What is invisible ink?
Invisible ink is a product used to conceal written information in plain sight. To the naked eye the information is invisible or obscured, however can be revealed (developed) if the correct method is known. It is also known as security ink or sympathetic ink.
Invisible ink is a form of Steganography; hiding a message inside something else (another message, an image, a computer file, etc.)
In this article I’ll give some examples of inks in use today and different types of invisible ink.
Common examples of invisible ink
Commercial examples
The most common commercial example of invisible ink I can think of is UV ink or pigment. This is invisible to the naked eye but glows under a UV lamp (also known as black-light).
It is used extensively to add security features to all sorts of things in modern life; paper money, cheques, medicinal packaging and tickets are some examples.
DIY Examples
The most common DIY example I know of is using household acids on paper, such as lemon juice or vinegar. The acids attack the cellulose fibres in the paper. When the paper is exposed to heat, the damaged fibres brown quickly exposing the message. Another method of revealing the message is to apply a base solution such as dissolved bicarb soda.
Spy craft examples
Invisible ink has been used in espionage from ancient times up to recent history. However, now information on methods is well known it is no longer viable as a means to hide anything important. That said the CIA only declassified their World War I invisible ink documents in 2011!
Various methods were used and spies had to hide both their messages and the means of creating them. It was suggested to heavily starch a collar of a shirt, then the collar could be soaked in water later to extract the starch to write a message. Ties could be soaked in chemicals that could be rinsed out later. Inks could be made from aspirin, a common headache medication. Although less pleasant urine and other body fluids can and were used to create invisible ink.
Types of Invisible Ink
Here are some examples of invisible inks you can try at home. This information is provided for entertainment purposes only, please use caution if you try any of these yourself.
Inks that develop with heat
Household acids work well for this type of invisible ink:
- Lemon juice
- Cola
- Vinegar
- Baking soda
There are also thermochromic inks which change colour or become transparent with a change in temperature.
Inks that develop with cold
Pilot Frixion pens use a combination of different substances to create their ink. When writing with one of these pens is heated above 60°C, it becomes transparent. However, the ink remains and if cooled down to -10°C it will reappear. See this article on pickypens.com for more information.
Inks that develop with a chemical reaction
These are mostly acid/base reactions.
- Vinegar (acetic acid), developed by red cabbage juice (base)
- Lemon juice (citric acid), developed by iodine solution (base)
- Starch, developed by iodine solution (base)
- Baking soda (sodium bicarbonate), developed by grape juice (acid)
Inks that become visible with light sources
These inks are revealed by shining a light of a certain spectrum on the ink.
Ultraviolet (UV) light
These inks are invisible to the naked eye but fluoresce (glow) under UV light. You can purchase UV lights/black lights fairly easily, however test your light/ink combo as LED UV lights do not produce the same spectrum as older tube lamps. Common household items that glow under UV light are:
- Sunscreen
- Some soaps
- Lemon juice
- Laundry detergents that contain optical brighteners (fluorescing agents to make clothes appear brighter)
Infrared (IR) light
This is a much trickier part of the spectrum to deal with, however you can purchase inks that are invisible to the eye but visible to infrared/near IR sensitive equipment. People use this method to cheat at playing cards – the cards look normal to the eye but the face value is printed on the back with IR ink. The cheater wears ‘sunglasses’ that expose the hidden marks.
Conclusion
Invisible ink is used to hide information from those who don’t know how to reveal it. It is in use commercially today, however it doesn’t play much of a role in espionage any more as it’s too well known.
There are many ways to try out your own invisible ink at home, give it a try and let me know how it goes!
