Development and Evaluation of Metal Salt-Based Invisible Inks for Forensic Document Examination on Different Paper Substrates

Abstract

There are many applications for invisible inks including security communications, verifying documents, and forensic evidence to name a few. The goal of this project was to determine how effective 3 different metal salt-based invisible inks (copper sulphate, ferrous sulphate, and zinc sulphate) on various types of paper. To develop the hidden writings 6 different chemical agents were used (sodium hydroxide, potassium ferrocyanide, sodium carbonate, ethanol, hydrogen peroxide, and ammonia). The invisible inks that were created were applied to white, coloured, and glossy papers, and after applying the reagents to the treated papers, the corresponding writings were evaluated under normal and ultraviolet (UV) light using the criteria of colour development, visibility, and fluorescence. The research found that copper sulphate was the best performing type of the three inks that were tested in terms of colour development and visibility on most paper types, and that all 6 reagents provided satisfactory results for successfully revealing the hidden writings. Ferrous sulphate was found to be moderately effective providing that most of the latent writings were not visible under normal light conditions, however, they were revealed when the papers were exposed to long wave UV light. At the same time, ultraviolet (UV) examination of latent writings on white and brown substrates had revealed materials which are otherwise undetectable through visible examination. This study illustrated the importance of the relationship between the metal salt, the detection reagent, and the paper substrate as factors in the efficacy of an invisible ink system. The results of this study showed that copper sulphate, then ferrous sulphate, were the most effective, with zinc sulphate demonstrating relatively low levels of efficiency when used as a metal salt with the corresponding detection reagent on paper substrates. The results of this study demonstrate the value of chemical visualization methods and ultraviolet (UV) examination as forensic document analysis and anti-counterfeiting techniques and how they can be applied in demonstrating the detection of invisible inks based upon metal salt.