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: A FORGED RECEIPT] [Illustration: PORTION OF THE SAME _By kind permission of "Knowledge"_] [Illustration: TESTS TO DISTINGUISH OLD FROM NEW INKS (SEE PAGE 110)] 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. The ease with which the notes could be imitated, and the readiness with which they could be circulated, caused hundreds of people to take up the trade of forgery, until at length whole days were occupied at the Old Bailey with the endless trials and convictions. Much indignation was expressed in the newspapers that the Bank had not issued notes which could not be imitated, and as a result of this outcry, a committee of scientific men was appointed to examine and report upon the best means of checking the evil. Their report stated that they had examined many specimens of engraving, but none that had been submitted to them was proof against skilful imitation. Most of the forged notes had been clumsily imitated, and from this it appeared that the public were quite ready to be deceived by them. It was commonly believed, though without foundation, that the Bank placed a private mark upon their notes by which they could subsequently be identified.