第39章

"The method I have hitherto found to answer best has been to spread the alkali thin with a feather or a bit of stick cut to a blunt point, though the alkali has occasioned no sensible change of colour, yet the moment that the acid comes upon it, every trace of a letter turns at once to a fine blue, which soon acquires its full intensity, and is beyond comparison stronger than the colour of the original trace had been. If now the corner of a bit of blotting paper be carefully and dexterously applied near the letters, in order to suck up the superfluous liquor, the staining of the parchment may be in a great measure avoided: for it is this superfluous liquor which absorbing part of the colouring matter from the letters becomes a dye to whatever it touches.

Care must be taken not to bring the blotting paper in contact with the letters, because the colouring matter is soft whilst wet, and may easily be rubbed off. The acid I have chiefly employed has been the marine; but both the vitriolic and nitrous succeed very well. They should undoubtedly be so far diluted as not to be in danger of corroding the parchment, after which the degree of strength does not seem to be a matter of much nicety.

"The method now commonly practiced to restore old writings, is by wetting them with an infusion of galls in white wine."(See a complicated process for the preparation of such a liquor in Caneparius De Atramentis, A. D. 1660, p. 277)

"This certainly has a great effect; but is subject, in some degree, to the same inconvenience as the phlogisticated alkali, of staining the substance on which the writing was made. Perhaps if, instead of galls themselves, the peculiar acid of or other matter which strikes the black with iron were separated from the simple astringent matter, for which purpose two different processes are given by Piesenbring and by Scheele, this inconvenience might be avoided. It is not improbable, likewise, that a phlogisticated alkali might be prepared better suited to this object than the common; as by rendering it as free as possible from iron, diluting it to a certain degree, or substituting the volatile alkali for the fixed. Experiment would most likely point out many other means of improving the process described above; but in its present state I hope it may be of some use, as it not only brings out a prodigious body of colour upon letters which were before so pale as to be almost invisible, but has the further advantages over the infusions of galls, that it produces its effect immediately, and can be confined to these letters only for which such assistance is wanted."The Society of Arts in 1830, received a communication from Dr. Bostock, in the course of which he stated that the "tannin, mucilage and extractive matter are without doubt the principal causes of the difficulty which is encountered in the formation of a perfect and durable ink and for a good ink the essential ingredients are gallic acid and a sesqui salt of iron." Owing to his working with galls he was unable to make decisive experiments, but he concludes, and that rightly, that in proportion as ink consists merely of gallate of iron, it is less liable to decomposition and any kind of metamorphosis.

In 1831 the Academy of Sciences in France took up the matter and designated a committee composed of chemists with instructions to study the subject of a permanent ink. After long research it reported that it was unable to recommend any better ink than the tanno-gallate of iron one then in use, but "it should be properly compounded."Peddington investigated, 1841-48, the ancient MSS. collected by the Asiatic Society of Bengal, Calcutta, and published the results in "Examination of Some Decayed Oriental Works in the Library of the Asiatic Society," which are of much interest as relating to "mineral" inks, the "gall" inks being unknown in Asia after the twelfth century.

Up to thirty-five years ago, the manufacture of "gall" inks necessitated a complicated series of processes and long periods of time to enable the ink to settle properly, etc. It was Professor Penny of the Anderson University who suggested the way to avoid one of the processes pertaining to ink-making by utilizing the known fact, that tannin is more soluble in cold than in warm or hot water. It was adopted all over the world and revolutionized the manufacture of ink, by doing away with boiling processes and hot macerations of ingredients. With hardly in exception the best tanno-gallate of iron ("gall") inks are now "cold" made.