, I want to thank you for taking the time to ask a Mad Scientist this question.

June Wingert
MadSci admin