260. YELLOW MADDER,
Cory's Yellow Madder, or Cory's Madder, is classed among the browns for the same reason that Italian Pink was ranked among the yellows. It was stated in the eighth chapter that no true madder yellow, brilliant and pure, exists as a pigment at the present day, and certainly this preparation can lay no claim to the title. Except in name, it is an orange-brown of the burnt Sienna hue, and might therefore with more reason have been called Orange Madder. It is a good and permanent colour, rich and transparent, at present used only in oil, we believe, and chiefly as a glaze.
261. Cadmium Brown.
By igniting the white carbonate of cadmium, among other methods, a cinnamon-brown oxide is obtainable, of a very clear and beautiful colour if the process be well conducted. It is, however, not eligible as a pigment, owing to the rapidity with which the oxide is acted upon by the air. In water, especially, we have found this brown so eagerly absorb carbonic acid from the atmosphere as to become in a few months once more a carbonate, and as purely white as before. The same result is observable when the powder is exposed: some shown at the International Exhibition of 1862, on a glass stand, had to be removed, its label marked 'Cadmium Brown' being at last found attached to a sample of cadmium white. In oil, the conversion takes place less readily, that vehicle having the property of protecting, to some extent, pigments from oxidation. It is curious that even in a book a water-rub of the brown slowly but surely changes to white.
262. Catechu Browns.
Catechu is an extract of the Khair tree or acacia catechu of Bombay, Bengal, and other parts of India. With the exception of such earthy matters as are communicated to it during the preparation, or are added purposely as adulterants, catechu is entirely soluble both in water and alcohol. An aqueous solution has a reddish-brown colour, and gives the following results:—protosalts of iron thrown down olive-brown and persalts greenish-brown precipitates; salts of tin and lead yield brownish-yellow and brick-coloured deposits respectively; while acetate of copper or bichromate of potash furnishes brown residues. To our knowledge, none of these have been introduced as pigments, but a brown prepared by Dr. Lyon Playfair some years back from the catechu bark has been described as exceedingly rich, transparent, and beautiful; and recommended for painting if not too thinly applied.
263. Chrome Browns
are produced by various methods of several hues, tints, and shades, both by wet and dry processes. We have obtained them by many methods, of different degrees of permanence. Some very intense in colour have stood well, while others paler and more delicate have gradually greened, but none possessed the strict stability of the green oxides. Presuming a paucity of browns, these preparations of chromium would be worth further attention; but, with the objection of being—for browns—somewhat expensive, they have the far more fatal objection of not being wanted.