The colorant is the substance that makes ink colorful. Colorants for ink are divided into two groups: pigments and dyes. The major difference between the two is the size of the particles; dyes have much smaller particles than pigments. Because of this, dyes will dissolve in liquid, whereas pigments are suspended in liquid. Water-soluble ink typically contains dyes rather than pigments; however, there are exceptions to this. Black printer ink often contains pigments, for instance.
All inks must have a solvent. The solvent is the material in which the colorant of an ink is dissolved or suspended, and enables the colorant to be applied to the desired item. There are many different kinds of solvents including water, petrochemical solvents and oils. Water-soluble inks usually have water as the main solvent, but there are other chemicals that can be used, any of which must also be water-soluble. Often, an ink will have a mixture of several solvents.
While an ink only requires a colorant and a solvent to be used, modern inks often contain many other chemicals that will change the quality of the ink. For instance, an ink may have a chemical that makes it dry more quickly or slowly, and inks often have chemicals that prevent the growth of bacteria or fungi. In printer cartridges, water-soluble ink often contains chemicals that keep pigment particles in suspension, since these particles do not normally dissolve and would otherwise clump together and be useless in a water-soluble ink.
Water-soluble inks are typically found in inexpensive printer cartridges and are also used for dip-style pens. Because they do not contain the chemicals that water-insoluble inks often have, they are generally non-toxic and environmentally friendly as well as relatively inexpensive to manufacture. They also do not clog the heads of printer cartridges, as the dye particles are very small. There are some disadvantages to water-soluble inks, however. For example, the small particle size of dyes that makes them desirable for printer cartridges also means that items printed with water-soluble inks tends to fade quickly.