ink, pigmented fluid used for writing and drawing, or a viscous compound used for printing, both of various colors but most frequently black. The oldest known variety, India ink or China ink, is still used in China and Japan for writing with small brushes instead of pens. All inks are composed of a colorant, a vehicle, a solvent, and additives; the colorant for India ink is carbon black, and the vehicle is water; various substances including glue and gum are used to stabilize suspension. Because of its rich blackness and permanence, India ink is used extensively by architects, engineers, and artists. In many early civilizations dyes obtained from plants, and sepia from the squid, octopus, and cuttlefish, were used as ink. The black and blue-black inks used today, composed of copperas (ferrous sulfate), gallic and tannic acids, and a preservative, were probably known as early as the 2d cent., the acids then being derived from oak or nut galls. Numerous master drawings made with ink containing the acids of gallnuts have been corroded by the ink itself. These inks, and also the colored inks used today, contain aniline and other soluble dyes instead of holding their pigments in suspension; they are accordingly filterable and flow easily even through ball-point pens. Felt-tip markers contain organic compounds in solution. The glutinous inks used by printers owe their various viscosities to such ingredients as linseed oil, synthetics of the alkyd type, mineral oil, and petroleum fractions. This reliance on hydrocarbons has raised concern about pollution, and led to the development of several alternatives: vegetable–based inks; toluene–based inks so that the solvent can be recovered; and water–solvent inks. Marking inks used to mark linen are composed of a salt of silver. Indelible or incorrodible ink is used for writing that is exposed to the weather or to strong acids or alkaline solutions. Fluorescent inks are used in printing maps to be read at night. The liquid in the ink used in newspaper printing is absorbed into the paper, while in many other sorts of ink the liquid evaporates leaving the pigment above the paper surface.