Decipherment of Secret Writing

Decipherment of Secret Writing

Secret Writing is the means of written communication that is hidden and cannot be seen by naked eyes; thus, require few developing methods. The purpose of secret writing is to make the written text unreadable by another person who is exposed to the message.

History – Secret writing is being used since the middle of the Twentieth century for hiding message. It was a source of communication for Prisoners. They used their Saliva or Sweat and sometimes Urine also as invisible ink to write the message. These writings were also used by the government during the times of war. In that era, Magnifying glass and Direct light were used to detect the hidden message, but later on, many other developing methods were introduced including both physical and chemical methods.

Secret Inks are needed for writing the secret message. These secret inks are also called as Invisible ink, Sympathetic inks, Disappearing inks, and Security inks. In most cases, any colorless fluids which do not evaporate immediately are used as secret inks.  The writing instrument which is used for secret writing can be paint-brush, pens with no ink, nibs, toothpicks, or other any sharp-pointed object; sometimes fingers and cotton ear-bud too. Mainly plain white papers are used as a writing surface. The invisible inks dry up after a few hours when applied on papers; thus, the written content becomes invisible and cannot appear until any revealing process is used.

Many destructive and non-destructive methods are available for forensic decipherment of these secret writing. Few of the revealing methods for invisible inks are Physical Methods like development by Heat, visualization under Ultra-Violet light and Oblique light and Chemical Methods like Iodine fuming, treatment with Silver Nitrate, pH indicator, and some other chemical treatments. All these developing techniques give different kinds of visualization and result in different kinds of inks due to the components present in invisible inks.

The fluids which are used as Invisible inks for secret writing are easily available and useable with a conventional writing instrument, these are categorized as Organic Fluids including Vegetable fluids; like Onion juice, Lemon juice and Fruit juices; like Apple juice,

Orange juice, Biological Fluids; like Milk, Saliva, and Chemical Fluids; like Vinegar.


The basic steps to be followed for decipherment are to prepare the sample first and then analyze them, i.e. the Decipherment methods.

Sample Preparation

Firstly, there is a need to prepare samples of secret writing. The simplest way to write with invisible inks is to dip the writing instrument into the liquid invisible ink and write on the paper. Being non-volatile in nature, the ink dries up and caused disturbances in the fibers of paper; that’s why it causes thinning of the paper, and it helps in the decipherment of written.


The decipherment techniques include both Physical and Chemical revealing methods. Most of these methods are reliable as they give good results, but few of them are destructive in nature. The basic concept for the development and revealing of the hidden message is to change the color contrast between the marked and unmarked area of the sample, here the marked and unmarked area simply means the area of paper which contains written text (by invisible ink) and the area without written text; respectively.

Physical Method

Most of the physical methods which are used for the decipherment of invisible inks are non-destructive in nature while a few of them are destructive as well. Firstly, decipherment by physical methods is preferred, but if those don’t work then the chemical methods are preferred.

  • Oblique Light: When the suspected sample is observed under the oblique light, the message is revealed due to the variability in the surface of the paper. This is because of the thinning of the paper which causes translucency when viewed under oblique light.
  • Ultra-violet light: It is also a non-destructive technique. The use of UV- lamp for the detection of secret ink is also useful as most of the invisible inks give fluorescence under UV-light when visualized. This fluorescence makes the impressions of writings visible. If the writings are visible, then these must be photographed.
  • Heat: There are several methods of developing the invisible ink by heat; such as: Place the paper in Oven, Hold the paper unto a hot Light Bulb, Hold the paper near the Candle Flame, Iron the paper. These developing methods help in revealing the message by the changed color, which is due to the thinning of paper.

Chemical method

If the decipherment of secret writing fails by Physical methods; then few chemicals methods can be used for development. Most of the Chemical developing methods are destructive in nature. Some of the chemical methods for development are mentioned below:

  • pH Indicator: pH indicators are the substances whose color changes when exposed to an acidic or basic solution. The species have a different color due to their different bonds and electric structures; thus, resulting in their different absorption spectra in the visible frequencies of lights. For the development of the pH indicator, the aqueous solution of pH solution is applied to both marked and unmarked regions of the document or sample.
  • Iodine Fuming: Iodine Fuming is a non-destructive technique. The fumes of Iodine pass through the sample containing writings. The writings on the surface of the paper are revealed because of the presence of starch in the paper.
  • Phenolphthalein: It is a destructive technique in which some of the invisible ink changes its color when treated with Phenolphthalein solution. As the ink is slightly acidic in nature and the Phenolphthalein does not change color in acidic medium, the unmarked area of paper changes its color. The variability in the color of the sample makes writings visible.
  • Silver Nitrate: Various invisible inks react with Silver Nitrate when treated because of the presence of some specific salts. These salts react with silver nitrate, which results in the formation of silver salts. These silver salts decompose into Silver metal when exposed to light and give color, which makes the text visible.

Results and Discussion

For Physical Developing Methods, the development by Heat and UV gives better results as compared to all other physical developing methods, even when observed for 30 days continuously. Ink developed by heat mostly gave

Brown color over the text or secret message. While inks visualized under UV light gave Blue fluorescence.

For Chemical Development Methods, the Silver Nitrate and Iodine Fuming gives the best results as compared to other chemical developing methods. The Iodine solution on reacting with organic fluids gave Purple color. While reacting with Biological and Chemical fluids it gives Brown color, where Saline is an exception as it gives Black color on reacting with Iodine solution. While Phenolphthalein gives Pink color when treated with Baking soda. The Silver Nitrate mostly gives Brown color when treated with different kinds of invisible inks.


Secret writing samples can be deciphered by both, Physical and Chemical methods. All the methods give different results with different chemicals.

Among all the Physical methods mentioned; development by Heat is the best method. Though; being destructive, it gives results universally with every kind pf invisible ink used. Even this developing method does not take much time and the visibility is good as well. The observation under UV light also a good technique as it is a non-destructive method and gives results most of the time.

While among Chemical methods, the Iodine fuming is known to be the best method as it is also a non-destructive technique; thus, it does not damage the paper. This is because the fumes of Iodine evaporate after a few minutes of development and leaves the treated paper as such, that is why it is a need to photograph the revealed message instantly after development. Whereas Silver Nitrate and Phenolphthalein give good results up to a month but both of these techniques are destructive; thus, less preferred.

“Hence, Heating and Iodine fuming are the best visualization and deciphering methods for secret inks.”


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