Volume 18, Issue 4 (October 1973)
Ink Differentiation with Infrared Techniques
There are often questioned document problems in which alterations, additions, and obliterations have been made by visually similar inks. Such cases of forgery have been found in checks, passports, birth certificates, receipts, currency notes, lottery tickets, account books, etc. In the examination of such documents, an expert is often not interested in the complete composition of the inks involved, but is basically concerned with discriminating between these inks which are visually similar but chemically different in composition. He is thus interested in comparative study of the properties of various inks at large. A chemical analyst provided with a bottle of ink can determine the complete composition of an ink by several physico-chemical methods, but examination of ink on paper presents a much more complex and difficult problem [1,2]. For instance, about ninety-eight percent of the ink evaporates on writing and the residue is an extremely small quantity covering a very large surface. Ink writings, therefore, are either tested on paper or physically removed by special techniques [1,3]. This approach, apart from other basic handicaps, results in physical damage to the document in varying degrees depending upon the method used. Removal of ink from the document by solvents or physical punching of paper, followed by chromatography, electrophoresis, spectrophotometry, etc. are some of the other recent techniques which may give some specific information about composition and comparative identification [1–5]. However, these micromethods are cumbersome and require a good deal of expertise, apart from the basic disadvantage that the document is bound to be physically altered—even though to a small extent.