The simplest invisible inks consist of organic compounds like lemon juice, vinegar, diluted honey, and even white wine– all substances that subtly weaken paper fiber. Once the liquid dries, it’s undetectable under normal circumstances. Apply heat, however, and the hidden message seems to appear out of nowhere.
Organic ink makes the painted area more susceptible to oxidation. When it’s heated, it browns before the rest of the page. If you try this at home, keep a close eye on the developing ink, and be careful not to actually set the paper on fire!
For an even more dramatic hidden ink DIY, paint your message in laundry detergent (dilute if necessary to make the ink invisible), then expose it to a blacklight. Your missive will glow!
Modern invisible inks take advantage of chemical reactions to hide, then reveal, secret communication. Phenolphthalein, for instance, is perfectly clear except in a slightly basic environment. Between pH 8 and 10, it turns pink! A dilute solution of this chemical makes a great hidden message medium– just spray it with watered-down ammonia to see the concealed writing take shape.
Note: phenolphthalein is toxic and should only be handled with proper safety gear and training. As always, ask for adult permission and have a fire extinguisher on hand when experimenting with heat & flames. Last but not least, have fun! Happy April Fools Day from Astrocamp!
Written By: Caela Barry