Take a sheet of test-paper of the same dimensions as the document to be examined, moisten it, and cover it underneath with a sheet of Swedish filter-paper. These two sheets together (the filter-paper underneath) are then applied to the document which has been moistened already. The whole is then laid between two quires of paper, covered by a weighted board, and left in this condition for about an hour. At the end of this time examine the test-paper to see if it has partly or altogether changed color. This examination finished, put the test-paper in contact with distilled water, to be afterwards removed and tried by appropriate tests to discover the nature of the alkali or acid present.
Silver nitrate is also used to discover whether the paper has been washed with chlorine or chlorites. A paper in that way becomes acid. The chlorine changes to hydrochloric acid, which dissolves in the water with which the suspected document or paper is moistened, and at the contact of silver nitrate little spots of silver chloride appear.
There are various other tests such as gallo-tannic acid or infusion of nutgalls prepared a short time before application and may be used with advantage to restore writings that have been removed by washing. Place the document or paper on a sheet of white paper and moisten the whole of its surface with a paint brush dipped in the reagent, taking care not to rub it or strongly press it. When the surface is well impregnated allow the solution to act for an hour, and at the end of this time examine the document again. Then moisten it a second time and the following day, examine the results. Repeat the moistening several times if necessary, for it often takes some time to make the traces of writing reappear.
Chevallier and Lassaigne experimented together on the effect produced by the vapor of iodine on the surface of the papers or documents upon which the alteration of writing was suspected. Take a bottle with a wide mouth from ten to eleven centimeters in height, and the opening from five to six centimeters in width. This last is covered by a disk of unpolished glass. Into the bottom of this vessel introduce from twenty to thirty grams of iodine in crystals.
Place the portion of paper on which the vapor of iodine is to act at the opening of the bottle, and cover it with the stopper of unpolished glass, on which put a weight so as to exert a slight pressure, and in order that the aperture may be hermetically closed. Then allow the vapor of iodine to act on the dry paper for three or four minutes at the temperature of 15° to 16° C. and examine it attentively. When the surface has not been spotted by any liquid (water, alcohol, salt water, vinegar, saliva, tears, urine acids, acid salts, or alkalis) a uniform pale-yellow or yellowish-brown tinge will be noticed on all parts of the paper exposed to the vapor of iodine.
Otherwise a different and easily distinguished tinge shows itself on the surface that has been moistened and then dried in the open air.
Machine-made papers with starchy and resinous sizing give such decided reactions that sometimes it is possible to distinguish by the color the portion of the paper treated with alcohol from that moistened with water. The spot produced by alcohol takes a kind of yellow tinge; that formed by water becomes a violet blue, more or less deep, after having dried at an ordinary temperature. As to the spots produced by other aqueous liquids, they approach in appearance, though not in intensity, those occasioned by pure water. Feeble acids, or those diluted by water, act like water; but the concentrated mineral acids, in altering more or less the substance of the sizing, produce spots that present differences.
Spots which become apparent by using vapor of iodine are due to chemical agents whose strength has altered either the fibers of the surface, or the paste uniting them.
In a word, the test of a document or paper by vapor of iodine has the double advantage of indicating the place of the supposed alteration and operating afterwards with appropriate reagents to bring back the traces of ink. It is only the reappearance of former letters or figures written or effaced that demonstrates forgery. Much time may be profitably spent in merely scanning each letter of a document, and the writing by lines, paragraphs, and pages before a closer scrutiny. Gradually, if the writing be genuine, its character will begin to reveal itself, and unconsciously a hypothesis as to the physical causes of the irregularities or characteristics will be formed.
When an entire document or page is forged, the ornamentation, flourishes, or the capitals at its head will often be seen to be out of keeping, either with its nature or with the supposed author's habits in similar cases. In a writing all must agree, place, day, year, handwriting, superscription or heading, signature, and material carrying the writing, especially paper, both as to constitution and color and ink.