May 1, 1995.
Viscosity is defined as internal friction .
The internal friction in high viscosity motor oil is due to long-chain hydrocarbon molecules. With ink, internal friction is caused primarily by the pigment and the binder. The pigment and the binder alone would have much too much internal friction to flow smoothly as an ink. A relatively large amount of a low viscosity component, the solvent, is needed to reduce internal friction. Thinner is primarily solvent, used to lower the viscosity of inks.