Ink

INK, in its widest signification, a substance employed for pro ducing 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 similar surfaces.

Writing Inks.

Writing inks are fluid substances which con tain colouring matter either in solution or in suspension, and com monly partly in both conditions. They may be prepared in all shades of colour, and contain almost every pigment which can be dissolved or suspended in a suitable medium. The most important of all varieties is black ink, after which red and blue are most commonly employed. Apart from colour there are special quali ties which recommend certain inks for limited applications, such as marking inks, ineradicable ink, sympathetic ink, etc. A good writing ink for ordinary purposes 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, not fading 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-poison ous, that it should as little as possible corrode steel pens, that characters traced in it should dry readily on the application of blotting paper without smearing, and that the writing should not present a glossy, varnished appearance.

Tannin inks

are prepared from galls, or other sources of tan nin, and a salt of iron, with the addition of some agglutinant in the case of the so-called oxidized inks, or a colouring matter in the case of unoxidized inks. Such mixtures form the staple black inks of commerce ; they are essentially an insoluble iron gallate in extremely fine division held in suspension in water or a soluble compound dissolved in water. On long exposure to air, as in ink stands or otherwise, tannin inks gradually become thick and ropy, depositing a slimy sediment.

Ordinary Black Ink.—The essential ingredients of ordinary black ink are—first, tannin-yielding bodies, for which Aleppo or Chinese galls are the most eligible materials; second, a salt of iron, ferrous sulphate (green vitriol) being alone employed; and third, a gummy or mucilaginous agent to keep in suspension the in soluble tinctorial matter of the ink. For ink-making the tannin has first to be transformed into gallic acid. In the case of Aleppo galls this 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. Chinese galls do not contain the ferment neces sary for inducing this change ; and 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 inti mately 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, ex tracts 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 water used being in such proportion to the galls as will produce as nearly as possible a solution having 5% 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% solu tion about 45 grams of the iron salt are used, or about ioo parts of tannin for go 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 ferrosoferric gallate in extremely fine division, sus pended in a coloured solution of ferrous gallate, is formed. To keep the insoluble portion suspended, a mucilaginous agent is em ployed, and those most available are gum senegal and gum arabic. An ink so prepared develops its intensity of colour only after some exposure ; and after it has partly sunk into the paper it be comes oxidized there, and so mordanted into the fibre. As the first faintness of the characters is a disadvantage, it is a common practice to add some adventitious colouring matter to give imme diate distinctness, and for that purpose either extract of logwood or a solution of indigo is used. When logwood extract is em ployed, a smaller proportion of extract of galls is required, log wood itself containing a large percentage of tannin. For making an unoxidized or blue-black ink indigo is dissolved in strong sul phuric acid, and the ferrous sulphate, instead of being used direct, is prepared by placing in this indigo solution a proper quantity of scrap iron. To free the solution from excess of uncombined 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 flows freely, is easily dried and is free from the glossy appearance which arises through the use of gum.

Chinese Ink.—China ink or Indian ink is the form in which ink was earliest prepared, and in which it is still used in China and Japan for writing with small brushes instead of pens. It is ex tensively 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 perfumed.

Logwood Ink.—Under the name of chrome ink a black ink was discovered by Runge, which held out the promise of cheapness combined with many excellent qualities. It is prepared by dis solving 15 parts of extract of logwood in goo parts of water, to which four parts of crystallized sodium carbonate are added. A further solution of one part of potassium chromate (not bi chromate) in ioo parts of water is prepared, and is added very gradually to the other solution with constant agitation. The ink so obtained possesses an intense blue-black colour, flows freely and dries readily, is neutral in reaction and hence does not cor rode steel pens, and adheres to and sinks into paper so that manu scripts written with it may be freely washed with a sponge with out danger of smearing or spreading. It forms a good copying ink, and it possesses all the qualities essential to the best ink; but 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 used especially as copying ink. A very strong decoction of logwood or a strong solution of the extract with ammonium-alum yields a violet ink which darkens slowly on exposure. Such an ink is costly, 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 expenditure of much less logwood. Either sulphate of copper or sulphate of iron may be used, but the former, which produces a pleasing blue-black colour, is to be preferred.

Aniline Inks.—Solutions of aniline dyestuffs in water are widely used as inks, especially coloured varieties. They are usually fugitive. Nigrosine is a black ink, which, although not producing a black so intense as common ink, possesses 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. Its ready flow from stylographic pens led to the name "stylographic ink." Other aniline inks are mentioned below.

Ink which yields by means of pressure an impression, 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 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 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, renders 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 be made stronger, i.e., that it should contain more pigment or body than common ink. It, therefore, is prepared with from 3o to 40% less of water than non-copying kinds; but otherwise, except in the presence of the ingredients above mentioned, the inks are the same. Copying ink pencils con sist of a base of graphite and kaolin impregnated with a very strong solution of an aniline colour, pressed into sticks and dried.

Red and Blue Inks.

The pigment most commonly employed as the basis of red ink is Brazil-wood. 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 in stead of the stannous chloride. Cochineal is also employed as the tinctorial basis of red ink; but, while the resulting fluid is much more brilliant than that obtained from Brazil-wood, it is not so permanent. A very brilliant red ink ma y be prepared by dissolv ing carmine in a solution of ammonia, but this preparation must be kept in closely stoppered bottles. A useful red ink may also be made by dissolving the rosein of Brook, Simpson and Spiller in water, in the proportion of one to from i5o to 200 parts.

For the production of blue ink the pigment principally used is Prussian blue. It is first digested for two or three days with either strong hydrochloric 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 eight parts of Prussian blue to one 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 one part of bleu de Paris in from 200 to 250 parts of water.

Marking Ink.—The ink so called, used principally for mark ing linen, is composed of a salt of silver, usually the nitrate, dis solved 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 brown ish-yellow. Many vegetable juices, e.g., of Coriaria thymifolia, Semecarpus anacardium, Anacardium occidentale (Cashew), are inks of this type.

Gold and silver inks

are writing fluids in which gold and sil ver, or imitations of these metals, are suspended in a state of fine division. In place of gold, Dutch leaf or mosaic gold is frequently substituted, and bronze powders are 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 one part of pure gum arabic and one part of soluble potash glass in four parts of distilled water, into which the requisite quantity of the metallic powder prepared 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. In general one part of foil to three or four 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. Another gold ink de pends upon the formation of purple of Cassius; the linen is mor danted with stannous chloride, and the gold applied as a gummy solution of the chloride.

Indelible or incorrodible ink

is the name given to various combinations 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 charac ters 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 of little practical utility. Characters written in a weak solution of galls develop 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 de velops colour on heating which fades as the paper cools. Among such substances are solutions of the chlorides of cobalt and 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. Writing with rice water and developing with iodine was a device much used during the Indian Mutiny.

Printing Inks.

Printing inks are essentially mixtures of a pig ment and a varnish. The varnish is prepared from linseed oil, rosin and soap ; the oil must be as old as possible ; the rosin may be black or amber and the soap, which is indispensable since it causes the ink to adhere uniformly to the type and also to leave the type clean after taking an impression, is yellow, or turpentine soap for dark inks, and curd soap for light inks. The varnish is prepared as follows : The oil is carefully heated until it "strings" properly, i.e., a drop removed from the vessel on a rod, when placed upon a plate and the rod drawn away, forms a thread about kin. long. The rosin is carefully and slowly added and the mixture well stirred. The soap is then stirred in. The ink is prepared by mixing the varnish with the pigment, and grinding the mass to im palpable fineness either in a levigating mill or by a stone and muller. For black ink, lamp-black mixed with a little indigo or Prussian blue is the pigment employed ; for wood engravings it may be mixed with ivory black, and for copper plates with ivory or Frankfurt black; for lithographic reproductions Paris black is used. Red inks are made with carmine or cochineal; red lead is used in cheap inks, but it rapidly blackens. Blue inks are made with indigo or Prussian blue ; yellow with lead chromate or yellow ochre ; green is made by mixing yellow and blue ; and purple by mixing red and blue.