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Invisible Printer Ink

invisible ink

What is invisible ink?

The name really says it all. Invisible ink is a substance that you can write with that is invisible upon application to a surface or soon thereafter, which can be made visible later by some means. Uses of invisible ink run the gamut from anticounterfeiting applications to children’s games. Technically the use of invisible ink is a form of steganography. 

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How is invisible printer ink used?

Invisible ink is applied to a surface with an instrument such as a stamp, stylus, pen, or even a toothpick. Soon after the ink is applied, the surface should return to its prior state. To make the ink visible again you can use one of several methods, depending on the type of ink used. There are primarily three methods: ultraviolet light, heat, and chemical appropriate to the ink type used.

Toy invisible ink pens typically have two tips: one for invisible ink writing, and the other for making the ink visible again. Also common are play books which include a special “decoder” pen that children can use to reveal messages hidden throughout the material.

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Invisible Printer Ink for Inkjet Printers

You may be wondering why you’d ever want to print in invisible ink. The truth is you’d be surprised how often business forms and other materials that you handle on a daily basis contain invisible printer ink. Most commonly it is used to print information on business forms that is to be used by a scanner or forms processor.

The idea here is to reduce the amount of clutter that’s actually visible on the pages by making these markings invisible to the eye. One of the biggest users of invisible ink is the United States Postal Service. These days most mailing sorting centers use UV visible ink to create scannable bar codes on envelopments and packages to supply important routing information to handling equipment and other sorting machines that come in contact with the collateral.

Inks visible under UV light

Invisible inks glow when placed under an ultraviolet light. Many ink formulations can be used on non-porous surfaces such as glass and plastics. Other invisible inks work in the reverse way: they actually absorb light. So when they are used on fluorescent paper, your writing actually fluoresces less than the surrounding paper when put under UV light. One of the most common applications of this invisible ink that responds to UV light is the security marker. These pens are used to mark property of value so if it’s ever stolen, the item can be traced. 

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Inks that are developed by heat

Several organics oxidize when heated, turning them brown. Most acidic fluids work for this purpose, especially when they are diluted with water. Some examples include: onion juice, soda/cola, vinegar, ad milk. To make the writing visible simply heat the surface

Inks that are developed by chemical reaction

This kind of invisible ink changes color when it is mixed with an acid or base. Some of the most common forms of this ink include:

  • Iron sulfate developed by sodium sulfide
  • Copper sulfate developed by sodium iodide, sodium carbonate, or ammonium hydroxide
  • Iron sulfate developed by sodium carbonate or potassium ferricyanide

Disappearing Inks

Not to be confused with invisible inks, disappearing ink is visible for some amount of time and is then lost forever. The most common disappearing inks rely on a reaction between the chemicals thymolphthalein and alcohol.

About William Elward

Founder of Castle Ink, William Elward has 20 years experience in the printer industry. He's been featured on CNN Money, Yahoo, PC World, Computer World, and other top publications and frequently blogs about printers and ink cartridges. He's an expert at diagnosing printer issues and has published guides to fixing common printer issues across the internet. A graduate of Bryant University and Columbia's Sulzberger Executive Leadership Program, he's held various leadership positions at The College Board, Bankrate, Zocdoc, and Everyday Health. Follow him on Twitter at William Elward's Twitter Profile