Stability of Consistency

Paint that is to be packaged in tubes must have a particular consistency. It should not be too liquid but should be of sufficient density to be pressed out of the tube. For impasto painting the paint needs to be buttery. As in all other respects, pigments vary considerably in their ability to maintain a certain consistency. While it is quite easy to produce a not-too-soft yet buttery paint by hand-grinding zinc white and poppy oil, a beginner would find this very difficult with lead white. When grinding ultramarine in oil or in some emulsions, one will find that the paint will long remain a dragging, puttylike mass. If more medium is added to remedy this, the material will suddenly start to flow off the slab. Different types of ultramarine vary in this respect. Beginners will discover that most pigments are difficult to prepare in casein emulsions.

Even if a good consistency is achieved, it may not remain stable. Some types of viridian will lose their consistency twenty-four hours after preparation. This can occur with other pigments in varying degrees. Therefore manufacturers first let their paints mature in vats and correct their condition by adding more pigment before packing them. Painters who prepare their own color sometimes put the freshly ground paint straight onto the palette but may find the next day that it has become too liquid. Occasionally additions of wax paste, aluminium stearate, and other materials are made to adjust the consistency of a paint but such remedies are used reluctantly. In water-base paint systems other means, which will be discussed later, can be employed. [p. 63]

Sometimes a slightly softer paint is required, one that is more brushable. This can easily be achieved by higher proportion of vehicle or by adding medium while painting.

The choice of certain pigments requires years of experience and paint manufacturers are always dependent on the qualities of different batches supplied by the pigment factories. This problem is closely linked with the different characteristics of various vehicles and with variations of quality within any particular type. One would conclude from Doerner's recommendations that self-prepared oil paints are not only cheaper but are also of better quality than commercial oil paints. This has not been confirmed by the author's long experience with students and professional painters. Furthermore, there is a great difference between grinding a small amount of color for immediate use and filling a supply of tubes. For the later purpose, certain difficulties regarding the storage life of tube paints have to be overcome. This is where the manufacturers have the advantage of experience. [pp. 63-64]