Science and the Criminal

by C. Ainsworth Mitchell

Skilful forgers guard against this obvious sign of alteration by treating the erased place with a solution of rosin in spirit, which leaves a fine shiny layer upon the paper similar to that of the original sizing.

A treatment first with hot water and then with alcohol will remove this coating of glue or rosin, and when the paper has been dried again it will be found that this part, which will now be free from its protective layer, will absorb a drop of water more rapidly than the rest of the surface.

Another simple test to reveal erasure is the use of iodine vapour, which will often cause a blue coloration (due to starch) upon the moistened surface from which sizing has been removed, but will only colour the remainder of the paper brown.

This test gave a very pronounced result in the examination of the letter to which reference has been made, in which erasure of the letter “y” had been suspected from the general appearance and microscopical examination of the surface of the paper.

The course of the tests described above should be followed under the microscope, although in some instances the fraud is so extensive as not to require any magnification. As a rule, however, it is preferable to use only one drop of a reagent, and to follow closely under a low power of the microscope, its action, both upon the material of paper and upon the ink of any writing, which it may render visible.

The detection of mechanical erasure, which as was mentioned above, is frequently indicated by the paper being thinner and more transparent at that place, is often rendered more certain by photography.

Thus if the document on which was the suspected erasure is placed between a strong light and the camera, the negative will show a darker area corresponding to the place where more light was transmitted through the paper.

A photograph taken in direct light would probably in such a case show nothing, but in a negative taken with the light falling obliquely upon the paper, the fibres that had been roughened by the erasure would be visible, unless a subsequent treatment with glue or rosin had been used to conceal the injury to the surface.

Ink applied to the surface of paper from which the sizing has been removed will show more or less tendency to spread, as upon blotting paper, and although this may be so slight as to escape the notice of the naked eye, it will be plainly visible under the microscope, and on a photographic enlargement the rough edges of the marks will be very pronounced.

Every little fault or attempt at touching up will be brought into prominence, and in cases where writing has been removed by the use of chemical reagents the slight yellow stain which is frequently formed upon the paper—a stain so trifling that it would not ordinarily attract notice—will appear as a dark blotch upon a photographic reproduction. It has frequently been claimed that it is possible to distinguish between different kinds of ink by means of photography. Since inks contain provisional colouring matters which cause the dried pigment upon the paper while apparently black to be in reality red-black, blue-black, etc., it was asserted that such differences would be made manifest in photographs taken on an ordinary plate, and still more by the use of colour-sensitive plates.