The first use of writing inks is credited separately to the Egyptians and the Chinese. Their inks, developed as early as 2500 B.C., were suspensions of carbon, usually lampblack, in water that contained a natural gum. Carbon is exceptionally resistant to the destructive effects of light, air, and moisture. Therefore many writings of great age are still legible. India ink, widely used for drawing purposes, is nothing more than a refinement of these early carbon inks. Modern colored "India" inks are similar in composition, but contain various mineral pigments in place of carbon.
Many modern washable writing inks are water solutions of dyes. They deteriorate from exposure to light and moisture. Permanent writing inks usually contain iron sulfate and gallic and tannic acids in addition to dye. These ingredients combine after the ink is applied to the paper to form a deposit which is resistant to both water and light.
The greatest progress in the development of writing inks has taken place since the 1940's. This has been due to the development during this period of several new writing instruments, such as the ball-point pen, the felt-tip marker, and the fiber-tip pen. The inks used in these various pens are solutions of dyes and water or organic chemicals such as propylene glycol, propyl alcohol, toluene, or glyco-ethers. Mixtures are employed in many inks. Most inks contain other ingredients such as resins, preservatives, or wetting agents.
The prototypes of today's printing inks were first developed in Europe during the 15th century. The original inks consisted of carbon black that had been ground into a varnish that was made by cooking linseed oil together with a natural resin.
Printing inks today must satisfy many more requirements than writing inks, and they vary widely in composition depending on the printing method used. The kind and speed of the press used are important in the selection of ink. Also printing inks must be suitable for application to a great variety of surfaces such as paper, rubber, textiles, metal, wood, or plastic. Printing inks must also serve widely diverse purposes; an ink suitable for a brightly colored circus poster, for example, is quite different from the ink used in printing this page. Each must last for a different period of time and must withstand different conditions during use.
Most modern printing inks are viscous liquids that contain synthetic pigments, binders, solvents, and various ingredients to aid drying. A necessary quality of all printing inks is the ability to remain wet while on the press, sometimes for several hours, and to dry very quickly after being applied. Inks dry by various processes, and the speed with which they dry can be controlled to a great extent with additives or by special treatment of the printed stock.
Some inks dry by oxidation. It was for this reason that the varnish of the original printing inks was cooked; heat partially oxidizes linseed oil and thus enables the air to complete the oxidation fairly rapidly. In modern inks, chemical driers, which frequently contain either lead or cobalt, are often used to further accelerate the oxidation reaction.
Some inks dry by absorption into the material upon which they are printed; ink used for newspapers is a good example of this type. In other inks, the liquid portion of the ink evaporates, leaving the solid matter as a dry film. Occasionally, low-boiling-point solvents, which disappear quickly at room temperature, are used, but more often drying is accomplished by passing the printed surface over an open flame or some other source of heat. Moisture in the form of a jet of steam is used for drying some printing inks. Such inks contain a resin that remains in solution as long as no moisture is present. However, as soon as the ink absorbs water from the steam jet, the resin precipitates to form an apparently dry solid.
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