Lime blue, a pigment made from solutions of copper salts with the addition of sal ammoniac and quick lime, really a mixture of copper hydroxide and calcium sulphate, has gone out of use and a cheap grade of ultramarine blue has taken its place.
Caeruleum is the name of a blue found on the decorations of ancient temples, but its preparation is a lost art, and what little is now sold under this name to artists appears to be a combination of the oxides of cobalt and tin. Indigo blue, too, does not interest the color grinder, excepting for its use in specialties, such as printers' ink or for pharmaceutical use. It is best purchased in the lump form, as the powder is often badly adulterated. The color and tone of indigo can be successfully imitated by a mixture of Prussian and ultramarine blue by adding sufficient ivory black. If the color grinder has occasion to use it he had best steep it in alcohol for a few days, beat the mixture up fine and evaporate the alcohol before adding the oil or varnish.