121. Uranium Red.
By treating the yellow sulphite of uranium with a prolonged current of sulphuretted hydrogen, and saturating gradually with ammonia, a red finally results. This colour is insoluble in water, and it has the objection of remaining partially suspended for an almost indefinite time, colouring the liquid light red. The product is brighter and more beautiful while moist; when dried and powdered, its tone—slightly approaching vermilion—is duller. The colour may be obtained of several degrees of brilliancy, but, apart from the question of expense, it would be inadmissible in oil, the red gradually altering by contact therewith. The most persistent tint at length resembles burnt Sienna.
122. Wongshy Red.
There was imported a few years ago from Batavia a new colouring principle, under the name of wongshy, and consisting of the seed-capsule of a species of gentian. The aqueous extract, freed from the pectin which it contains, yields with baryta- and lime-water yellow precipitates, from which acids separate the colouring matter of a vermilion hue. When thus prepared it is insoluble in water, and would so far be adapted for a pigment. The red has not, however, been employed as such, and we are unacquainted with its habitudes.
The concluding remarks appended to the chapter on yellow apply equally to red, and indeed to all other colours. It is not assumed that the list is exhausted: there are other reds, but they are, like some we have mentioned, ineligible as pigments, either by reason of their fugacity, their costliness, the difficulty of producing them on a scale, or the sources whence they are derived being commercially unavailable. While endeavouring throughout the work to render complete the collection of pigments actually in use, it is our object to give a selection only of numbered italicised colours; ample enough, however, to include those which have become obsolete or nearly so, and full enough to afford some insight into our resources. The nearer we approach perfection, the more eager we are to arrive at it: the path before us, therefore, cannot fail to be of interest.
Looking back, and noting those pigments commonly employed, we find that the reds like the yellows are divisible into three classes—the good, bad, and indifferent; or the permanent, the semi-stable, and the fugitive.
Among permanent reds, rank cadmium red, madder reds, Mars red, the ochres, and vermilions.
In the second or semi-stable class, must be placed cochineal lakes, Indian lake, and red chrome.
To the third division, or the fugitive, belong dragon's blood, pure scarlet, red lead, and the coal-tar reds.