F. Carre also gives another method for determining, approximately, the age of ink-writings.  If the writing is in iron ink, and is moistened with a solution of one part of hydrochloric acid to eleven parts of water and put in letter-copying press and copy transferred to copy paper it should give a strong copy, if but ten years old; a hardly legible copy, if thirty years old; and if sixty years old, a few marks will be copied, but they will not be legible.

If the same solution be used in place of water, as in the ordinary letter-copying process and the copying paper be saturated with it, the result will be the same.

To determine the age of writing by applying bleaching acids and watching results and counting the seconds is a dangerous method.  Thick inks will respond to the acids slower than thin, and the time comparisons are misleading.

Safety inks, so-called, designed to resist the action of acids and alkalies have been repeatedly put upon the market, but no such ink has ever successfully challenged the world and proved its title of safety.

Many chemicals are recommended as restorations for faded writing, but these should be avoided as far as possible, as they are liable to stain, disfigure the paper, and in the end make matters materially worse.  Familiarity with particular handwritings after some practice will enable the reader to make out otherwise unintelligible words without any other assistant than a powerful magnifying glass.

If the ink is very faint, the simplest and most harmless restorative is sulphate of ammonia, but its loathsome smell once encountered is not easily forgotten.  The experiment in consequence is very seldom repeated for the result is scarcely good enough to risk a repetition of so horrible a smell.

The writing on old and faded documents may be restored, by chemical treatment, turning the iron salt still remaining into ferrous sulphate.  A process which will restore the writing temporarily is as follows:  A box four or five inches deep and long and broad enough to hold the document, with a glass, is needed.  A net of fine white silk or cotton threads is stretched across the box at about one half the depth.  Two saucers containing yellow ammonium hydrosulphide are placed in the bottom of the box.  By means of a clean sponge or brush, moisten the paper with distilled water; then place it on the net with the writing side down.  The action of the vapor of the ammonium hydrosulphide will cause the obliterated writing to slowly turn brown, then black.  But within a short time after removal from the box the writing will again disappear.

Another method is to wash the document carefully in a solution of hydrochloric acid, one part, and distilled water, one hundred parts.  Dry the moistened paper somewhat, leaving it just moist enough to hold a uniform layer of fine yellow prussiate of potash.  A plate of glass with a light pressure should be placed on this.  In a few hours dry the paper thoroughly, and carefully brush off the yellow prussiate of potash.  The writing should come out a Prussian blue.  This restored writing will be permanent unless exposed too much to the light.