This is a mixture of Prussian blue, alumina, magnesia, and zinc oxide, in various proportions. It is prepared like Prussian blue, except that the zinc, magnesia, and alum are added to the lye of crude potassium ferrocyanide.
This might replace ultramarine, but for its defect of having a violet hue by artificial light. Its full intensity of colour is only developed after long exposure to the air, when it acquires a slightly green tint. The mode of preparing it is as follows: - Roasted cobalt ore is heated with excess of dilute nitric acid, and the solution is evaporated nearly to dryness in a porcelain vessel. The residue is boiled with water, and filtered, in order to remove the precipitated arseniate. Into the filtrate is poured a solution of basic phosphate of soda, which produces a precipitate of basic phosphate of cobalt. This is washed and collected on a filter. While still gelatinous, 1 part of it is thoroughly mixed with 8 parts hydrated alumina, recently precipitated from a solution of potash-alum by ammonia. The mixture is dried to brittleness, and calcined at a cherry-red heat for 1/2ur in a covered clay crucible. The resulting pigment is kept in glass jars. It is essential that the alumina be prepared with sufficient excess of ammonia, and washed several times with very clear water. Arseniate of cobalt may replace the phosphate, in which case, only half the quantity of the cobalt salt is needed.
It is asserted by Boullai-Marillac that by substituting lime for the alumina a richer and more velvety blue is produced.
This blue, for oil- and water-colour painting, is introduced by Rowney and Co. It is entirely soluble in hot hydrochloric acid, and the light-blue tint of the solution becomes violet-red on diluting with water. The original colour reappears by concentration, and the pigment is restored if the solution be evaporated to dryness. Nitric acid dissolves the cobalt and leaves a white residue, mostly composed of stannic acid. The green colour shows the presence of small proportions of iron and nickel. Concentrated sulphuric acid does not dissolve it, but, diluted with 4 volumes of water, produces partial decomposition. Acetic acid and caustic potash do not act upon it. Coeruleum is principally a combination of a tin oxide with cobalt oxide. Ber-zelius mentions a stannate of cobalt, prepared by adding a solution of potash stannate to one of cobalt. The bluish precipitate formed becomes light-red after washing, and then brown. Calcined at white heat, its colour changes to a light blue. The composition of coeruleum is -
Oxide of tin (stannic acid).
Oxide of cobalt
Sulphate of lime and silica
A very beautiful shade of blue is noticeable upon many ancient monuments found in the tombs of Egypt. Analysis proves the colour to be formed by a combination of soda, sand, and lime, with certain proportions of copper, from which substances the Egyptians prepared 3 different products: - (1) A peculiar red, green, and blue glass; (2) a brilliant enamel; (3) the colour to which reference is made, and which was used for painting. Peligot has succeeded in reproducing this shade of blue by heating together 73 parts silica with 16 oxide of copper, 8 lime, and 3 soda. If the temperature exceed 800° F. (426 1/2° C), a valueless black product results.
In preparing calcium chloride by calcining a mixture of chalk and chlorine residues, Kuhlmann found in the furnaces a slag of bright blue colour, consisting of manganate of lime. It resembles ultramarine, but, though not soluble in water, is not durable when in contact with it.
(a) A thorough mixture of 2 parts sulphur and 1 dry soda carbonate is gradually heated in a covered Crucible to redness or till fused; a mixture of soda silicate and aluminate is sprinkled in, and the heat is continued for an hour; the little free sulphur present may be washed out by water. (6) An intimate mixture of 37 parts China-clay, 15 soda sulphate, 22 soda carbonate, 18 sulphur, and 8 charcoal, is heated in large crucibles for 24 to 30 hours; the mass is re-heated in cast-iron boxes at a moderate temperature till the desired tint appears, and is finally pulverized, washed, and dried, (c) Gently fuse 1075 oz. crystallized soda carbonate in its water of crystallization; shake in 5 oz. finely-pulverized orpiment, and, when partly decomposed, as much gelatinous alumina hydrate as contains 7 oz. anhydrous alumina; add 100 oz. finely-sifted clay, and 221 oz. flowers of sulphur; place the whole in a covered crucible, and heat gently till the water is driven off, then to redness, so that the ingredients sinter together without fusing; the mass is cooled, finely pulverized, suspended in river-water, and filtered. The product is heated in a covered dish to dull redness for 1 to 2 hours, with occasional stirring.
Colourless or brownish patches may occur, and must be removed.
This is a hydrated oxide of copper, which resists boiling water and a heat of 212° F. (100° C). It is prepared from any soluble salt of copper, but preferably the sulphate, a dilute solution of which is treated with excess of ammonia and precipitated by potash or soda.
China-clay, calculated dry........
83 to 100
In course of manufacture, a lye of sodium sulphide is produced, and a portion may be substituted for part of the sulphate or carbonate. It is introduced either evaporated and dry or in solution. In solutions, the proportion of sodium, and not that of sulphur, is alone considered; 100 parts anhydrous soda carbonate may be replaced by 80 of dry sodium sulphide; and 100 of dry sulphate, by 60 of dry sulphide.
The principal operation is calcination. The mixture must be brought to the pioper temperature without contact of air, and the heat maintained long enough to uniformly penetrate the whole mass. Iriegular and defective calcination never gi\es advantageous results. Under the best conditions, vessels resembling the ciucibles or seggars of porcelain works are heated in ovens built of fireclay, in the shape of small porcelain ovens. There is great waste of heat in these furnaces, and in many works it is partly utlized in evaporating the mother-liqiors or wet mixtures. The calcining vessels are made of good fireclay, formed upon a potter's wheel,like flowerpot; if their shape is that of seggars, the diameter is 5 in., and the height 3 in. The top edge should be level. Only a snail number of fiat covers are needed, because the bottom of one becomes the cover of that upon which it rests. When crucbles are employed, the cover is depressed so as to receive the bottom of the next. This appears convenient, because though the crucibles be placed close to each other, enough free space is left between them to permit the heat to circulate.