INK (French, encre; German, Tinte), in its widest signification, is the medium employed for producing graphic tracings, inscriptions, or impressions on paper or similar materials. The term includes two distinct conditions of pigment or colouring matter :the one fluid, and prepared for use with a pen or brush, as writing ink; the other a glutinous adhesive mass, printing ink, used for transferring to paper impressions from types, engraved plates, and other like surfaces. The latter class may be more properly dealt with under LITHOGRAPHY and PRINTING (q.v.).
Writing inks are fluid substances which contain colouring matter either in solution or in suspension, and very com-monly partly in both conditions. They may be prepared in all shades of colour, and contain in their composition almost every pigment which can be dissolved or suspended in a suitable medium ; but by far the most important of all varieties is black ink, after which red and blue are the colours most commonly employed. Other colours are only occasionally employed; but apart from colour there are special qualities which recommend certain inks for limited applications, such as marking inks, ineradicable ink, sym-pathetic ink, &c.
Black Ink.Among the qualities which should char-acterize a good black ink for ordinary purposes, the follow-ing are important. It should continue limpid, and flow freely and uniformly from the pen; it should not throw down a thick sludgy deposit on exposure to the air ; nor should a coating of mould form on its surface. It should yield distinctly legible characters immediately on writing, which ought to become a deep blue-black, not fading or decaying with age ; and the fluid ought to penetrate into the paper without spreading, so that the characters will neither wash out nor be readily removed by erasure. Further, it is desirable that ink should be non-poisonous, that it should as little as possible corrode steel pens, that characters traced in it should dry readily on the applica-tion of blotting paper without smearing, and that the writing should not present a glossy varnished appearance. To obtain these characteristics is the chief object of the ink manufacturer; and upon the whole they have hitherto been found best combined in ink prepared from galls, or other sources of tannin, and a salt of iron, with the addition of some colouring matter. Such a compound indeed forms the staple black ink of commerce, which is essentially a ferroso-ferric gallate in extremely fine division held in sus-pension in water.
The essential ingredients of this ordinary black ink are first, tannin-yielding bodies, for which Aleppo or Chinese galls are the only eligible materials; second, a salt of iron, the ferrous sulphate (green vitriol) being alone employed ; and third, a gummy or mucilaginous agent to keep in suspension the insoluble tinctorial matter of the ink. For ink-making the tannin, from whatever source obtained, has first to be transformed into gallic acid; and, as has been shown by Viedt, in the case of Aleppo galls that change takes place by fermentation when the solution of the galls is exposed to the air, the tannin splitting up into gallic acid and sugar (see GALLIC ACID, vol. x. p. 41). Chinese galls, which formerly were considered unsuitable for the manufacture of ink, do not contain the ferment necessary for inducing this change; and therefore to induce the process yeast must be added to their solution. To prepare a solution of Aleppo galls for ink-making, the galls are coarsely powdered, and intimately mixed with chopped straw. This mixture is thrown into a narrow deep oak vat, provided with a perforated false bottom, and having a tap at the bottom for drawing off liquid. Over the mixture is poured lukewarm water, which, percolating down, extracts and carries with it the tannin of the galls. The solution is drawn off and repeatedly run through the mixture to extract the whole of the tannin, the quantity of water used being in such proportion to the galls as will produce as nearly as possible a solution having 5 per cent, of tannin. The object of using straw in the extraction process is to maintain the porosity of the mixture, as powdered galls treated alone become so slimy with mucilaginous extract that liquid fails to percolate the mass. For each litre of the 5 per cent, solution about 45 grammes of the iron salt are used, or about 100 parts of tannin for 90 parts of crystallized green vitriol. These ingredients when first mixed form a clear solution, but on their exposure to the air oxidation occurs, and an insoluble blue-black ferroso-ferric gallate in extremely fine division, suspended in a coloured solution of ferrous gallate, is formed. To keep the insoluble portion suspended, a mucilaginous agent is employed, and that most available is gum Senegal. An ink so prepared develops its intensity of colour only after some exposure ; and after it has partly sunk into the paper it becomes oxidized there, and so mordanted into the fibre. But, as the first faintness of the characters is a dis-advantage, it is a common practice to add some adventitious colouring matter to give immediate distinctness, and for that purpose either extract of logwood or a solution of indigo is used. When logwood extract is employed, a smaller proportion of extract of galls is required, from the fact that logwood itself contains a large percentage of tannin. Black ink in which the provisional colouring matter is indigo was introduced about the year 1856, under the name alizarin ink, although the substance alizarin has nothing whatever to do with the preparation. The indigo for this ink is dissolved in strong sulphuric acid, and the ferrous sulphate, instead of being used direct, is prepared by placing in this indigo solution a proper quantity of scrap iron. In order to free the solution from excess of uncom-bined acid, chalk or powdered limestone is added, whereby the free acid is fixed and a deposit of sulphate of lime formed. A solution so prepared, mixed with a tannin solution, yields a very limpid sea-green writing fluid, and as all the constituents remain in solution, no gum or other suspending medium is necessary. In consequence the ink i flows freely, is easily dried, and is free from the glossy appearance which arises through the use of gum. C. H. Viedt of Brunswick, who has written very exhaustively on all kinds of ink, gives the following as the standard in-gredients of these three varieties of ink:
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On long exposure to air, as in inkstands, or otherwise, all these varieties of ink gradually become thick and ropy, depositing a slimy sediment. This change on exposure is inevitable, resulting as it does from the gradual and at the same time essential oxidation of the ferrous compound, and it can only be retarded by permitting access of air to as small surfaces as possible. The inks also have a tend-ency to become mouldy, an evil which may be obviated by the use of a minute proportion of carbolic acid ; or, should that body be objectionable on account of its smell, salicylic acid may be used.
Under the name of chrome ink a black ink was discovered and prepared by the chemist Runge, which held out the promise of cheapness combined with many excellent quali-ties. It is prepared by dissolving 15 parts of extract of logwood in 900 parts of water, to which 4 parts of crystal-lized sodic carbonate are added. A further solution of 1 part of chromate (not bichromate) of potassium in 100 parts of water is prepared, and is added very gradually to the other solution with constant agitation. Thereby is obtained an ink which possesses an intense blue-black colour, which flows freely and dries readily, which being neutral in reac-tion does not corrode steel pens, and which so adheres to and sinks into paper that manuscripts written with it may be freely washed with a sponge without danger of smearing or spreading. It forms a good copying ink, and in short it possesses all the qualities essential to the best ink; but unfortunately on exposure to air it very readily undergoes decomposition, the colouring matter separating in broad flakes, which swim in a clear menstruum. It is affirmed by Viedt that this drawback may be overcome by the use of soda, a method first suggested by Bottger.
Logwood forms the principal ingredient in various other black inks which are now much used, especially as copying ink. A very strong decoction of logwood or a strong solu-tion of the extract with ammonia-alum yields a violet ink which darkens slowly on exposure. Such an ink is costly, however, on account of the concentrated condition in which the logwood must be used. If, however, a metallic salt is introduced, a serviceable ink is obtained with the expendi-ture of much less logwood. Either sulphate of copper or sulphate of iron may be used, but the former, which pro-duces a pleasing blue-black colour, is to be preferred. The following is the formula most highly recommended for this ink. A clear solution of 20 kilos of extract of logwood in 200 litres of water is obtained, to which is added, with agitation, 10 kilos of ammonia-alum dissolved in 20 litres of boiling water. The solution is acidified with 0'2 kilo of sulphuric acid, which has the effect of preventing any deposit, and finally there is added a solution of 1 '5 kilos of sulphate of copper dissolved in 20 litres of water. This compound is exposed to the air for a few days to allow the colour to develop by oxidation, after which it is stored in well-corked bottles. The acid condition of this ink has a corrosive influence on steel pens ; but in all other respects it is a most valuable writing fluid.
A black ink under the name of nigrosin is prepared from a soluble aniline colour, which, although not producing a black so intense as common ink, is possessed of various advantages. Being perfectly neutral, it does not attack pens; it can easily be kept of a proper consistency by making up with water; and its colour is not injuriously affected by the action of acids.
Copying Ink.Ink which yields by means of pressure an im-pression, on a sheet of damped tissue paper, of characters written in it is called copying ink. Any ink soluble in water, or which retains a certain degree of solubility, may be used as copying ink. Runge's chrome ink, being a soluble compound, is, therefore, so available ; and the other logwood inks as well as the ordinary ferrous gallate inks contain also soluble constituents, and indeed are essentially soluble till they are oxidized in and on the paper after exposure to the air. To render these available as copying inks it is only necessary to add to them a substance which will retard the oxidizing effect of the air for some time. For this purpose the bodies most serviceable are gum arabic or Senegal, with glycerin, dextrin, or sugar, which last, however, has the disadvantage of rendering the ink sticky. These substances act by forming a kind of glaze or varnish over the surface of the ink which excludes the air. At the same time when the damp sheet of tissue paper is applied to the writing they dissolve and allow a portion of the yet soluble ink to be absorbed by the moistened tissue. As copying ink has to yield two or more impressions, it is necessary that it should he made stronger, i.e., that it should contain more pig-ment or body than common ink. It, therefore, is prepared with from 30 to 40 per cent, less of water than non-copying kinds ; but otherwise, except in the presence of the ingredients above alluded to, the inks are quite the same.
Red Ink.The pigment most commonly employed as the basis of red ink is Brazil-wood, a dye-stuff which yields a good durable ink. Such an ink is prepared by adding to a strong decoction of the wood a proportion of stannous chloride (tin spirits), and thickening the resulting fluid with gum arabic. In some instances alum and cream of tartar are used instead of the stannous chloride. Cochineal is also employed as the tinctorial basis of red ink ; but, while the re-sulting fluid is much more brilliant than that obtained from Brazil-wood, it is not quite so permanent. A very brilliant red ink may be prepared by dissolving carmine in a solution of caustic ammonia, but it is necessary to keep this preparation in closely stoppered bottles. A useful red ink may also be made by dissolving the rosein of Brooke, Simpson, and Spiller in water, in the proportion of 1 to from 150 to 200 parts.
Blue Ink.For the production of blue ink the pigment princi-pally used is Prussian blue. To render this colour soluble in water it is first digested for two or three days with either strong hydro-chloric acid, sulphuric acid, or nitric acid, the digested mass is next very largely diluted with water, and after settling the supernatant liquid is siphoned away from the sediment. This sediment is repeatedly washed, till all traces of iron and free acid disappear from the water used, after which it is dried and mixed with oxalic acid in the proportion of 8 parts of Prussian blue to 1 of the acid, and in this condition the material is ready for dissolving in water to the degree of colour intensity necessary. An aniline blue ink may be prepared by dissolving 1 part of bleu de Paris in from 200 to 250 parts of water.
China ink or Indian ink is the form in which ink was earliest prepared, and in which it is still made and used in China and Japan for writing with small brushes instead of pens. It is exten-sively used by architects, engineers, and artists generally, and for various special uses. China ink is prepared in the form of sticks and cakes, which are rubbed down in water for use. It consists essentially of lamp-black in very fine condition, baked up with a glutinous substance; and the finer Oriental kinds are delicately per-fumed. The following description of the manufacture as conducted in Japan is from a native source : "The body of the ink is soot obtained from pine wood or rosin, and lamp-black from sesamum oil for the finest sort. This is mixed with liquid glue made of ox skin. This operation is effected in a large round copper bowl, formed of two spherical calottes, placed 1 inch apart, so that the space be-tween can be filled up with hot water to prevent the glue from hardening during the time it is being mixed by hand with the lamp-black. The cakes are formed in wooden moulds, and dried between paper and ashes. Camphor, or a peculiar mixture of scents which comes from China, and a small quantity of carthamine (the red colouring substance of safflower), are added to the best kinds for improving the colour as well as for scenting the ink. There is a great difference both in price and in quality of the various kinds of ink, the finest article being rather costly." It is said that the size used in Chinese kinds is of vegetable origin.
Marking Ink.The ink so called, used principally for marking linen, is composed of a salt of silver, usually the nitrate, dissolved in water and ammonia, with a little provisional colouring matter and gum for thickening. The colour resulting from the silver salt is developed by heat and light ; and the stain it makes, although exceedingly obstinate, gradually becomes a faint brownish-yellow. The following yields a good marking ink. Equal parts of nitrate of silver and dry tartaric acid are triturated in a mortar, and treated with water, when a reaction takes place, resulting in the formation of tartrate of silver and the liberation of nitric acid. The acid is neutralized, and at the same time the silver tartrate is dissolved by the addition of ammonia, and this solution with colouring matter and gum forms the ink, which may be used with an ordinary steel pen.
Gold and silver inks are writing fluids in which gold and silver, or imitations of these metals, are suspended in a state of fine division. In place of gold, Dutch leaf or mosaic gold is frequently substi-tuted, and bronze powders are also used for preparing a similar kind of ink. The metallic foil is first carefully triturated into a fine paste with honey, after which it is boiled in water containing a little alkali, and then repeatedly washed in hot water and dried at a gentle heat. A solution is prepared consisting of 1 part of pure gum arabic and 1 part of soluble potash glass in 4 parts of dis-tilled water, into which the requisite quantity of the metallic powder prepared as above is introduced. Owing to the superior covering nature of pure gold, less of the metal is required than is necessary in the case of silver and other foils ; but the proportion varies according to the colour and condition of the surface upon which the ink is to be used. In general 1 part of foil to 3 or 4 parts of solution is sufficient. The metallic lustre of writing done with this solution may be greatly heightened by gently polishing with a burnishing point.
Indelible or incorrodible ink is the name given to various combi-nations of lamp-black or other carbonaceous material with resinous substances used for writing which is exposed to the weather or to the action of strong acids or alkaline solutions. An ink having great resisting powers may be conveniently prepared by rubbing down Indian ink in common ink till the mixture flows easily from the pen. Other combinations have more the character of coloured varnishes.
Sympathetic inks are preparations used for forming characters which only become visible on the application of heat or of some chemical reagent. Many chemicals which form in themselves colourless solutions, but which develop colour under the influence of reagents, may be used as sympathetic ink, but they are all of little practical utility. Characters written in a weak solution of galls de-velop a dark colour on being treated with a solution of copperas; or, vice versa, the writing may be done in copperas and developed by the galls solution. Writing done in various preparations develops colour on heating which fades as the paper cools. Among such substances are solutions of the acetate and the chloride of cobalt and of the chloride of nickel. Very dilute solutions of the mineral acids and of common salt and a solution of equal parts of sulphate of copper and sal-ammoniac act similarly. Characters traced in a weak solu-tion of nitrate of silver darken on exposure to light, and terchloride of gold solution develops a purple colour on exposure. (J. PA.)