Printing and writing inks have different formulations, and even within those categories there are variations, but in general inks comprise pigment and vehicle, with the pigment supplying the color and the vehicle consisting of chemicals used to stabilize it and prevent early drying. Ninety percent of ink produced is printer ink, with writing ink composing a small minority.
Modern printer ink contains pH modifiers or humectants that prevent early drying and binding and allying polymeric resins and anti-foaming agents that prevent ink from foaming. Rheology modifiers to thicken composition, surfactant wetting agents to regulate the properties of the ink's surface and biocides to keep fungus and other colonizing organisms from damaging the ink are also included.
The major distinction between writing and printing inks is that writing inks make use of dyes while printer inks make use of pigments. Dyes are soluble materials, making them ill-suited to the rapid needs and fixing solutions of printers, though they function perfectly with the longer timescale involved in handwriting. Pigments, however, are insoluble and do not smear as readily.
Inorganic substances such as clay are sometimes added to ink solutions as extenders: materials meant to take up space and provide body while reducing the cost of loading in pigment. These substances sometimes comprise significant percentages of a given ink formulation.