There are different methods of making invisible ink. Obviously, which method you will use depends on the purpose of the invisible ink will it be used by children playing games or by secret agents? As secret agents are unlikely to be reading my Helium article, I will describe only the methods using everyday materials. It will be necessary to make the invisible ink itself, but you may also need a specific substance to read your secret messages.
Surprisingly, any acidic fluid will work as invisible ink. This includes vinegar, diluted Coke, milk or orange juice. The invisible ink can be applied using a paintbrush, cotton swap or even a finger, and will become invisible as it dries. To reveal your message, you must heat it. This causes the ink substance to oxidise and turn brown. The safest method of heating is probably putting the paper on a radiator or holding it up to a lightbulb, but placing it briefly in the oven, under an iron or in George Forman’s Lean Mean Grilling Machine will work just as well.
But what if you’re only concerned with passing messages in class and are not conveniently seated near to a radiator? You can use an acid or an alkali as ink, and reveal the message using a pH indicator. Ink may be made from white vinegar or baking soda solution. They can be applied to the paper as before. To reveal your message, you must paint over it with a pH indicator, such as iodine solution, red cabbage water, purple grape juice or apple juice.
If you want to write secret messages without making your own invisible ink, you could purchase an invisible ink pen. Invisible ink pens can be bought from many toy, stationery or magic shops. The ink from these pens is usually visible only under ultraviolet light, and a UV light will often be included with the pen.
For your invisible ink to work best, remember to use plain paper rather than lined, and be careful if you are heating your paper. If you are hoping to play around with invisible ink for fun or for a science experiment, hopefully you will use my methods and have a good time. If you are making invisible ink because you are an important government official, hopefully you will ignore this article as I do not want to have endangered national security in any way.
