This section is from the book "Chromatography; Or, A Treatise On Colours And Pigments, And Of Their Powers In Painting", by George Field. Also available from Amazon: Chromatography, or A Treatise on Colours and Pigments, and of Their Powers in Painting.
Ultramarine, Outremere de Guitnet, Bleu de Ga-rance, and Gmelin's German Ultramarine. In some of the latter numbers of Brande's Journal are accounts of a process for producing factitious ultramarine; and a variety of these, English, French, and German, have been before the public under various names. These pigments are in general of deep rich blue colours, darker and less azure than fine ultramarine of the same depths, and answering to the same acid tests, but variously affected by fire and other agents: none of them, however, possess the merits of genuine ultramarine, and their relative value to other blues remains to be determined by mature experience. An experiment in which this blue was tried by an ingenious artist and friend of the author, however, speaks little in its favour. He took a picture, the sky of which had been recently painted in the ordinary manner with Prussian blue and white; and having painted on the clear part of the sky uniform portions with tints formed of the best factitious ultramarine, co-ball blue, and genuine ultramarine, so as to match the ground of the sky, and to disappear to the eye thereon by blending with the ground, when viewed at a moderate distance, he set the picture aside for some months; after which it appeared upon examination that the colour of these various blue pigments had taken different ways, and departed from the hue of the ground: - the factitious ultramarine had blackened, - the cobalt blue greened, - the true ultramarine appeared of a pure azure, like a spot of light, - and their ground, the Prussian blue sky, appeared by contrast with the ultramarine of a grey or slate colour. Fire generally darkens these colours, but the best way of distinguishing factitious ultramarine from the natural is by the violent effervescence of the former when dropped into nitrous acid; while not a bubble escapes in such case from the true ultramarine: unless, indeed, as sometimes happens, it retain a portion of alkali, with which it may have been combined in the preparation, but from which it should have been freed.
All the chemical combinations of iron with inflammable bases, under proper management, afford blue colours; and in this respect the factitious ultramarines and Prussian blue are analogous: nevertheless, the first may be regarded as a great improvement upon the factitious blues of the palette, rivalling in depth, although not equalling in colour, the pure azure of genuine ultramarine.