Another test is to weigh out equal quantities of the two leads, and then to take a very small quantity of the same color, say black, and add to each sample, thoroughly mixing. The lead that is the lightest in color will be the finest. The explanation of this is somewhat interesting. Suppose that we have a number of cubes of white lead each measuring one inch side. This will give us six superficial inches to be colored. Now suppose that we break up these inch cubes into half inch cubes, which will give eight half inch cubes to each inch cube. Now as each half inch cube has six faces measuring half an inch by half an inch, it has a superficial surface of three square inches, and as there are eight of the half inch cubes, there are twenty-four superficial inches to be colored against six in the inch cubes. It will be seen, therefore, that by increasing the fineness of a pigment a greater surface is presented to be colored, and hence more color is required.
Another test for fineness is to paint different samples thinned in turpentine on plate glass; when dry the two specimens may be compared and the difference of fineness between them will soon be apparent.
Still another test, and one frequently used by painters, is to place a quantity of the color ground in oil that is to be tested upon a level surface such as a piece of glass, and to run the blade of a spatula or palette knife over it, and then over another sample with which it is to be compared, noticing carefully the difference in appearance of the two samples. By these means the presence of grit is discovered.
Spreading Capacity or Covering Power. The spreading capacity of pigments and their body are very nearly related, although of two equal in body one may possess greater covering power or spreading capacity than the other. A practical method of testing covering power is to mix a small quantity of a standard paint and an exactly similar quantity of the pigment to be tested, taking care to use precisely the same amount of oil and thinners in each case. Then,
