To such an ink a slight amount of some one of the water soluble coal-tar colors is usually added to give the desired initial color to the ink when used in writing.
Cheaper grades of black writing-ink are pro duced by substituting for the nutgalls other tannin containing substances, or by using log wood. In these other iron salts, or salts of other metals are sometimes used as of copper, aluminum or chromium. For special purposes some of these have certain advantages. For copying, for instance, the ink made from log wood with alum cake and chromate has the highest efficiency known. This ink, however, fades out after a few years' exposure to the open air and daylight.
Within modern times colored liquid solu tions have come much into general use as inks, made up with aniline and other dyestuff colors. They are easily and cheaply made, flow nicely from the pen, and allow of a great variety as to choice in coloring, but none of them have the permanency of the ancient nutgall iron ink on exposure to light and air.
The usual basis of commercial marking inks, for use on textile fabrics, is some salt of silver. The permanent color of this ink is developed through the action of light, heat or some chem ical, after the ink has been applied. The usual basis of India ink is an exceedingly finely di vided solid carbon, mixed with a size to hold it in suspension when the ink is prepared for use by being ground up with water. (See INDIA Inx). Printing ink consists of dry pigment, black or colored, ground very fine and mixed with varnish and a few minor chemicals as it may be desired for various purposes. For black printing ink, lampblack and linseed oil were originally employed, but gas-black has taken the place of the former, and the scarcity and high cost of linseed oil has led to much substitution; one of the most common is rosin oil. For pigments, the inorganic or earthy bases are preferred, but of late years a line of coal-tar products gas been evolved that yield exceedingly brilliant colors though less per manent than the earth bases. Pigments require
very careful grinding. The mills are water cooled, but nevertheless heat under the pressure and friction on the rolls when grinding is con tinued too long, to remove the last vestige of grit; the ink thickens, and becomes so pasty that it has to be thinned down before it will flow through the fountain of a printing press. This thinning process tends to lighten the color, that is render a dense black slightly gray. Manufacturers of printing inks are required to produce very thin, cheap ink for rapid printing on newspaper presses, and whole series of better grades of black adapted to presses operating at speeds varying from 500 to 20,000 impressions per hour. They are also called upon to supply all conceivable shades of color in all qualities, and adapted to all varieties of paper. The re sult is an almost infinite number of grades and kinds of ink for typographic and lithographic work. The temperature of a press room much affects the working of ink. A desirable ink must not dry readily on the type from the ink rollers, but must dry quickly on the paper. To hasten drying, the ink makers insert material called a udryer,' and in recent years it is com mon in high-grade printing to pass the freshly printed sheet quickly over a gas flame to set the ink. Previous to the World War, the best colors for inks came from Germany. When importation ceased, American manufacturers set to work, and within two years a very fine large line of coal-tar colors of home make were on the American market.
Bibliography.—Astle, 'Origin and Progress of Writing) (1803); Carvalho, 'Forty Centuries of Ink) (1904) ; Champour and Malepeyre, 'Fabrication des Encres' (1895); Dieterich, Manual) (1904) ; Fehling, 'Hand warterbuch Chemie (Vol. VII, 1903) ; Francis, 'Printing for Profit' (1917).