A simple and useful method is the rub-out test, in which the liquid paint film is rubbed with the finger. If there is discolouration, dispersion is inadequate. This can be due to insufficient grinding of the paint during manufacture, or to flocculation (re-agglomeration) of the pigment if the paint has been stored for too long. Here is another test: if the pigment has been insufficiently dispersed it will show a marked tendency to settle out.
The formation of a hard mass at the bottom of the container, which is difficult to re disperse, is a good indication. The degree of dispersion also influences flow. If, therefore, no changes in viscosity are apparent when the paint is passed through the mill, maximum dispersion will have been achieved.