In the case of printing inks it is rarely possible to render the characters visible again, except when, as in blue printing ink, some iron pigment was present.
The limits of this method of reading writing upon charred paper have recently been investigated by Habermann, who finds that a main essential for the successful working of the process is that the paper itself shall yield a coherent white ash. In the case of common varieties of paper, especially printing paper, which are loaded with china clay and other mineral matter, this condition is admirably fulfilled.
With pure rag papers, however, the ash is much less coherent and is too small in quantity to leave a background. Even in such cases it is possible to increase the amount and coherence of the ash by painting the reverse side of the charred paper with a solution of a mineral fixative agent, such as aluminium acetate. On now drying the paper and continuing the ignition the added substance leaves its own white ash which binds together the ash of the paper.
The fragments of white ash upon which writing has been made visible will obviously be extremely fragile, but they may be rendered firm enough to handle by applying a solution of collodion to the reverse side, which on evaporation leaves a layer of nitro-cellulose similar to that with which incandescent gas mantles are coated.
Any shrinkage or distortion of the letters in the writing caused by the contraction of the ash of the paper during the incineration is obviated or minimised by burning the carbonised paper very slowly.
A record of the revivified writing may also be made by means of photography.
The one pound notes issued by the Bank of England until as late as 1826, appear to have afforded peculiar temptations to forgery, judging by the number of persons convicted of the offence.
[Illustration: TESTS TO DISTINGUISH OLD FROM NEW INKS
The statistics on this point are very remarkable. Between the years 1797 and 1811, 471 people were convicted of uttering the notes or having them in their possession.
In 1814, the number of fraudulent one pound notes detected was 10,342; in 1815, 14,085; in 1816, 21,860; in 1817, 21,241; and during the first three months of 1818, 8,937.