ABSTRACT
The spy crossed the street, dodging carriage wheels and horse carts. The Ancient Greeks and Romans made invisible ink from the milk of the tithymalus plant. During the Renaissance, inventor Giovanni Battista Della Porta developed a recipe for invisible ink using an ounce of alum and a pint of vinegar. Lemon juice or milk can both be used as inks. The handy part about this type of invisible ink is that you can write a regular letter right over the message. When the spy is delivering the letter, it will look like a common everyday shopping list or a letter to a sweetheart. Scenes like this played out during the American Revolution on both sides of the conflict. Spies had to get messages back and forth, and to protect the identity of their partners, they often left their messages in secret places called dead drops.
