=Chrome Green.= Chrome green is prepared as a paint pigment from nitrate of lead, Chinese blue, and bichromate of soda. It has a gravity of 4 and is liable to contain slight traces of lead salts. The particles when magnified appear very fine and flocculent. This color is often precipitated on pigments, such as barytes, which do not reduce its tone. =Bone Black.= By grinding the carbonaceous matter resulting from the charring of bones, in iron retorts, the pigment bone black is prepared. It contains about 15% of carbon and 85% of calcium phosphate. It has a gravity of 2.7. Comparatively large particles of charred bone can be seen scattered throughout the mass, resulting from the difficulty of grinding to a uniform size. =Carbon Black.= This form of very pure carbon results from the combustion of gas. Its gravity, 1.09, is lower than that of lampblack, which shows a gravity of 1.8. It is used in much the same way and for the same purposes as lampblack. In physical appearance it shows great similarity to the particles of lampblack. =Lampblack.= This pigment, made from the combustion of oils, consists very often of more than 99% carbon. It has wonderful tinting value. The particles show a fine, fibrous structure with a tendency toward agglomeration. They differ greatly in physical appearance from those of either graphite or bone black, being exceedingly more uniform than the latter. [Illustration: Zinc Chromate] [Illustration: Prussian Blue] [Illustration: Ultramarine Blue] [Illustration: Chrome Green] [Illustration: Bone Black] [Illustration: Carbon Black] =Graphite.= Graphite, both in the natural and artificial form, contains impurities such as silica, iron oxide and alumina, but the natural form has a much greater percentage of these foreign materials, in some cases as high as 40%. Graphite is usually mixed with other pigments, such as red lead and sublimed blue lead, thus serving better as a paint coating. The difference in physical appearance of the various carbon pigments is interesting, as each pigment has characteristics of its own. In graphite we find a great tendency toward agglomeration or massing of particles. =Mineral Black.= Mineral black is a pigment made by grinding a black form of slate. It contains a comparatively low percentage of carbon and consequently has low tinting value. It finds use as an inert pigment in compounded paints, especially for machine fillers. The pigment has a flocculent appearance, the particles showing a strong tendency to mass. Photomicrographs of two combination paint pigments are here given, to show the various pigments as they appear under the microscope, when in combination. PERCENTAGES OF OIL REQUIRED FOR GRINDING VARIOUS DRY PIGMENTS INTO AVERAGE PASTE FORM