116. Silver Red.
By adding monochromate of potash to an acid solution of nitrate of silver, a particularly fine ochre-red is obtained. It is, however, apt to be injured both by foul air and exposure.
117. Sorgho Red.
Some nine years back there was found to be a carmine colouring matter in most parts of the Chinese sorgho, chiefly in the unpressed stem. The red, which is extracted in an impure state, is dissolved in weak potash-ley, thrown down by sulphuric acid, and washed with water. This purified product, soluble in alcohol, caustic alkalies, and dilute acids, has been employed in Austria, Baden, &c., for the dyeing of silks and woollens with the common tin mordants. The colours produced from it are unchanged, they say, by warm soapsuds or light. We do not know whether the red found its way to England, but it has certainly not appeared here as a pigment.
118. Thallium Red.
The orange-yellow precipitate formed by mixing a neutral salt of protoxide of thallium with bichromate of potash, is converted by nitric acid into an orange-red. The latter compound, which is a terchromate, is almost insoluble in cold water, 2814 parts being required to dissolve it. If the colour be boiled in a large excess of moderately strong nitric acid it is dissolved, yielding magnificent cinnabar red crystals on the solution cooling. These crystals likewise seem to be the terchromate.
119. Tin Pink.
By igniting strongly for some hours a mixture of stannic oxide, chalk, chromate of potash, and a little silica and alumina, a dingy red mass is obtained, which acquires a beautiful rose-red colour on being washed with water containing hydrochloric acid. For the same reason that the pinks of cobalt are superfluous as artistic pigments, this tin product is commercially ineligible. Having, however, the advantage of being cheap, and being probably durable, it would be well adapted for the common purposes of painting, in place of the fugitive rose pink.
120. Ultramarine Red?
In Gmelin's Handbook of Chemistry it is remarked that "Hydrogen gas passed over ignited ultramarine, colours it light red, from formation of liver of sulphur, hydrosulphuric acid gas and water being evolved at the same time." On most carefully making the experiment with a sample of native blue (the variety referred to) we did not succeed in effecting this change: no alteration to red or even to purple took place, the only result being that the colour was entirely spoilt, having assumed a leaden slate-gray hue. At our request, the trial was kindly repeated by well-known chemists, who took every precaution to ensure success. Several specimens of ultramarine were acted upon, but in no case was a red or anything like a red obtained, the products ranging from a slate-gray to a drab-grey. Sufficient hydrosulphuric acid gas was evolved to blacken paper moistened with acetate of lead, a fact which proved that the blue had lost some of its sulphur. Seeing that not only no red was produced, but that no tendency to red was imparted, is it possible the change described by Gmelin occurred under exceptional circumstances? All conversant with chemical matters will admit that results are obtained occasionally which cannot be repeated, owing it may be to some slight difference in the materials employed, or some slight variation of the process. Perhaps a link, considered of no importance at the time and overlooked, has been lost, and thus the whole chain of proceeding becomes useless. It is, therefore, within the bounds of probability that the red ultramarine of the great German chemist was furnished either by a peculiar specimen of blue, or by a modified form of the method he gives. We have noticed the subject at some length because if a red ultramarine, brilliant and durable, could be obtained, the colour might prove of value. A permanent artificial compound corresponding to French blue would certainly be an acquisition.