The extenders, some of which are now also classed as reinforcing pigments, and which are mostly used in "letting down" white paste paints, as well as colors in oil, ready mixed paints, and are also useful in dipping paints, and special paint coatings are generally mixed and ground together with whatever white or colored pigments they are to be added to. But there are some exceptions to this rule, and under certain conditions or for certain reasons they are ground in oil or other vehicle in paste or semi-paste form to a certain degree of fineness and kept in that form to be used alone or added to other whites or to colors as occasion may require. We shall only describe those that are most commonly used, omitting the rare materials as being of little or no practical interest.

Those that are of real practical interest to the paint maker and color grinder comprise natural barytes and precipitated barytes or blanc fixe, carbonate of barium and magnesia, the various grades of whiting, gypsum, marble dust, china clay or kaolin, silex or silica, asbestine and starch.