The ink used by printers is compounded mainly of two ingredients, colouring matter and varnish. The former varies according to the quality and tint of the ink; the latter may be obtained by natural resinous substances, or by mixing oil, rosin, and soap.
(A). Black. - (a) The chief colouring matter in black printing-ink is vegetable lampblack. The price of the best qualities precludes their use except for specially fine ink; nevertheless, good ink cannot be made with inferior samples. An undue proportion of lampblack in the ink will cause it to smear, however long it may have been printed, and to "set-off" during bookbinding operations. Thus the thickest inks are not the best, if the lamp-black is more than the varnish can bind. Ivory-black is too heavy to be used alone; but a proportion ground up with the other ingredients makes a valuable ink for producing the best possible effect with wood-engravings. Only the best and blackest is admissible. Prussian blue, ground exceedingly fine, and used sparingly, deepens the colour of ink; in excess, it gives a cold appearance. Indigo may replace Prussian blue. Perhaps the blackest tint is produced by equal quantities of each. To give a rich tone, and remove the coldness caused by indigo and Prussian blue, the addition of a little Indian red is strongly recommended.
The natural resinous substances employed as a source of varnish are balsam of copaiba and Canada balsam. The former is superior, and when old and pure may be used without any preparation. The latter is much thicker and dries more quickly, and cannot therefore be used alone: but for a strong ink, a small proportion may with advantage be added either to the balsam of copaiba or to the artificial varnish now to be described.
The basis of the artificial varnish is linseed-oil, which should be as old as possible. Of all other oils, the only one recommended as a substitute is nut-oil. The rosin used may be either black or amber. It melts in the boiling oil, and combines with it, preventing its separation from the colouring matter and staining of the paper, and binding the ink to prevent its smearing. The properties possessed by soap, which render it such an indispensable ingredient of printing-ink, are that it causes the ink (1) to adhere uniformly to the face of the type, (2) to coat it completely with the smallest quantity, (3) to leave the face of the type clean, and attach itself to the surface of the damp paper by the action of pressure, and that repeatedly, (4) to wash easily off the type, and (5) to never skin over, however long it may be kept. For all dark inks, well-dried yellow or turpentine soap may be employed; for light-tinted inks, curd soap is preferable. Used in excess, soap tends (1) to render the colour unequal, where a large surface is printed, (2) to spread over the edges of the type, so as to give them a rough appearance, and (3) to prevent the ink drying quickly, and cause it to "set off" when pressed. It is thus opposed to the binding quality of the rosin.
Its due proportion is when the ink works clean, without clogging.
The combination of these several ingredients is effected in the following manner: - Into an iron vessel having 2 to 3 times the capacity of the materials it is to receive, put 6 qt. linseed-oil, and make a fire under it. After a time, the oil simmers and bubbles up, but as the temperature increases the surface resumes placidity; next it commences to smoke, and then to boil, emitting a very strong odour; as the boiling continues, a scum arises. At this stage, repeated tests should be made to ascertain whether the escaping vapours will ignite. At the moment when they will do so, the pot is removed from the fire and placed on the ground, and the contents are stirred with an iron spatula, and kept burning. The pot is covered occasionally to extinguish the flame, while samples are withdrawn to test the consistence. When drops of the oil let fall upon a porcelain surface will draw out into strings about 1/2 in. long, the oil is suited for ink for ordinary book-work. The flame is then extinguished by firmly replacing the cover.
On removing it, there is a great escape of strong-smelling smoke, and much froth; the latter is made to subside by thorough stirring, and when this is accomplished, but not before, 6 lb. of amber or black rosin is gradually introduced and stirred in.
When the rosin is dissolved, 1 3/4 lb. of dry brown or turpentine soap, in slices, is stirred in gradually and cautiously, as it froths copiously. When all the soap is in, and the frothing has ceased, the pot is returned to the fire till its contents boil, constant stirring being maintained. This completes the varnish. Into an earthenware pot, or a tub, of sufficient capacity, is put 5 oz. of Prussian blue or indigo, or the two combined; then 4 lb. of the best " mineral lampblack," and 3 1/2 lb. of good lampblack; next add the varnish by degrees while warm, stirring meantime and until all the ingredients are thoroughly mixed; finally pass it through a levigating mill, or between the stone and muller, and reduce it to impalpable fineness. (6) A fine, intensely black, strong ink, without the use of oil - and rosin, may be made in the following manner: - 9 oz. balsam of copaiba, 3 oz. lampblack, 1 1/4 oz. indigo or Prussian blue, or equal proportions of each, 3/4 oz. Indian red, 3 oz. dry turpentine soap, ground between a muller and a stone to impalpable fineness.
This is an excellent ink for giving good effect to highly-finished wood-engravings. (c) In Germany an ink, prepared as follows, has been used, and is said to yield a very clear and fine impression when properly prepared: - Venice turpentine, 2 1/4 oz.; soap, in thick paste, 2 1/2 oz.; oleine, rectified, 1 oz.; carbon black, 1 1/4 oz.; Paris blue, 1/4 oz.; oxalic acid, 1/8 oz.; water 1/4 oz. The three last ingredients are mixed into a paste. The turpentine and oleine are mixed at a gentle heat, the soap and carbon then introduced, and, after cooling, the blue paste is added, the whole betng ground beneath a muller to a very fine and smooth paste. (d) Kercher and Ebner's printing-ink is prepared by first dissolving iron in sulphuric, hydrochloric, or acetic acid. Half the solution is oxidized by means of nitric acid, after which the two halves are mixed, and precipitation is produced by the oxide of iron. The precipitate is filtered, washed, and mixed with equal parts of tannic and gallic acid, which produces a black bordering on blue.