Following is a general outline of their manufacture: One hundred parts, by weight, of washed clay in paste form are finely suspended in 6 to 8 times the volume of water and acidulated with about 1 1/2 parts, by volume, of 5 per cent hydrochloric or acetic acid, and heated by means of steam almost to the boiling temperature. There is next introduced, according to the shade desired, 1 to 2 parts, by weight, of the dyestuff, such as auramin, diamond green, Victoria blue, etc., with simultaneous stirring and heating, for 1 to 2 hours, or until a sample filtered off from the liquor shows no dyestuff. Next the clay dyed in this manner is isolated by filtration and washed with hot water and dried. The colors thus obtained may be used as substitutes for mineral colors of all description.
The method of manufacture varies greatly. According to the Bennett and Mastin English patent the procedure is as follows: Grind together to a paste in water, substances of a clayey, stony, earthy, or vitreous nature, and certain metallic oxides, or “prepared oxides,” such as are commonly used in the pottery trades; dry and powder the paste, and subject the powder to the heat of a furnace, of such a temperature that the requisite color is obtained, and for such length of time that the color strikes through the whole substance. For example, 8 parts of black oxide of cobalt, 12 parts of oxide of zinc, and 36 parts of alumina, when incorporated with 20 times their combined bulk of clay and treated as described, yield a rich blue pigment in the case of a white clay, and a rich green in the case of a yellow clay. Long-continued firing in this case improves the color.
Many minerals included in formulas for pigments have little or no coloring power in themselves; nevertheless they {556} are required in producing the most beautiful shades of color when blended one with another, the color being brought out by calcination.
Mixing Oil Colors And Tints.
Unless otherwise indicated, the materials are understood to be ground fine in paste form.
NOTE.—The majority of the following are by Joseph Griggs, in the Painters’ Magazine:
Grounds For Graining Colors:
Ash Ground.—Four hundred parts white lead; 4 parts French ocher; 1 part raw Turkey umber.
Ash.—Raw umber; raw sienna; and a little black or Vandyke brown.
Hungarian Ash.—Raw sienna and raw and burnt umber.