Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/58

50, and that the pureness of the grays is attributable to the property of charred substances to prevent discolorment.

Blue black is a well-burnt and levigated charcoal, of a cool neutral color, and not differing in other respects from the common Frankfort black. Blue black was formerly much employed in painting, and, in common with all carbonaceous blacks, has, when duly mixed with white, a preserving influence upon that color in two respects, which it owes chemically to the bleaching power of carbon, and chromatically to the neutralizing and contrasting power of black with white. A superior blue black may be made by calcining Prussian blue in a close crucible, in the manner of ivory black, and it has the important property of drying well in oil. Innumerable black pigments may be made in this way by charring.

Vegetable black is a pigment now very extensively employed, superseding to a great extent the use of lamp black, to which it is in every way superior. The best way to procure it is to buy it in a dry state, in which it resembles soot, and is so exceedingly light that an ounce or two will fill a gallon measure. It is free from grit, and only requires to be rubbed up with a palette knife on a marble slab, instead of grinding. It should never be diluted with linseed oil, because, if it were, it would never dry, and it is not advisable to employ turpentine, but always the best boiled oil, and a little varnish will improve it. A small quantity of driers should be added, to ensure its drying with a uniformity of surface.

Blues. Prussian blue is made by mixing prussiate of potash with a salt of iron. The prussiate of potash is obtained by calcining and digesting old leather, blood, hoofs or other animal matter with carbonate of potash and iron filings. This color is much used, especially for dark blues, making purples and intensifying black. It dries well with oil. Slight differences in the manufacture cause considerable variation in tint and color, which leads to the material