An ink is a watercolor that is already diluted for convenience and whose colorant tends to remain in solution. An ink is composed of a pigment or dye that is suspended and stabilized in water. Glue, gum arabic, gelatin, shellac with borax, and soap are some of the more common ingredients used to maintain a suspension and provide a binding agent.
The most common form of ink, dating from about 250 B.C. to approximately 1100, was a carbon black ink with a glue or gum binder. At this time, it was found that the mixing of a natural iron salt-ferrous sulfate-with an extract from nutgalls, which are high in tannic acid, resulted in a chemical reaction that produced a blue-black ink. This method of ink production was considered an improvement; however, it was not lightfast and the particles that remained in the ink from the chemical reaction did nothing to improve flow characteristics. It was the development of synthetic dyes, during the nineteenth century, that paved the way for the fountain pen. Inks could now be made from true solutions, with no particles to clog a writing instrument.