Scientific American 5, 1.8.1863
The blue pigment known as ultramarine, was formerly the product of a mineral, which, on account of its beauty, was also employed as a gem in jewelry. Being obtained in a few countries only, as an agent of painting it was very costly; but now- thanks to the great improvements made in synthetical chemistry - it is produces artificially at quite a moderate establishments of Germany; and the composition and processes involved are described as follors, in Dinglers Journal:-
"The composition for a dark aluminous ultramarine consists of 100 parts of slightly burned kaolin (porcelain clay), 90 parts of soda-ash (95 p. c.), 100 parts of refined roll sulphur, 6 parts of rosin, and 4 of dry pine charcoal. Each of these ingredients is powdered, with the exception of the rosin, which is only added in pieces the size of a walnut when the materials have been mixed, and the whole is rolled together for the space of four hours. It then forms a smooth gray powder, and is loosely packed into fire-proof boxes, which are covered up, properly luted, and placed on the lower floor; and after closing up all the apertures of the furnace, it is rapidly brought to a point of temperature equivalent to the fusing point of an alloy of equal parts of gold and silver, at which temperature the oven is kept for from five to six hours. By means of small tubes inserted in the front of furnaces, the process is warched: samples being taken from time to time, by means of hollow cylinder screws. When these samples remain of a green color on cooling, the fire is gradually slackened, and afterwards the draught is shut off; the furnace being left to cool for 28 hours. Two days afterwards the mass is removed from the boxes. It is forst broken up under mill-stones, then finely powdered, filled into cast iron annealing boxes (1½ feet hight, 2 feet long, and 1 4/5 feet wide on top, somehwat narrower in the bottom, the iron 1/5 of an inch thick), the covers of which overlap the sides. These boxes are placed on the upper floor of the furnace, at the same time that a fresh charge is placed on the lower floor; and are removed about twelve hours after the firing has ceased. This annealing or coloring, which changes the green to blue, by partly oxidising, and partly removing an excess of sulphur, is similar to the process of coloring red-lead.
"The blue pigment now obtained is lixiviated, and then, while moist, ground between granite or quartz millstones. When the desired fineness is obtained, the pulp is run into draining bags, and afterwards put in cast-iron dishes, which are also placed in the upper floor of the furnaces to dry, whenever the iron annealing boxes have been removed. On the Rhinw, some factories are supplied with reverberstory furnaces, the soles of which are heated from below by the fire, which then again passes over the charge before reaching the vlue. Such furnaces hold as much of the crude materials as will yield about 1,300 pounds of ultramarine.
"Another method consists in mixing the materials in smaller quantities, and forming them into batches, in boxes containing only about 700 pounds each. These boxes are placed in pairs on the benches of a double floor reverberatory furnace, heated by one fire, which first passes around the boxes on the lower floor, and from underneath them to the upper floor. The masonry of the lower floor is fire-brick, the supports of both soles and arches being stone, and the upper floor is formed of iron plates. The boxes are made from fire-proof tiles, one inch thick, grooved and let in at the edges. The fuel used is bituminous coal."