When the difference between a written and printed signature has been once noticed it is hardly likely that an observant person will be deceived. It is, however, as well to be carefully on guard against this contingency, for modern photography and process printing have been brought to such a degree of imitative perfection that it is easy for a not too keen-eyed person to experience great difficulty in forming an opinion in the absence of the acid test. Fortunately that is infallible.
It must, however, be admitted that up to the present no great success has attended efforts to determine how long an interval has passed between the writing of the original and the suspected addition. Broadly speaking, the most that the expert can hope to gain from an examination of ink under these circumstances are hints, clues and suggestions rather than definite, reliable facts. Fortunately it often occurs that a suggestion so obtained proves of immense value to the trained or careful observer, though it might convey no conviction to others.
As in the case of nearly all deductive reasoning the handwriting expert becomes sensitive to slight suggestions. If called upon, as he sometimes is, to explain to others how and why one of these slight and almost imperceptible signs fit in with his theory, he fails. Therefore the cautious expert, like a good judge, is careful in giving reasons for his judgment only to cite those which are self-evident.
Many an expert has made a poor exhibition in the witness-box by failing to convey to a jury the impression produced on his own mind by a slight piece of evidence, the proper understanding and interpretation of which can only be grasped by those who have learned how to recognize faint signs.
The process of chemically testing inks for the purpose of ascertaining the points mentioned is quite simple, and is distinctly interesting. In a very important case the services of a qualified chemist will probably be requisitioned, but the cost of the necessary material and the time required to make oneself proficient as a capable tester are so slight that even the small fee that would be charged by a chemist is scarcely worth paying.
The materials necessary are a few test tubes, some bottles of lime water, diluted muriatic acid, a solution of nitrate of silver in distilled water, in the proportion of ten grains to the ounce, some camel hair pencils, and clean white blotting and litmus paper. The whole need not cost more than half-a-crown.
The method of using these materials is best illustrated by describing a test often needed by autograph collectors.
A very common method employed by forgers to give an appearance of age to the ink used in spurious old documents is to mix with ordinary ink, muriatic acid, oxalic acid, or binoxalate of potash. The presence of these colouring agents can be detected in the following manner.
In the first place, washing the letter with cold water will make the ink become darker if acid has been used to brown the ink, but the following test will settle the point beyond dispute:
With a camel's-hair brush wash the letter over with warm water. If, as sometimes happens, a sort of paint or coloured indian ink has been used, this will be immediately washed away and disappear, leaving a rusty smudge. If not, apply the litmus paper to the wetted ink, and the presence of acid will be shown in the usual way by the litmus paper changing colour. If genuine, wetting makes no difference.