169. Madder Orange,
or Orange Lake. It has been said that the yellows so-called produced from madder are not remarkable for stability, differing therein from the reds, purples, russets, and browns. Like them, this 'orange' is of doubtful colour and permanence, and not to be met with, brilliant and pure, on the palette of to-day. The russet known as Rubens' madder has a tendency to orange.
170. Orange Lead,
of a dull orange colour, is an orange protoxide of lead or massicot. Like litharge, it may be employed in the preparation of drying oils, and, being a better drier than white lead, may be substituted for it in mixing with pigments which need a siccative, as the bituminous earths.
Minium sometimes leans to orange; and there is made from ceruse a peculiar red, Mineral Orange.
Orange Orpiment,
or Realgar, has also been called Red Orpiment, improperly, since it is a brilliant orange, inclining to yellow. There are two kinds, a native and an artificial, of which the former is the sandarac of the ancients, and is rather redder than the latter. They possess the same qualities as pigments, and as such resemble yellow orpiment, to which the old painters gave the orange hue by heat, naming it alchemy and burnt orpiment. Orange orpiment contains more arsenic and less sulphur than the yellow, and is of course highly poisonous. It is often sophisticated with brickdust and yellow ochre.
172. Thallium Orange
is produced when bichromate of potash is added to a neutral salt of the protoxide of thallium, as an orange-yellow precipitate. The scarcity of the metal precludes their present introduction as pigments, but if the chromates of thallium were found to resist the action of light and air, and not to become green by deoxidation of the chromic acid, they might possibly prove fitted for the palette. It is a question whether their very slight solubility in water would be a fatal objection; and, although they would be liable to suffer from a foul atmosphere, we are inclined to think the effects would not be so lasting as in the chromates of lead. Like lead sulphide, the sulphide of thallium ranges from brown to brownish-black, or grey-black; and, like it too, is subject to oxidation and consequent conversion into colourless sulphate. It is, however, much more readily oxidized than sulphate of lead; and hence the thallium chromates would doubtless soon regain their former hue on exposure to a strong light.
Mr. Crookes, who discovered this new metal in 1861, believes that the deep orange shade observable in some specimens of sulphide of cadmium is due to the presence of thallium. He has frequently found it, he says, in the dark-coloured varieties, and considers the variations of colour in cadmium sulphide to be owing to traces of thallium. That thallium affects the colour is most probable, but it is not necessarily the cause of the orange hue. The tint of cadmium sulphide is a mere matter of manufacture, seeing that from the same sample of metal there can be obtained lemon-yellow, pale yellow, deep yellow, orange-yellow, and orange-red. With deference to the opinion of a chemist so distinguished, we hold that thallium rather impairs the beauty of cadmium sulphide than imparts to it an orange shade, the thallium being likewise in the form of sulphide, and therefore more or less black. On chromate of cadmium, made with bichromate of potash, thallium would naturally confer an orange hue.