Prof. Dr. Dhrubo Jyoti Sen*


Heat, Chemical Reactions and UV light the three musketeers who have capability to make invisible ink write-ups to visible. When you hear the words invisible ink, you probably think of James Bond, magic shows and secretive government operations. While the idea seems complex, reserved for individuals with access to fancy and expensive tools, it is actually quite an old and often simple idea. Writing messages that cannot be seen at first but appear later when the paper is developed (that is, treated in some special way) is a very old idea going back 2,000 years and you can use some pretty mundane techniques to do it. In fact, invisible ink is not so mysterious, once you stop thinking of it as ink. Invisible ink is usually not ink at all. What we think of as ink is a fluid containing a pigment or a dye, which is usually permanent. Invisible ink does not use dye to produce writing or images, but instead uses chemistry. This means you can write a message on paper (or sometimes on other surfaces) and it won’t be visible until it is developed, depending on what the invisible ink is that you used. There are basically three types of invisible ink. Each type is defined by how the message can be developed in order to be seen. Heat: The fluid used oxidizes when heated (that is, the compounds that make up the liquid lose some electrons therefore changing their chemical makeup) and this oxidation turns the compounds brown. Acids work well for this type of ink, because they not only may oxidize themselves, but they change the chemical makeup of the paper also, causing it to burn and char more easily. Chemical Reactions: The fluid used as ink will be exposed to another chemical, either in liquid or gas form, in order to be developed. This exposure will cause a chemical reaction to occur, which changes the color of the original ink, making it visible. Some of these inks are also acids or bases, but others are metal compounds like iron sulfate (developed using sodium carbonate) and cerium oxalate. UV Light: The fluid does not have any color visible to the naked eye, but will fluoresce (give off light) when put under a UV light. Many organic compounds do this, as do laundry detergents and sunscreens. Of the three types of invisible ink, those developed by heat are the easiest and the safest to use (developing chemicals used for the second type and UV light for the third type can be very dangerous). Let’s explore more carefully what these invisible inks are really doing. Paper is made up of a compound called cellulose, which comes from wood and consists of a long chain of linked sugar molecules. The cellulose fibers are pressed together and dried, leaving a thin flexible sheet behind. This flexible sheet is your sheet of paper. One type of invisible ink developed by heat is weak acid, like lemon juice or cola. When these are brushed on the paper they are not visible, but they slowly begin to change the chemical composition of the paper by breaking down the cellulose the paper is made of. The compounds created by this breakdown oxidize more easily than the original cellulose and when they oxidize they turn brown. Heat speeds up chemical reactions, so heating these compounds makes them turn brown faster (give them enough time and they will turn brown without heat...that’s why old books sometimes look yellow, they have acid in the paper!). Other types of heat-developed invisible ink contain sugar, which caramelizes when heat is applied.

Keywords: Heat, Chemicals, UV light, Invisible ink, Disappearing ink, Scantegrity, Fluorescence, SOE.

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