305. Prussian Black.
The same Prussian blue which gives a brown when burnt in the open air, yields a black when calcined in a close crucible. Very intense, very soft and velvety, and very agreeable to work, this bluish-black dries much more promptly than most other blacks, and scarcely requires grinding. On account of its extreme division, however, it would probably be found more energetic as a decolourising agent in admixture with organic pigments than most carbonaceous blacks.
Another Prussian black, containing copper, and made by a wet process, is obtained when a dilute solution of cupric sulphate and ferrous sulphate, in proper proportions, is mixed with a quantity of ferrocyanide of potassium not in excess. A very bulky deep black precipitate is formed, which is difficult to wash, and is deep black when dry. It is insoluble in water, and appears to be a compound analogous to Prussian blue. As a pigment, this black is inferior to the preceding.
306. Purple Black
is, or rather was, a preparation of madder, of a deep purple hue approaching black. Powerful and very transparent, it glazed and dried well in oil, and was a durable and eligible pigment. Its tints with white lead were of a purple cast.
307. Spanish Black,
or Cork Black, is a soft black, obtained by charring cork, and differs not essentially from Frankfort black, except in being of a lighter and softer texture. "Some of my friends," says Bouvier, "call it Beggars' Ultramarine, because it produces, by combinations, tints almost as fine as ultramarine." A blue but not a velvet black, where intensity is required some other is to be preferred. For mixtures, however, it is stated to be admirable, and especially for linen, skies, distances, and the various broken tints of carnations, &c.
Besides those blacks which have been mentioned, there are others furnished by several of the metals and by many organic substances employed as dyes; but as the blacks in common use are all permanent, and have been found sufficient for every purpose, it is scarce needful to swell the list. Nor is it more needful, the Editor considers, to swell the book; lest his aim be defeated of reflecting in a moderate-sized mirror the palette as it is and might be at the present day. Arrived at age, as it were, in its twenty-first chapter, this treatise may fitly conclude with Black, the last of the series of colours. Let us hope the maxim of Sir Joshua Reynolds, that success in some degree was never denied to earnest work may apply here.
Still, by way of finale, we would offer a few remarks. In no branch of the science, perhaps, is it more hazardous to commit oneself to a positive dictum than in the chemistry of colours, so liable are theory and practice to clash, and so often does the experience of one person or one time differ from that of another. He who has turned his attention to pigments, finds nearly every assertion must be qualified, for to nearly every rule there is some exception, and learns that theory alone may mislead. For example, a colour known to be fugacious may last, in certain cases, a surprisingly long time; while, on the contrary, a pigment permanent when used alone, may be rendered fugitive by improper compounding. Again, what holds good of a colour produced by one process, or employed in one vehicle or by one artist, may not be true of the same colour made by a different mode, or used in another vehicle or by another artist. It is because, then, colours are of every degree of durability, from the perfectly stable to the utterly fugitive, and because each one is liable to influence by every condition of time, place, and circumstance, that the chemist's theory is opposed as often to the painter's practice as the experience of artists themselves varies. This may explain the charges of inconsistency and contradiction which have been brought against writers on pigments, faults that lie rather with the nature of the subject than with the authors.