The preparing of engraver's ink does nut require the same delicacy with that of printing ink. The black, indeed, requires to be well chosen ; but when this is done, every engraver mixes up his own ink, which is not the case with the printer.
Our common writing ink is a substance of a totally dif ferent kind, both in its colouring matter, and its menstruum. The colouring matter is a tenuious substance, formed by precipitating iron from a saline solution, by the gallic acid and tannin, and the vehicle is water, slightly impregnated with a mucilage. But, though this is the radical principle on which ink is made, the preparing of good ink requires considerable nicety in the choice of materials, as well as skill in the manipulation. Although any salt of iron would give a dal k precipitate, when treated with astringent mat ter, the only one which gives a good black is the sulphate, and the only form of astringent which answers is the gall nut. It is, indeed, often improved by conjoining it with a little logwood. It has been observed that many inks are liable to become pale, or to disappear.
The learned and laborious Dr. Lewis instituted a series of minute experiments, for the purpose of determining the just proportion of the different ingredients, and obtained some interesting chemical results. Ile found that equal parts of this salt, and of blue or Aleppo galls, gave an ink which, though of a good black when first used, became yellowish brown when the writing was kept for a moderate time ; that in proportion to the quantity of the sulphate, the inks were less durable in colour ; and that those in which the galls were in excess were most durable. Thus it ap peared that the galls were the most perishing article in ink after it is applied to the paper, and therefore ought to be in largest proportion ; that a durable ink should contain at least three parts of galls for one of the sulphate of iron ; but when much above this proportion, they render the co lour too pale.
The degree of dilution of ink admits of considerable lati tude. The fullest and blackest colour is produced, when the water is just sufficient to cover the powdered ingredi ents. But 40 or 50 ounces of water to one ounce of the metallic salt, and three of galls, make an ink sufficiently black for common purposes. Distilled or rain water was found, by Lewis, preferable to common water. But white wine formed a blacker ink than water, and vinegar formed one still blacker than wine. Spirit injured the colour, and occasioned a precipitation of part of Inc colouring matter. A decoction of logwood instead of water improved both the beauty and the deepness of the black. A piece of iron kept in ink fur a length of time after it is made, improves i:s colour, probably by uniting with the sulphuric acid dis engaged by the galls. In order to give ink a greater con sistence, and enable it better to keep the colouring matter suspended, g-uol arable is added, and it probably preserves the black matter in a state of greater tenuity, preventing, by its mechanical qualities, the attraction of cohesion from forming that matter into larger particles, and thus preserv ing it nearer a state of solution than it would otherwise be.
The gum also prevents it from spreading on the paper. A greater body of colour is thus collected on each stroke, and the writing retains its blackness much longer than when none is added. A common addition is sugar ; but its only good quality is that of communicating a shining gloss to the writing. It renders the ink more tedious in drying, and a sufficient glossiness is obtained by the gum alone. Dr. Lewis found that the addition of other metallic substan ces which had been sometimes recommended, such as sul phate of zinc or of copper, injured ultimately the quality of the ink ; and that the galls ought to be finely powdered, and not merely bruised, as some had directed.
The recipe of Dr. Lewis, deduced from his numerous trials for making good ink, was, one ounce of powdered sul phate of iron, one of powdered logwood, and one of gum, with three of powdered galls, and a quart of white wine or vinegar, although water will serve common purposes. These ingredients are to be put in a glass or other conve nient vessel not metallic, and the mixture shaken four or five times a-day. In ten or twelve days it will be fit for use, and sooner if in a warm situation ; but it continues for a long time to improve if left without decantation. When it is separated from the powdery residue, it will be kept in a good state with greater certainty, if soma broken galls, freed from the fine powder, and some pieces of iron, are put in it. Iron, however, is the only metal which it is safe to retain in contact with ink. Leaden or copper vessels, when used for containing it, become dissolved by its acid ingre dients, and part of the iron is precipitated. This fact ap peared an anomaly during the prevalence of the doctrines of elective affinity, as explained by Bergman. As iron pre cipitates other metals from their solutions, it appeared con trary to all chemical principles to suppose that these other metals were capable of precipitating iron. But, since the investigations of Berthollet have imparted a new, and in some respects more correct, light to the doctrines of che mical affinity, we acknowledge that the acid exerts a simul taneous attraction for both metals, and dissolves a propor tion of each, depending on their relative affinities. These precautions relating to ink were clearly laid down by Dr. Lewis as the result of practical experience, while the doc trines of Berthollet were as yet unknown, and in that state of science such precautions appeared inexplicable ; but, in its present state, they are recognised as furnishing a beauti ful illustration of them.