Ink is a general term for any fluid substance which, when applied to a suitable surface, leaves upon it a partially or wholly indelible impression. Any such fluid may be used for writing purposes; but, as the recording material is generally paper, this fluid must have either an affinity for the fibrous matter of which the paper is made, or for the sizing material used to produce upon it a homogeneous surface. This is necessary to prevent the removal of the ink by water; and this power of mordanting itself is one of special importance, as upon it depend the permanency and indelibility of the records. Certain salts have this property, especially salts of iron, which when exposed to the air absorb oxygen, the result of which is that the pale blue- green solution produced by mixing protosalts of iron with vegetable matters containing tannic or gallic acid is converted into a dense blue-black insoluble compound, which cannot be removed from the paper unless it is tampered with by means of chemicals capable of decomposing or destroying it. It is owing to the formation of this insoluble compound that writing-ink, when left in open vessels exposed to the air, becomes thick andropy, and unfit for use. Other black inks are prepared from salts of chromium and vanadium. These inks are in some cases more suitable than the simple writing-inks described above. Sulphate of indigo is also used as a colouring matter. A black ink which lays claim to indelibility is prepared from nigrosine, one of the aniline compounds; but the colour is much inferior to that of ordinary inks, and is not absorbed by the paper fibre to the same extent. Writing-inks are generally acid in character, which causes the corrosion on metal pens; but this property rather tends to enhance the value of the ink, as it retards the bleaching action noticeable in old documents. Creasote, or common wood vinegar, is added to most inks to prevent moulding.
The following will be found excellent recipes for the manufacture of black writing-ink on a small scale: 'With galls and sulphate of iron.—1 lb. bruised galls, 1 gal. boiling water, oz. of sulphate of iron (copperas) in solution, 3 oz. gum-arabic, previously dissolved, and a few drops of an antiseptic, such as carbolic acid. Macerate the galls for twenty-four hours, strain the infusion, and add the other ingredients. With Logwood.—Boil 10 oz. logwood in 20 oz. of water; boil again in 20 oz. more water, and mix the two decoctions; add 2 oz. chrome alum, and boil again for quarter of an hour; and 1 oz. gum-arabic. The product is 25 oz. deep black ink.'
Copying-inks are prepared by adding sugar, gum, or glycerine to ordinary writing-inks. These substances protect the colouring matter (combined with the iron) from the oxidising influences of the air, by forming a skin or impervious varnish over the writing. Thus, when the damp 'tissue' is pressed upon the writing, sufficient unoxidised ink is transferred to stain the paper from back to front, and expose a legible copy on the upper side. Aniline colours dissolved in water holding gum or sugar in solution are also used as copying fluids. Owing to their intense colouring power these inks are useful where many copies from one document are required, but only for temporary use, as on exposure to air or light these colours quickly fade, and the record is lost. Copyable printing-ink is prepared from these materials; and, when written and printed matter is to be copied, as on way-bills or invoices, its use is a great convenience. Copyable pencils are prepared from the same materials (in a solid form). But there is a very serious objection to the use of such inks and pencils: the printed or written matter can be entirely removed by means of alcohol or other solvents.
Many attempts have been made to produce writing-inks which would hinder or render impossible tampering with documents, but without much success. The necessity for such inks seems exaggerated, as it has been found that even with the best manipulative skill and chemical knowledge it is practically impossible wholly to remove writing produced with the common iron and tannin ink, such as is almost universally used.
Coloured Inks.—These are essentially solutions of colouring matters. Red ink is best prepared by dissolving pure carmine in ammonia; blue, by dissolving Prussian blue in oxalic acid; green, by dissolving one per cent. methyl green in warm water. Other colours can also be simply prepared, but not being in demand are not usually met with in commerce. The desirable properties in all writing-inks are that they shall flow freely and not gum or clog the pen, that they shall remain perfectly fluid (without depositing the colouring matter), and that they shall be reasonably permanent in character.
Sympathetic Inks.—These are of great variety, and although possessing an interest to the lover of the marvellous, are not in common use. When weak solutions of cobalt are used (chloride or nitrate), the writing remains invisible until the paper is heated; it then assumes a red colour, which on being exposed to damp air (or breathed upon) changes to green. Salts of lead or bismuth, on being treated with sulphuretted hydrogen, give a black impression. When a weak solution of galls or tannic acid is used the paper on being plunged into a bath of a per-salt of iron will show the characters in black. This is a useful method of restoring faded old writing, and in cases where chemicals have been used with the purpose of removing it.
Printing-ink.—This is a greasy or oily compound in which solid pigments are held in suspension, and is altogether different both in appearance and composition from the writing-inks we have described. It also is usually applied to paper surfaces, and amongst other qualities it must possess the property known to printers as 'distribution'—i.e. of being easily spread out in a layer, the tenuity of which will not cause it to fill up the interstices of and between the types; it must also attach itself to the paper when the type is pressed upon it, detaching itself from the latter entirely; it must possess the apparently opposite properties of drying on the paper within reasonable time, whilst it shall not do so on the type, rollers, or ink-tables; and lastly, it should be permanent in so far as the impressions on paper should have no tendency to change. The various qualities of printing-ink may be described under three heads—viz.: (1) newspaper inks; (2) bookwork inks; and (3) lithographic inks. With the common and consequently cheap printing-inks it is not necessary that the more expensive drying oils be employed as a vehicle or varnish for the colouring matter. Common oils made from paraffin and resin are used, mixed with ordinary lampblack. From this compound, when applied to the surface of printing-paper, the oily matter is absorbed, leaving the colouring matter as a stain on the outside, which does not 'set off' to such an extent as to prevent its employment for cheap periodicals and newspapers. The better class of printing-inks, however, must actually dry upon the surface of the paper in the same manner as paint will dry when applied to a wall. This result is obtained by the employment of drying oils—that is, oils which have the property of absorbing oxygen and becoming gum-like or resinous in character. Thus, when the ink, prepared from linseed, poppy, or walnut oil, is exposed to the atmosphere, especially if assisted by heat, the colouring matter becomes imprisoned or varnished over with a resinous coating, which prevents it setting off or staining any substance brought in contact with it. In preparing the varnish of such inks the oils already mentioned are heated to 500° or 600° F., at which temperature they are kept for a period varying with the degree of viscosity or thickness of the varnish required. In this operation the oil (a compound of fatty acids with glycerine) gradually thickens, without much loss of colour or weight, pungent fumes of acrolein, due to the decomposition of the glycerine, being given off. The varnish so produced is mixed with lampblack, prepared either from coal or burning oil, or from the imperfect combustion of gas, and after very careful grinding is in a condition for use.
The manufacture of coloured inks is practically much the same as for black inks, only great care must be exercised to secure the purity of the varnish, and to see that the chemicals employed do not react upon one another. For example, when vermilion (which is a compound of sulphur and mercury) is employed with colours containing iron or lead, the splendid colour of the vermilion is entirely destroyed by the formation of black sulphur compounds with the iron and lead. It is impossible in such an article to give more than the general outline of this manufacture, with which are connected many mysterious processes for which there is no chemical explanation. The technical manuals are said to contain reliable recipes to guide to its manufacture, and we must refer our readers to these for details. The chief drawback in regard to coloured printing-inks is their tendency to fade on exposure to the air and light. This drawback has become more accentuated since the introduction of coal-tar colours, with which it seems to be the rule that the more brilliant and beautiful they are the less are they fit to resist these destructive influences.
In lithography both writing and printing inks are employed, these being of a peculiar character. The former consists of a soapy fluid holding in suspension fatty matters (shellac, white wax, and tallow), which on being transferred to the stone are absorbed and retained by its porosity. The subsequent application of lithographic printing-ink (which is only the finest variety of printing-ink prepared in an especial manner) to the damp surface of the stone causes it to collect and form a layer on the portions which the lithographic writing-ink has penetrated. As in the case of letterpress-inks, those for lithographic purposes are prepared in black and coloured varieties.
Special inks are prepared for collotype and tin-plate printing. Stamping or obliterating inks may either be prepared by thinning down black or coloured letterpress printing-inks with linseed-oil or turpentine, or by grinding aniline colours with glycerine and treacle.
Indian Ink or China Ink.—This is a mechanical mixture of the purest and densest lampblack, with a solution of gum, gelatine, or of agar-agar. The black paste is dried and pressed so as to form cakes, in which condition it is sold. The lampblack is prepared by burning sesame or other oils, controlling the supply of air so that in place of a clear flame the carbon from the burning oil is deposited in fine flakes in the form of lampblack. For the very finest varieties the material used for this purpose is camphor. The lampblack or carbon so produced is amorphous, and of an intensely black colour. In this condition it is seldom used for the purpose of ordinary writing, but when rubbed down with water forms a material used by draughtsmen for plans, &c. Inks of a similar nature can be prepared by mixing the solutions already mentioned with colouring matter.
Marking-ink.—When certain salts of silver or platinum are applied to textile fabrics these materials are reduced in the fibres of the fabric, and the writing so produced is not removed by the ordinary scouring process to which such articles are subjected. Aniline in the presence of oxidised substances also produces a useful indelible ink.
Ink-stains.—The removal of writing-ink stains from linen is easily effected, by alternately dipping the parts in a solution of oxalic acid and hypochlorite of lime (or soda). If the stains be old and have assumed the brown colour of iron mould, warm diluted muriatic acid will be found effectual in their removal. Where the fabric is coloured the removal of ink-stains is more difficult, as the chemicals employed in the former case are inadmissible. In this case a solution of pyrophosphate of soda may be used with advantage, as this salt does not seriously affect even delicate colours. It is of course necessary to thoroughly wash the fabric after the removal of the stains.