INK is also an appellation given to any coloured liquor used in writing. Different kinds of these inks may be prepared by the directions given under the article COLOUR-MAKING.
Sympathetic Ink, a liquor with which a person may write, and yet nothing appear on the paper after it is dry, till some means are used, as holding the paper to the fire, rubbing it over with some other liquor, &c.
These kinds of ink may be divided into seven classes, with respect to the means used to make them visible; viz. 1. Such as become visible by passing another liquor over them, or by exposing them to the vapour of that liquor. 2. Those that do not appear so long as they are kept close, but soon become visible on being exposed to the air. 3. Such as appear by strewing or sifting some very fine powder of any colour over them. 4. Those which become visible by being exposed to the fire. 5. Such as become visible by heat, but disappear again by cold or the moisture of the air. 6. Those which become visible by being wetted with water. 7. Such as appear of various colours.
I. The first class contains four kinds of ink, viz. solutions of lead, bismuth, gold, and green vitriol, or sulphate of iron. The two first become visible by the contact of sulphureous liquids or fumes. For the first, a solution of common sugar of lead in water answers very well. With this solution write with a clean pen, and the writing when dry will be totally invisible; but
Ink. if it be wetted with a solution of hepar sulphuris, or of orpiment, dissolved by means of quicklime; or exposed to the strong vapours of these solutions, the writing will appear of a brown colour, more or less deep according to the strength of the sulphureous fume. By the same means the solution of nitrate of bismuth will appear of a deep black.
The sympathetic ink prepared from gold depends on the property by which that metal precipitates from its solvent on the addition of a solution of tin. Write with a solution of gold in nitro-muriatic acid, and let the paper dry gently in the shade; nothing will appear for the first seven or eight hours. Dip a pencil in the solution of tin, and draw it lightly over the invisible characters, they will immediately appear, of a purple colour.
Characters written with a solution of green vitriol, will likewise be invisible when the paper is dry; but if wetted with an infusion of galls, they will immediately appear as if written with common ink. If, instead of this infusion, a solution of an alkaline prussiate be used, the writing will appear of a deep blue.
II. To the second class belong the solutions of all those metals which are apt to attract oxygen from the air, such as lead, bismuth, silver, &c. The sympathetic ink of gold already mentioned belongs also to this class; for if the characters written with it are long exposed to the air, they become by degrees of a deep violet colour, nearly approaching to black. In like manner, characters written with a solution of nitrate of silver are invisible when newly dried, but being exposed to the sun, appear of a gray colour like slate. To this class also belong solutions of sugar of lead, nitrates of copper and of mercury, acetate of iron, and muriate of tin. Each of these has a particular colour when exposed to the air; but they corrode the paper.
III. The third class of sympathetic inks contains such liquids as have some kind of glutinous viscosity, and at the same time are long in drying; by which means, though the eye cannot discern the characters written with them upon paper, the powders strewed upon them immediately adhere, and thus make the writing become visible. Of this kind are urine, milk, the juices of some vegetables, weak solutions of the deliquescent salts, &c.
IV. This class, comprehending all those that become visible by being exposed to the fire, is very extensive, as it contains all those colourless liquids in which the matter dissolved is capable of being reduced, or of reducing the paper, into a sort of charcoal by a small heat. Sulphuric acid diluted with as much water as will prevent it from corroding the paper makes a good ink of this kind. Letters written with this fluid are invisible when dry, but instantly on being held near the fire appear as black as if written with the finest ink. Juice of lemons or onions, a solution of sal-ammoniac, green vitriol, &c. answer the same purpose.
V. The fifth class comprehends only a solution of muriate of cobalt; for the properties of which, see CHEMISTRY, No 1608, p. 627.
VI. This class comprehends such inks as become visible when characters written with them are wetted with water. They are made of all such substances as deposit a copious sediment when mixed with water, dissolving only imperfectly in that fluid. Of this kind
are dried alum, sugar of lead, vitriol, &c. We have therefore only to write with a strong solution of these salts upon paper, and the characters will be invisible when dry; but when we apply water, the small portion of dried salt cannot again be dissolved in the water. Hence the insoluble part becomes visible on the paper, and shows the characters written in white, gray, brown, or any other colour which the precipitate assumes.
VII. Characters may be made to appear of a fine crimson, purple, or yellow, by writing on paper with solution of muriate of tin, and then passing over it a pencil dipped in a decoction of cochineal, Brazil-wood, log-wood, yellow wood, &c.