To assist in determining the ages of writings by one and the same ink, it is to be observed that the older the writing the less soluble it is in dilute ammonia.  If the writing be lightly touched with a brush dipped in ten-per-cent ammonia, the later writing will always give up more or less soluble matter to the ammonia before the earlier.  In case of inks of different kinds this test is not serviceable, for characters written in logwood ink, for instance, will always give up their soluble material sooner than nutgall inks, even if the last named be later applied.  To estimate the age of writing from the amount of bleaching in a given time by hydrochloric or oxalic acid is very precarious, because the thickness of the ink film in a written character is not always the same, and the acid bleaches the thinner layer sooner than the thicker.

The determination of the age of a written paper is a problem difficult of solution.  According to F. Carre the age can be approximately determined if the characters written in iron ink are pressed in a copying press and a commercial hydrochloric acid diluted with eleven parts of water is substituted for water; or, if the written characters are treated for some time with this diluted acid.

The explanation is that the ink changes in time, its organic substance disappears little by little, and leaves behind an iron compound, which in part is not attacked even by acids.

An unsized paper is impregnated with the described diluted acid, copied with the press, and a copy from writing eight or ten years old can be obtained as easily as one by means of water from a writing one day old.

A writing thirty years old gives, by this method, a copy hardly legible, and one over sixty years old, a copy hardly visible.  In order to protect the paper against the action of the acid, it should be drawn through ammoniacal water.

To determine the exact age of writings by the ink is not easy.  The approximate age may be determined with some degree of certainty.  If ink-writings are but a few days old, it is easy to distinguish them from other writing years old.  But to tell by the ink which of two writings is the older, when one is but two months and the other two years, is, as a rule, impossible.

Where during the progress of a trial a document purporting to be years old is introduced in evidence, and it can be shown that it is but a few days old, having been prepared for the occasion, ordinarily the age of the writing will be comparatively easy of demonstration by the expert.  Oxidization will not have set in to any extent, if the ink is very fresh, and this, with a careful watching of the color for any darkening, will determine whether or not the ink is fresh.  This ink study should be a question of the utmost interest to bankers and bank employes.

A ten-per-cent solution of ammonia applied to two inks in question will show which is the fresher.  The older ink will resist the action of the ammonia longer and give up less soluble matter than the newer writing.  Nutgall, and logwood inks, of course, should not be tested comparatively by this method, as the logwood ink will respond to the ammonia sooner than the nutgall ink.