The term ink is usually restricted to the fluid employed in writing with a pen. Other kinds of ink are indicated by a second word, such as red ink, Indian ink, marking ink, sympathetic ink, printed ink, &c. Common ink is, however, sometimes distinguished as writing ink; its usual ingredients are a decoction of galls and a solution of sulphate of iron, together with gum, to prevent the black precipitate from subsiding. By exposure to the air, the proto-gallate and proto-tannate of iron become per-gallate and per-tannate, a change which accounts for the subsequent blackness of writing executed with pale ink.
In making ink, it is important to regulate the proportion of sulphate to the galls. Mr Lewis gives the proportion of 3 parts galls to 1 part sulphate of iron, but this will vary with the quality of the galls. For 12 gallons of ink, 12 lbs. of nut galls should be bruised and boiled for 3 hours in a copper with 9 gallons of water, the loss by evaporation being made up by occasional additions of water. The decoction is allowed to settle in a tub, the clear liquor is drained off, and the lees are strained. 5 lbs. of gum Senegal are to be dissolved in hot water just sufficient for the purpose, and the mucilage is to be filtered and added to the clear decoction. 5 lbs. of green sulphate of iron is also to be dissolved separately, and well stirred in. When the ink thus formed is tolerably dark in tint, the clear portion should be drawn off into bottles, and be well corked up. To prevent the ink from becoming mouldy, 1 grain of corrosive sublimate, or 3 drops of kerosene, may be added to each pint of ink. Some makers use bruised cloves, and various perfumes: logwood, sumach, and other vegetable astringents, and also sulphate of copper have been used, but not with good effect.
Mr Brande gives the following recipe—Aleppo galls, bruised, 6 oz.; sulphate of iron, 4 oz.; gum-arabic, 4 oz.; water, 6 pints. Boil the galls in the water, then add the other ingredients, and keep the whole in a well stopped bottle, occasionally shaking it. In two months strain off the ink into glass bottles which are to be well corked.
Such ink as the above is liable to become yellow in the course of time, from the decay of the vegetable matter, while mere rust or per-oxide of iron remains. By the careful application of an infusion of galls the writing may be made legible, a plan which has been adopted with success in the recovery of certain ancient manuscripts. Ink is also liable to fade on paper made from inferior rags and bleached by an excess of chlorine; this substance, together with acids and alkalies, effectually destroys writing ink, an objection which did not apply to the writing ink of the ancients, for this was made of finely divided charcoal mixed with some mucilaginous fluid, and resembling a black paint, while modern ink has more of the character of a dye. Indian or China ink may be made with lamp-black, purified with potash lye, mixed with a solution of glue, and dried. If the lamp-black be levigated with pure gelatine, an ink of good colour is produced, although it has not the shining fracture and is not so permanent on paper as genuine China ink. Camphor and other perfumes are added to this ink, and the lamp-black is said to be formed by collecting the smoke of the oil of sesame.
The want of duration in common writing inks has led to many suggestions for superseding vegetable astringents in their composition. The permanent character of carbonaceous inks would probably obtain a demand for them, if they could be made sufficiently fluid for rapid writing. Some years ago, Professor Traill suggested an acetic solution of gluten as the basis of an indelible writing ink. The gluten is obtained by kneading the dough of wheat flour in a stream of water until the starch is completely separated. The gluten should be kept for from 24 to 36 hours in water, and then be digested in acetic acid of sp. gr. 1·033 to 1·034, in the proportion of 3 parts gluten to 20 of the acid. By means of a gentle heat a greyish-white saponaceous fluid is obtained, which may be kept for a long time. Colour is given by from 8 to 12 grains of the best lamp-black and 2 grains of indigo, well incorporated with each fluid ounce of the vehicle. An aroma may be imparted to the ink, by digesting bruised cloves, pimento, or cinnamon, in a portion of the original acid. This ink is of a beautiful black colour, cheap, and cannot be removed by water, chlorine, or dilute acid. It is not adapted for writing on parchment, but may be used on paper with a steel pen, which should be washed after being used.
Another kind of ink, said to be well adapted for steel pens, is made as follows—Exhaust 10 parts of logwood with a sufficient quantity of boiling water to furnish 80 parts of liquid. To 1000 parts of this decoction add gradually 1 part of yellow chromate of potash; the liquid becomes first reddish-brown, and finally bluish-black; gum, or any other addition, is injurious. This liquor is an actual solution, and may be filtered; no deposit is formed in it, and the writing is not removed by immersing the paper in water.
Blue writing inks have been largely substituted for the black. Stephen's ink is made by dissolving Prussian blue (previously washed in dilute muriatic acid) in a solution of oxalic acid. According to Hornung, blue ink is best prepared as follows—Mix 4 parts of perchloride of iron in solution with 750 parts of water; add 4 parts of cyanide of potassium in solution; collect the precipitate, wash it with several additions of water, and allow it to drain until it weighs about 200 parts; add to this 1 part of oxalic acid, and promote the solution of the cyanide by agitation. Such an ink may be kept for a long time. The Rev. J. B. Reade prepares a blue writing ink which is free from acid, and therefore can be used with steel pens as follows—to a solution of iodide of iron add half the weight of the iodine employed in obtaining it; pour this mixture into a semi-saturated solution of yellow prussiate of potash, employing a weight of this salt nearly equal to the whole weight of iodine used in the above iodide solution. Decomposition takes place; the cyanogen of the prussiate of potash and the iron combine, and are precipitated in a solid form; the potassium of the prussiate and the iodine combine to form a neutral iodide of potassium, which remains in solution with a slight excess of iodide of iron. Filter and wash the solid precipitate of cyanogen and iron, and dissolve it in water, thus forming the blue ink required. A portion of the precipitate added to ink made from galls greatly improves its quality. Mr Reade has formulae for various other inks, the whole of which form the subject of a patent.
A permanent writing ink is said to be obtained by triturating the finely divided charcoal employed in the manufacture of Indian ink with a solution of silicate of potash. A similar kind of ink may be obtained by heating leather with caustic potash, and adding gelatinous silica to the black carbonaceous matter thus produced, so as to saturate the potash. A decoction of cochineal mixed with a solution of silicate of potash furnishes a red ink of a very permanent character.
Sir George Stewart Mackenzie some years ago patented a plan for writing with a colourless ink on paper prepared so as to produce either black or blue characters. The clear fluid used as ink is a dilute solution of per-muriate of iron, or some other per salt of iron. The paper is prepared in the course of its manufacture, after the sizing and before the finishing, by brushing in certain powders, the powder for producing dark writing with a clear fluid ink consisting of 3 parts by weight of powdered gall-nuts, 1 part anhydrous ferro-cyanide of potassium, 1 part carbonate of lime and 3 parts rice flour; for blue writing 1 part anhydrous ferro-cyanide of potassium, and 6 or 7 parts rice flour. The same patent also contains a formula for preparing a black indelible ink, by making a stiff paste of the finest lamp-black with a strong mucilage of gum-arabic, and diluting this paste with a solution of 1 part per-muriate of iron to 7 parts water. Copies may be taken on prepared copying paper of writing executed with this ink. The usual plan for copying writing by means of a copying machine is to write the original document with common ink containing a little sugar.
A good red ink may be prepared from 2 oz. of the best Brazil wood, ½ oz. pounded alum, ½ oz. crystals of tartar, boiled with 16 oz. of rain or distilled water down to half; ½ oz. gum-arabic is to be dissolved in the strained liquid; and lastly, there is to be added to it a tincture made with 1 drachm of cochineal, and 1½ oz. of alcohol, of sp. gr. 0·893.
There are many recipes for coloured inks for which there is little or no demand. Sympathetic ink is a solution of a salt of cobalt; the chloride, the nitrate, and the sulphate of cobalt, form inks which are colourless and invisible on paper, but if held near the fire the writing becomes of a beautiful blue. As the paper cools moisture is absorbed and the colour disappears; but it may be reproduced by heat. The addition of a salt of nickel gives a green colour, so that, in preparing what are called magic or chemical landscapes, the sky may be painted with pure chloride, and the trees and the grass with a solution containing nickel. Acetate of cobalt gives an azure blue; chloride of copper a gamboge yellow; chloride of cobalt an olympian green; and bromide of copper a fine brown.
Sympathetic inks. A diluted solution of muriate of copper forms a delicate sympathetic ink, which is invisible until gently heated, when the letters assume a fine yellow colour; and they become again invisible when the paper cools.
Letters written with a solution of acetate of lead are invisible till exposed to the action of sulphuretted hydrogen.
A weak infusion of nut-galls, or other astringent vegetable, will form letters that become legible when touched with a solution of some salt of iron.
Letters written with ferro-cyanide of potass become legible when touched with a solution of sulphate of iron.
The ink used for marking linen is commonly a solution of nitrate of silver written with a pen upon the linen, previously moistened with a solution of potash or soda. In this process the oxide of silver is precipitated upon, and combines with the cloth very firmly. It may, however, be removed by dipping the linen into chlorine water until the silver is converted into a chloride, which may be known by the white colour of the stain; then rinsing in soft water, and washing in water containing ammonia, which quickly dissolves the chloride of silver. Mr Reade prepares a marking ink by rubbing together in a mortar nitrate of silver and the proper equivalent of tartaric acid in a dry state. By the addition of water, crystals of tartrate of silver are formed, and the nitric acid is set free; this acid is neutralized by the addition of liquor ammonia, which also dissolves the tartrate of silver. Gum, colouring matter, and water, can next be added, in quantities which may be varied at pleasure.
Printers' ink is prepared by boiling linseed or nut oil in an iron pot, kindling it and allowing it to burn for half an hour; the vessel is then closely covered over to extinguish the flame; it is again boiled, and the last remains of the grease are by some makers removed by means of slices of bread; that which remains is a fine elastic drying oil, which may be mixed with lamp-black for black ink, with vermilion for the finer works in red ink, or the pigment may be varied according to the colour. For copperplate printers' ink the oil is not boiled so much, and Frankfort black is used for the carbon. For further details on the subject of printers' ink we must refer to the article Printing. A curious account of inks is contained in the work of Caneparini, De Atramentis cujuscunque generis, Rotterdam, 1718.