Home >> Science & Nature >> Science

Making Invisible Signature Ink

Spies have devised ways to keep their messages hidden from their enemies for centuries. One method, which was used up through the early 20th century, was invisible ink. These inks are made of substances that disappear when they dry, but which can be made visible again using heat, chemical treatments or ultraviolet light. Many of these inks are created using acids and bases, which weaken the paper where they are applied and cause treatments to affect those areas more quickly than the rest of the paper.

Baking Soda Ink
- Combine baking soda with an equal portion of water and mix the solution until the soda is completely dissolved. Dip a paintbrush or cotton swab into the liquid and use it to sign your name on a thick sheet of paper. Avoid tissue paper, tracing paper and other thin sheets, as these can crinkle when they become too wet and may not respond well to heat. Let the ink dry thoroughly, then put the paper on a cookie sheet and place it in a warm over or hold it near a 100-watt lightbulb, over a toaster or in front of a hot iron to reveal the signature. You can also paint over the writing with purple grape juice, allowing the acid in the liquid to react with the alkaline soda to reveal your handwriting.
Corn Starch Ink
- Combine one part corn starch with two parts water and cook the mixture over low heat, stirring constantly until it becomes smooth. Let it cool, then dip a cotton swab or toothpick into it and use it to write your name on a thick piece of paper. Let the paper dry completely while you mix one part iodine with 10 parts water. Dip a cloth or sponge into the solution and gently wipe across the page to reveal your signature. The letters will appear dark blue or purple against a lighter colored blue paper.
Acidic Liquid Ink
- Pour undiluted lemon juice, apple juice, vinegar, milk or white wine into a dish, then dip a paintbrush, cotton swab or toothpick into the liquid. Use it to write your name on a piece of thick paper and let it dry thoroughly while you cut one or two red cabbage leaves into pieces and place them in a pot. Cover the leaves with water and place it on the stove over high heat. Turn the heat down as necessary to keep the water simmering, but not boiling, until the water turns deep purple. Let it cool, then pour it into a spray bottle. Lightly spray the paper and wait for the natural pH indicator in the cabbage to react with the acid in the invisible ink, making your signature appear in color. You can also reveal the letters by heating the paper in the same way as you would when writing with baking soda ink.
CIA Invisible Ink
- In April 2011, the CIA released its outdated invisible ink formulas from World War I. These include a German formula that was discovered by the French intelligence. These inks were more sophisticated than those made from household liquids, but include many of the same ingredients. One formula, for example, calls for water, fabric starch and sodium nitrate, which is an ingredient in fertilizer. While some were revealed with complex chemical treatments, others were made visible using water and a hot iron like many elementary school inks.

Science