By trying different proportions of galls and sulphate of iron, it was found, that when about in equal quantities (the galls being powdered, and boiled fully to ex. tract their soluble parts) they appeared to he mutually saturated, so that the mixed liquors would receive no additional blackness from a further dose of one or the other.
This, however, was only a rough, ap. proximation to accuracy, for the same ef fect was produced when either substance was also in a small degree superior in quantity to the other. But Dr. Lewis found that an ink, with equal parts of the two, though very black at first, changed to a yellowish brown, upon exposure to the sun and air only for a few days. This was again blackened by washing with fresh gall infusion, and hence it appears in fair inference that the galls are a perishable substance, so that to insure durability, a much greater proportion must enter into the ink than is required for mere saturation in the first instance. Thus it was found that two parts of galls and one of vitriol make a much more du rable ink than with equal parts, and three of galls with one of vitriol was still more durable. When the galls were increased beyond this point, the colour was indeed quite permanent, but it was not of so full a black.
The proportion of water or ether liquid to the solid ingredients will admit of great variation. One part of vitriol, three of galls, and fifty parts of water, gave an ink black enough for common use ; but the finest and blackest was made when only ten of water were employed ; nor was any deficiency in the gallic acid ob. served after fifteen years, though the water was scarcely more than sufficient to cover the galls, and therefore could hardly be supposed capable of extracting all the soluble part of them ; and though the vitriol, from its greater solubility, woidd probably be dissolved entirely, and thus be in greater proportion than usual. Other liquors besides water were tried. or these, white wine and vinegar appeared to answer somewhat better; but any considerable proportion of spirit of wine, or brandy, obviously did harm, owing to the insolubility of the sulphate of iron (as of all the other sulphates) in alcohol, and therefore its diminished so lubility in any liquor is in proportion to the alcohol it contains. A decoction of
logwood used instead of water sensibly improved the beauty of the colour.
Instead of galls other astringents were employed, such as sloes, oak-bark, tor mentil root, &c. ; but though they all gave. a good blue black, with the salt of iron, none of them was equal to the gall nut in this respect.
Other salts of iron were also substitu ted to the sulphate. The muriate and nitrate of iron nearly equalled the sul phate in colour, but proved too corrosive to the paper, and as they were in no re spect preferable to the sulphate, there is no reason for abandoning it.
Imagining that there must be some ex cess of sulphuric acid in common ink, to which the fading might be imputed, Dr. Lewis tried to neutralize it by lime and alkalies, but with manifest injury, the colour being rendered thereby extreme ly fugitive. Another ingenious idea for avoiding the supposed excess of acid was, to separate the black atramentous preci pitate, wash it, and again diffuse it with water thickened with gum. This, indeed, makes a very good ink, but with the capi tal detect of not remaining so long sus pended in the liquor, and especially of not fixing itself to the paper like com mon ink, but rather only slightly adhering like a weak gum varnish, and was readily washed off by water. Hence it appears that the acid of the salt of iron acts as a kind of mordant, or intermede, between the atramentous precipitate and the pa per, and causes a degree of chemical union between them ; a real advantage which this species of ink possesses over all the lamp-black, or China inks, which, indeed, are rather black varnishes.
with regard to the gummy ingredient, the effect of which is rather mechanical, it was found that any other gum-mucilage would answer as well ; but not glue, isin glass, nor animal jelly of any kind. Be sides, as these latter petrify by keeping, this alone would be a strong objection.
Sugar is sometimes added to ink. It makes it flow somewhat easier from the pen, and gives it when dry a gloss which is admired by some, It has this quality, however, of making it very slow in dry ing, which in most cases is an inconve nience.