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
| White lead (corroded) | 9% | Chrome green, 25%, color extra dark | 17% |
| White lead (sublimed) | 10% | Graphite (pure) | 40% |
| Zinc lead (American) | 12% | Indian red, (98%) | 20% |
| French process zinc oxide | 17% | Ochre, yellow, American | 26% |
| American process zinc oxide | 16% | Ochre, yellow, French | 28% |
| Blanc fixe | 30% | Ochre, golden | 28% |
| Barytes (natural) | 9% | Red, Venetian | 23% |
| Paris white (whiting) | 20% | Red, Oxide | 25% |
| Terra alba (gypsum) | 22% | Red, Tuscan | 27% |
| Floated silica or Silex | 26% | Red, Turkey | 28% |
| Kaolin (China clay) | 28% | Red, lead | 12% |
| Asbestine | 32% | Red, lake | 55% |
| Blue, ultramarine | 27% | Sienna, Italian, raw | 52% |
| Blue, Chinese or Prussian | 50% | Sienna, Italian, burnt | 45% |
| Black, gas carbon | 82% | Sienna, American, burnt | 38% |
| Black, lamp | 72% | Sienna, American, raw | 40% |
| Black, drop | 60% | Ultramarine green | 28% |
| Black, bone | 50% | Umber, Turkey, raw | 48% |
| Brown, mineral | 24% | Umber, Turkey, burnt | 47% |
| Brown, vandyke | 50% | Umber, American, burnt | 36% |
| Chrome yellow, lemon | 23% | Umber, American, raw | 38% |
| Chrome yellow, medium | 30% | Verona green (terra verte or green earth) | 32% |
| Chrome yellow, orange | 20% | Vermilion, English (quicksilver) | 14% |
| Chrome yellow, dark orange | 15% | Vermilion, American (chrome red) | 16% |
| Chrome green, Chem. pure light | 21% | Paris green, American | 23% |
| Chrome green, Chem. pure extra dark | 25% | Zinc chromate (permanent yellow) | 15% |
| Chrome green, 25%, color light | 13% |
