True cobalt blue is only interesting to the color grinder so far as its use by artists is concerned, while for all other purposes the artificial ultramarine, known to the trade as imitation of cobalt blue, which can be obtained in several grades and shades, is being placed at the disposition of the consumer. While true cobalt blue is of great permanency and unaffected by the most destructive agencies, it is too high in cost for general use, and really the artificial ultramarine, sold as imitation of cobalt blue, gives a much more brilliant color effect. However, the latter should not be put up for the use of the artist, excepting when label states plainly that it is artificial or imitation. True or genuine cobalt blue is a compound of the oxides of cobalt and alumina with some phosphoric acid, occasionally. It works better as a water color than it does in oil, and is highly valued on that account among moist colors used by artists. Grinding it for artists' use in oil to be put up in tubes will require 35 per cent dry pigment to 65 per cent by weight of bleached poppyseed or nut oil, either of which is preferable to linseed oil for this purpose. Cobalt blue has a greenish tone, that when viewed under gaslight is more or less violet. True cobalt blue is readily distinguishable from its imitation by being unaffected in contact with acids and strong alkalies that affect the artificial brands. It has also been known as Thenard's blue and azure blue. Its cost being prohibitive, it is not placed on the market as a coach color or as an enamel.
Cobalt blue smalts, a glass consisting of a double silicate of cobalt and potash with such impurities as the oxides of iron and calcium, was very largely ground up in various vehicles and sold as cobalt blue before the advent on the market of the lower-priced imitation of cobalt blue made by the manufacturers of artificial ultramarines. The latter show a far more brilliant color effect, much stronger hiding and staining power and are not affected so much in tone by electric or gaslight. Imitation of cobalt blue is offered to the color grinding trade in various depths of shade, the marks of the best quality being C. S. or C. 9, another lower-priced brand being marked C. C. or C. I. When pale shades are required, a portion of French process zinc oxide added to the blue will produce very fine light effects. These blues are for many purposes more desirable than some of the artificial ultramarine blues, as they make colder blue tints with white, not leaning so much to the purple or violet rays. Nor are these imitations of cobalt blue so readily affected by the presence of alkalies, as is the case with the soda ultramarines. For mixing and grinding the best grades of imitation of cobalt blue in oil, figure on 35 per cent oil to 65 per cent pigment, but when zinc is added to produce a lighter shade reduce the percentage of oil proportionate to quantity of zinc used. For making an azure blue color in oil or japan, when zinc oxide can be employed as base, imitation of cobalt blue should serve best as coloring matter, but when white lead forms the base, Chinese blue will prove superior for standing exposure. This blue is favored to some extent for carriage painting, especially for striping and should be ground in pale gold size or rubbing varnish, and when toned with a small portion of zinc oxide produces a rather pretty blue stripe. But it will prove very effective as a body color for some small vehicles. To mix and grind the pigment for this purpose, figure on 54 per cent by weight of Super. Imitation Cobalt Blue, 6 per cent of best French zinc (no more) and 40 per cent of pale gold size japan, or 35 per cent of this and 5 per cent good pale rubbing varnish. Imitation of cobalt blue is not favored as a base for blue enamel on account of its settling tendency in liquid form. To grind it in water for distemper work, figure on 70 per cent pigment and 30 per cent water. As cobalt blues do not appear in lists of second or third grades of oil colors, we need not mention extenders for this blue, but we may say that it is very necessary to examine these blues as well as artificial ultramarine blues for the presence of free sulphur before mixing the pigment in any oil, japan or varnish vehicle. Either free sulphur or free soda will give trouble in grinding, as either will act on the vehicle, producing a gummy paste of livery tendency that also tends to cake in the containers. The writer has had occasion to reject supplies of ultramarine blue that had such a strong odor of sulphur that it required no chemical test. However, it is always best to take no chances when the material gives the least cause for suspicion.
Ultramarine blue in its true or native state is found in Tibet, Persia, China, Siberia and in the Andes of South America as a mineral, called "lapis-lazuli." It is mostly found in pebbles, associated with a gangue of iron pyrites, limestone or other rocky substance, according to the formation of the earth in which it is found. The process by which the mineral is made into a workable pigment is not interesting to the color grinders, as it may be said to be a classic or dead, rather than a live paint material, excepting perhaps for the ceramic art and a small class of artist painters. Suffice it to say that the native product lacks the brilliancy of the best grades of artificial ultramarine blue. It is very harsh and granular in texture and somewhat refractory in grinding. In oil it is rather transparent, while the artificial product is more opaque and has far greater tinting power. To grind true ultramarine blue in oil for the use of the artists it will be found that 60 per cent of oil to 40 per cent of pigment are a good average to figure on, but as the specific gravity varies considerably these figures are not to be depended upon. True ultramarine blue is not put up as a distemper color, and if any one desired it in that form and did not mind paying the price it would have to be manipulated specially for such demand. As such occurrence is hardly liable to happen, we will pass on to the consideration of artificial ultramarine blues. There are two distinctive processes or methods of preparing this pigment, one being known as the sulphate ultramarine, the other as soda ultramarine, the latter having the violet undertone, the former leaning to a more greenish tinge. In either process the constituents are nearly similar, comprising kaolin (china clay), sodium sulphate, sodium carbonate, sulphur, coal or charcoal, rosin, quartz and infusorial earth. All of these are not used in one operation, if, for instance, quartz is used, infusorial earth is omitted and vice versa, or when charcoal is used, coal is omitted and so on. When sodium sulphate is used less sulphur is added to the batch, but in that case the sodium sulphate must be increased proportionately, and the more sulphur is used in soda ultramarine the deeper the shade and tone.