Invisible Ink Markings
Many types of invisible ink exist for marking items with a message that can be seen only when the item is exposed to heat. These inks are often made from organic liquids such as clear soda, lemon juice, tomato sauce, vinegar or milk. When heated, the liquid turns a different color and reveals the message. Other types of invisible inks react to chemicals that cause the marking to change colors when exposed to certain light sources. These inks are used for a variety of applications including product identification, quality control, assembly and inspection, tagging, ownership tracking and marking metal objects.
A special type of invisible ink is a heat-sensitive marker that disappears when it cools. This type of invisible ink is commonly used on clothing and shoes to mark where seams, pockets or logos should be added. When the item is placed in a cooler environment, these invisible marks reappear, revealing the instructions for assembly or inspection. This type of invisible ink is also known as thermal sensitive invisible ink or temperature-sensitive ink.
During WWII, both the Allies and the Axis powers worked hard to develop invisible inks. The holy grail of invisible inks was one that was odorless, reacted only to a few reagents such as heat or phenolphthalein and could be developed in the dark without exposing the writing to sunlight. This type of invisible ink is now available as a lead-free alternative and can be developed with simple chemistry: simply add salt to the ink.