689. An Indelible Ink for marking Linen.—Pour a little aqua-fortis into a cup, and add to it a small piece of pure silver; when the effervescence ceases, filter the solution through a piece of blotting-paper, and put it into a small phial; then add to it a little gum-arabic and a little of the paint, called sap-green. After the whole is perfectly combined it is then fit for use.
690. To take out Writing.—When recently written, ink may be completely removed by the oxymuriatic acid (concentrated and in solution). The paper is to be washed over repeatedly with the acid; but it will be necessary afterwards to wash it also with lime-water, for the purpose of neutralizing any acid that may be left on the paper, and which would considerably weaken it. But if the ink have been long written, it will have undergone such a change as to prevent the preceding process from taking effect. It ought therefore to be washed with liver of sulphur (sulphuret of ammonia), before the oxymuriatic acid is applied. It may be washed with a hair pencil.
691. To make Old Writing legible.—Take six bruised gall-nuts, and put them to a pint of strong white wine; let it stand in the sun for forty-eight hours. Dip a brush in it and wash the writing, and by the color you will discover whether the mixture is strong enough of the galls.
692. Sympathetic Ink.—With a clean pen write on paper with a solution of muriate of cobalt, so diluted with water, that the writing when dry will be invisible. On gently warming the paper, the writing will appear of a blue or greenish color, which will disappear again when cool. A solution of muriate of copper forms a yellow and sympathetic ink, and acetate of cobalt a rose or purple. If a landscape be drawn representing a winter scene, the paper being overlaid where the foliage should be with the green sympathetic ink, then, on gently warming the drawing, it will represent summer. Sky and water may be drawn with the blue, and standing corn with the yellow ink.
693. Blue Ink.—Dissolve a small quantity of indigo in a little oil of vitriol, and add a sufficient quantity of water, in which gum-arabic has been dissolved.