Nature And Types Of Invisible Ink

Invisible ink is the term applied to a number of fluids that remain invisible to the eye after being written in over ordinary paper, until some means are used to make the writing visible. Curiously employed in espionage, invisible ink is yet another form of steganography (the art of writing hidden messages) used in marking property, hand stamping for the purpose of readmission, and special marking for identification in manufacturing.

Processes For Making Invisible Ink

The fluid for writing invisibly is applied with a common fountain pen, a toothpick, or some other device fit for writing, on blank paper. After drying, the paper appears blank. The writing is later made visible by subjecting the invisibly written material to heat, viewing under ultraviolet light, or by applying particular chemicals. These methods develop the ink on the paper back to visibility. However, the writing might be seen without developing if it has been scribed in any of the fluorescent invisible ink varieties. This will need only illuminating the paper with black or ultraviolet light. Some invisible inks depend on chemical reactions commonly on acid-base reactions. In this case, a developer fluid is used like a spray bottle or ammonia vapors (for writing done in phenolphthalein ink) to bring the ink to visible form.

Varieties Of Invisible ink

Depending on the process to make them visible to the eye, invisible inks can be divided into three types i.e. those made visible by: (1) applying heat, (2) chemical reaction, and (3) ultraviolet light.

Inks Developed By Heat

As a general rule, any acidic fluid works as invisible ink provided it is diluted with water. The writing in such fluids becomes visible in brown color, which is the result of oxidation of organic substances present in them. These substances include milk, onion extract, lemon or orange juice, apple juice, diluted honey, sugar solution, diluted cola drink, soap water, vinegar, and wine. Heat can be provided by a radiator, electric iron, or oven. Drying in sunlight will work poorly because of the intensity of light.

Inks Developed By Chemical Means

These inks are developed by means of an acid or a base. Vinegar, phenolphthalein, copper sulfate, iron sulfate, ammonia, starch, lemon juice, sodium chloride etc. all serve as invisible inks. Common developers for these include red cabbage water, sodium carbonate, ammonia fumes, iodine solution, silver nitrate etc.

Inks Visible Under UV Light

Many substances have the property to glow faintly under ultraviolet light. These are commercially available as inks in different colors. Besides paper, these inks can be used on non-porous surfaces like glass and plastic etc. Hence their use for marking valuable home items to foil burglaries. Other uses of such invisible inks include property marking purposes and identification, and readmissions.