Fluid or paste of various colours (usually black or dark blue) used for writing and printing, composed of a pigment or dye in a liquid vehicle. Early inks used lampblack (a form of carbon) or coloured juices, extracts, or suspensions of plant, animal, and mineral substances. Modern writing inks usually contain ferrous sulfate (see iron) with a small amount of an acid; on paper, they darken and bond, becoming permanent. Coloured and washable inks usually contain soluble synthetic dyes. Printing inks are formulated for various requirements (including colour, opacity, fade resistance, pliability, odourlessness, drying behaviour, and health and environmental safety) for uses in offset, letterpress, screen, ink-jet, laser, and other printing.