Kinds of Ink

• Invisible inks fall into two general categories. Chemical inks can be revealed by heat or a chemical. Ultraviolet sensitive inks, which are revealed using UV light (black light), are often used as security markers.
  
  

Revealing Chemical Inks

• Some chemical inks can be revealed using a substance that reacts to acids or bases (alkalies). One example of a revealing agent is a pigment found in some fruits and vegetables -- natural purple grape juice or the juice of a boiled red cabbage turns pink in the presence of an acid and blue in the presence of a base.
  
  

Lemon Juice and Vinegar

• Two of the simplest invisible inks you can use are vinegar and lemon juice. You need real lemon juice, not lemonade or soda. Take a piece of plain white paper, and write or draw on it with lemon juice using a toothpick or a fine paintbrush. Let the juice dry. To reveal the secret message, warm the paper (for example by ironing), or paint with grape juice or red cabbage juice. These will turn the message pink.
  
  

Baking Soda

• Make a strong solution of baking soda: half baking soda, half water. Paint or draw on plain white paper with the baking soda solution and let it dry. You can reveal the message with heat or by painting the paper with grape or cabbage juice. The baking soda will turn the juice blue.
  
  

Black Light Ink

• You can find black light security markers in the hardware store. It's also possible to make your own ink by mixing laundry whitener in water and painting with the mixture. To reveal your message, you will need a black light. Some security stores sell portable black lights. You could also use a jeweler's loupe with a black light built in. The ink glows blue-white under the black light. This phenomenon is called fluorescence.