Science and the Criminal

by C. Ainsworth Mitchell

In more than one instance the results of a scientific examination of the inks have failed to support the claim, for they have proved conclusively that the ink was of recent origin.

It is a simple matter to distinguish between the ancient types of ink that were in use during the early centuries of the Christian era until they were gradually replaced by iron-gall inks and modern writing inks. For the basis of all these ancient inks is lampblack, or some other form of carbon, which is very resistant to the action of reagents. It is for this reason that printing ink, the pigment of which is carbon, is so much more stable than any ordinary writing ink can be. In fact, in order to increase the permanence of writing inks it has frequently been recommended to add a small amount of some carbon ink.

The most easily obtained preparation of the kind is the commercial Indian or Chinese ink, which consists essentially of a mixture of glue with lampblack in the finest possible state of division.

In order to distinguish between a carbon ink of this nature and an ordinary writing ink all that is necessary is to apply a dilute bleaching agent. The blue-black pigment of the writing ink will then gradually disappear, whereas the fine particles of carbon in the other ink will show little, if any alteration, and may still be discerned under the microscope as minute black granules resting upon the fibres of the paper.

It was by a method similar to this that Sir Humphrey Davy proved that the writing upon papyri found in the ruins of Herculaneum, which was destroyed in A.D. 79, had been done with a carbon ink, of the same nature as that used by the ancient Egyptians and by the Chinese and Japanese at the present day. On none of the Herculaneum MSS. could any trace of iron ink be detected.

The same tests may be applied to determine whether the writing upon a document has been lithographed or has been written with ordinary ink.

An amusing instance of the kind came within the present writer’s experience. A sheet of paper upon which was some writing that was believed to have been written by Nelson had been handed down in a family for several generations as an heirloom, and had always been looked upon as a genuine document. The ink had the faded yellow tone of old iron ink, and there was nothing to show that the writing was not what it professed to be.

Its present owner, however, happened to notice in a museum what appeared to be a duplicate of the manuscript in his possession, and when a chemical test was applied to the ink upon the latter the pigment was quite unaffected. Hence there could be no doubt as to its being a copy of the original reproduced by lithography.

Cases in which it is necessary to distinguish between iron-gall writing inks and printing or other carbon inks occur from time to time in criminal investigations. As a recent example a case that was tried a few months ago may be mentioned. The chief clerk of a firm of merchants had for a considerable time been defrauding his employers, and when suspicion at length fell upon him, endeavoured to conceal his doings by falsifying the entries of previous years in the ledger.

In order to do this it was necessary to abstract certain pages in a particular part of the ledger and to substitute the necessary alterations. Then, finding that the ink of the writing would appear too new, and thus invite inquiry, he added a small amount of Indian ink to an ordinary writing ink, and thus obtained a mixture, which gave an immediate effect of age to the writing. To the naked eye there was nothing to show that these pages had not been written on the dates mentioned on them, three or four years previously, but on applying a weak bleaching agent the fraud was at once made obvious. The iron-gall part of the pigment faded away, but the particles of carbon that had formed the basis of the Indian ink were left, and their nature could easily be recognised under the microscope. The entries on the other pages in the ledger, which had been written in ordinary writing were completely bleached in the test.