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The viscosity of a mill base must be adjusted to suit the dispersing unit. If the viscosity of the mill base is excessive, the unit may be damaged. If it is too low, shear forces will be inadequate to break down the pigment agglomerates. The viscosity is also an important indicator of the stability of a pigment concentrate. Any change in rheology during storage indicates inadequate pigment stabilisation.

An easy method to determine viscosity is by measuring the efflux time. For mill bases, however, the viscosity is usually simply too high to use a flow cup.

A rotational viscometer is often used to determine the viscosity more precisely. The resultant complete flow curves provide information on flow characteristics of the particular material, from the manufacturing process through transport to the final application. During development of a pigment concentrate, its flow characteristics over the entire shear rate spectrum are of great importance. For quality control purposes, measurement at two points, e.g. at low and medium shear rate, is usually sufficient.