A Study on Common Visualization Methods for Secret Writing done by Invisible Inks and their Sensitivity Over a Period of Time

Research Article

Austin J Forensic Sci Criminol. 2017; 4(2): 1061.

A Study on Common Visualization Methods for Secret Writing done by Invisible Inks and their Sensitivity Over a Period of Time

Hussain SS and Sahu M*

Department of Forensic Science, Guru Ghasidas Vishwavidyalaya, Bilaspur-492010, Chhattisgarh, India

*Corresponding author: Manju Sahu, Department of Forensic Science, Guru Ghasidas Vishwavidyalaya, Bilaspur-492010, Chattisgarh, India

Received: March 29, 2017; Accepted: April 20, 2017; Published: May 10, 2017


Secret writing is a skill of concealed writing. The art of secret writing is projected to make the written text unreadable by other person who is exposed or deciphered by different revealing process to make the messages readable. Techniques of secret writing are many, but this paper mainly focuses on the use of invisible inks. The current study comprises of secret writing samples which contain hidden messages written using some fruit juices (lemon, apple and grape juice), chemical fluids (detergent, sugar solution and vinegar) and biological fluids (milk, saliva and sweat). They were then visualized using common physical and chemical methods in a specific interval of time; examined from 1st day to 30th day after regular interval of five days. Heating and Iodine fuming were found to be the best methods for visualizing invisible inks, among physical and chemical methods, respectively. Both of these methods showed good results up to one month.

Keywords: Secret writing; Hidden messages; Invisible ink; Deciphered; Forensic science


Secret writing is any means of written communication whereby a spy conceals the actual written text, whether it is enciphered or encoded or not [1]. It is the art of covered or hidden writing [2]. Techniques of secret writing include use of invisible inks, carbon copies, miniature writings, latent photographs, codes and ciphers. Among these, the use of invisible inks is the most primitive method; others are the result of developments in the field of secret writing with the advancement of time. Invisible inks are also known as Sympathetic inks or Security inks [3]. It is a liquid used to create concealed messages that don’t seem unless uncovered by a noteworthy procedure. Invisible inks can be ordered into three principle classes: those that are uncovered by heat, those uncovered by chemical reactions and those that are instantly recognizable under ultraviolet light [4]. The nature of the inks varies tremendously. It may be a simple substance, like water, body secretions, fruit juice, soap, etc. On the other hand, they may be highly complex chemicals. Generally, organic fluids and sympathetic inks are classified as invisible inks. Organic fluids include lemon juice, sweat, saliva, vinegar, milk, onion juice, urine, diluted blood and can be developed through heat. These fluids alter the fibers of the paper and when exposed to heat at lower temperature, the secret writing turns brown faster than the surrounding paper. On the other hand sympathetic ink contains at least one chemical and requires the utilization of a particular “reagent” to be developed [5]. Invisible ink is one form of steganography i.e. concealing a message. The experiment of secret writing comprises of dipping a paint brush in fruit juice, biological fluids and different chemicals and writing on a piece of blank white paper. At the point when the “ink” dries, it will be undetectable to the bare eye, however in the event that the paper is held up to a direct warmth source, for example, a light, a radiator or an iron utilizing UV light to disentangling imperceptible composition. Some basic invisible inks are diluted fruit juices, vinegar and clothing cleanser, all which can be connected by a paintbrush, special invisible ink pen or even a toothpick. Historically, invisible inks were utilized as a part of times of war by governments and extremists alike [4]. The utilization of secret writing has declined since the middle of the twentieth century for some reasons, most critical of which is the sheer volume of information that knowledge administrations must transmit and process [6]. Secret writings are also employed by international spy rings [3]. The inclusion of codes and ciphers under secret writing has been disputed sometimes as they just hide the meaning of a message, unlike secret writing which hides the original message [6].

Materials and Methods

The present study was carried out at Department of Forensic Science, Guru Ghasidas Vishwavidyalaya, Bilaspur (Chattisgarh) to study the common visualization methods for secret writing done by invisible inks and to find out the sensitivity of these methods over a period of time. Fruit juices (lemon juice, grape juice, apple juice), chemical fluids (vinegar, sugar solution, detergent), and biological fluids (sweat, saliva, milk) were used as invisible ink in the present study. A4 size blank papers were utilized for preparing secret writing samples and a ball point pen without ink, was used as a writing instrument. The nib of this pen was dipped in the required invisible ink and messages were written over the blank paper. The papers with secret messages were marked at the top, with date of sample preparation, name of the invisible ink and the respective day on which it needs to be examined. Extra samples of secret messages were also prepared in the same manner. The papers containing written messages were kept under a running fan, so that the inks dry up and the writings become invisible. These samples were then kept at normal room temperature and conditions. They were then analysed at regular interval of five days for one month, using different physical (application of heat and visualization under ultraviolet lamp) and chemical (iodine fuming, treatment with phenolphthalein solution and treatment with silver nitrate solution) methods.


The results of the study on common visualization methods (physical and chemical), for decipherment of secret writings done using different invisible inks, varied as shown in the Tables 1-3.