All inks have the same basic composition: a colorant which is either suspended or dissolved in a liquid of some type. Colorants can be either pigments or dyes, and the difference between a pigment and a dye is the particle size. Pigments have much larger particles than dyes. Pigments and dyes also require different solvents, or liquids. Dyes are typically dissolved in water or water-soluble substances, whereas pigments are usually suspended in oils or petroleum distillates. Some inks have a mixture of dyes and pigments. While a basic ink only requires two ingredients, most inks have additives that give the ink additional qualities, such as buffering agents to control the pH of the ink or fungicides to prevent fungal growth.
Inks for drawing are typically water-based and contain dyes. These inks usually require a dip pen, which is a very basic pen with a nib that is dipped into ink. Inks for drawing need to be thin in order to not clog the nib, and can be thinned further with water, which is a desirable quality for an artist who may wish to control the opacity of the ink. Drawing inks often contain shellac. A problem with inks containing dyes is that dyes tend to fade quickly. Modern drawing inks may or may not be light-fast. Inks for drafting and calligraphy often have similar characteristics to drawing inks.
Fountain pens have a nib like a basic nib pen, but they are different in that they contain a cartridge that holds a small amount of ink. This means that the ability of ink to flow out of the cartridge smoothly and not clog the nib is the most important characteristic of the ink. Because of this, fountain pen inks are water-based and contain dyes as the pigment in the pen. Water-based inks are thinner and clog writing instruments less than other types of ink. Fountain pen inks also contain a surfactant, which helps to control the flow of ink from the pen.
Ballpoint pens contain ink in a cartridge, like a fountain pen. But instead of a nib, these pens have a small ball in the tip that rolls the ink across paper. Ballpoint pens are different from other pens in that they typically use an oil-based solvent or are otherwise not soluble in water, and use a pigment. The thickness of ballpoint pen ink is the most important characteristic. Ink that is too thin, as water-based ink usually is, will flow out of the pen too quickly and make a mess. Ink that is too thick will clog the ball in the tip of the pen. Ballpoint pen ink tends to be very paste-like.