第38章

"As to the durability of the more ancient inks, it seemed, from what occurred to me in these experiments, to depend very much on a better preparation of the material upon which the writing was made, namely, the parchment or vellum; the blackest letters being those which had sunk into it deepest. Some degree of effervescence was commonly to be perceived when the acids came into contact with the surface of these old vellums. Iwas led, however, to suspect, that the more modern;for in general the tinge of colour, produced by the phlogisticated alkali in the acid laid upon them, seemed less deep; which, however, might depend in part upon the length of time they have been kept: and perhaps more gum was used in them, or possible they were washed over with some kind of varnish, though not such as gave gloss.

"One of the specimens sent me by Mr. Astle, of the fifteenth century, and the letters were those of an engrossing hand, angular, without any FINEstrokes, broad and very black. On this none of the above-mentioned re-agents produced any considerable effect; most of them seemed to make the letters blacker, probably by cleaning the surface;and the acids, after having been rubbed strongly on the letters, did not strike any deeper tinge with the phlogisticated alkali. Nothing had a sensible effect toward obliterating these letters but what took off part of the surface of the vellum, when small rolls, as of a dirty matter, were to be perceived. It is therefore unquestionable, that no iron was used in this ink; and from its resistance to the chemical solvents, as well as a certain clotted appearance in the letters when examined closely, and in some places a slight degree of gloss, I have little doubt but they were formed with a composition of a black, sooty or carbonaceous powder and oil, probably something like our present printer's ink, and am not without suspicion that they were actually printed (a subsequent examination of a larger portion of this supposed MSS. has shown that it is really a part of a very ancient printed book).

"Whilst I was considering of the experiments to be made, in order to ascertain the composition of ancient inks, it occurred to me that perhaps one of the best methods of restoring legibility to decayed writing might be to join phlogisticated alkali with the remaining calx of iron, because, as the quantity of precipitate formed by these two substances very much exceeds that of the iron alone, the bulk of the colouring matter would thereby be greatly augmented. M. Bergman was of opinion that the blue precipitate contains only between a fifth and a sixth part of its weight of iron, and though subsequent experiments tend to show that, in some cases at least, the proportion of iron is much greater, yet upon the whole it is certainly true, that if the iron left by the stroke of a pen were joined to the colouring matter of phlogisticated alkali, the quantity of Prussian blue thence resulting would be much greater than the quantity of black matter originally contained in the ink deposited by the pen, though perhaps the body of colour might not be equally augmented. To bring the idea to the test, I made a few experiments as follows:

"The phlogisticated alkali was rubbed upon the bare writing in different quantities, but in general with little effect. In a few instances, however, it gave a bluish tinge to the letters, and increased their intensity, probably where something of an acid nature had contributed to the diminution of their colour.

"Reflecting that when phlogisticated alkali forms its blue precipitate with iron the metal is first usually dissolved in an acid, I was next induced to try the effect of adding a dilute mineral acid to writing besides the alkali. This answered fully to my expectations, the letters changing very speedily to a deep blue colour, of great beauty and intensity.

"It seems of little consequence as to the strength of colour obtained, whether the writing be first wetted with the acid, and then the phlogisticated alkali be touched upon it, or whether the process be inverted, beginning with the alkali; but on another account I think the latter way preferable. For the principal inconvenience which occurs in the proposed method of restoring MSS. is, that the colour frequently spreads, and so much blots the parchment as to detract greatly from the legibility; now this appears to happen in a less degree when the alkali is put on first, and the dilute acid is added upon it.