Prussian blue, also known as Paris blue or Berlin blue, requires the same general treatment and precautions in mixing and grinding as Chinese blue, differing from the latter only in the method of making the dry blue and in the more or less marked absence of bronze luster. The paler shades of Prussian blue resemble Chinese blue to a great extent, producing the clear sky-blue tint with white, while the darker shades usually give the lavender-gray tint with white. For admixture with black and yellow in making composite greens, however, the dark Prussian blues are best adapted for the reasons above mentioned. Before mixing Prussian or Chinese blue in oil or japan or varnish it is essential to have the pigment almost bone dry, and unless it can be procured in that state the pigment should be mixed in a steam-jacketed apparatus at a temperature of not over 140 deg. F. to drain off all the moisture possible. Where much of the blue is being manipulated, a so-called sifter and drier will not prove very expensive, while in smaller establishments the pigment may be dried on metal pans in a drying oven or drying room. This is especially necessary when the blue is to be ground in gold size japan or varnish, but it is also beneficial for ordinary oil color. Equal weight of pigment and linseed oil will produce a satisfactory paste, when good pure Prussian blue is used, while for the coach color trade it should be mixed and ground in gold size japan, when about 40 per cent by weight of pigment and 60 per cent of japan will be the average proportion. For making blue enamel a practically bronzeless blue should be ground in, say, 75 pounds rubbing or finishing varnish to 25 pounds of the dry blue, the quality of the varnish to depend upon the purpose for which this varnish base is required, but also upon its properties as a grinding vehicle.
When Prussian blue as an oil color or paste paint in oil is to be extended, alkaline extenders must not be used. This bars out the use of whiting, asbestine and blanc fixe or barytes, unless the last two pigments are well washed and prove to be inert by test. Floated silica, gypsum, china clay, floated barytes and blanc fixe (the two last named if free of alkali) will serve as extenders, the best being a mixture of china clay and gypsum. Whiting and asbestine always show appreciable traces of alkalies, and the presence of these is very apt to change the tone of the blue to a reddish tinge. While Prussian or Chinese blue cannot be made use of by decorators in distemper painting, so long as they employ a whiting base there is some slight demand for these blues and color grinders who have a color-making establishment connected with their factory will not mix and grind the dried blue in water, but will use the pulp instead, simply making it smooth by running it through a mill.
Bronze blues, reflex blues, steel blue and night blue, or whatever may be the fancy names given to that class of pigment, are of the Prussian blue group and characteristics and used for special purposes, principally for the preparation of printing and lithographing inks. The manufacture of these inks is usually carried on as a special line of business, and the larger establishments prepare their own colors from the raw materials. So, for instance, in manipulating Chinese or Prussian blue, as well as the special blues referred to in the last paragraph, the pulped blue is not dried by heat, unless there is an excess on hand, which it is desired to market in the dry state. The pulp blue that is about to be used for ink is either well drained of its water (the so-called liquor) or filter-pressed and then placed in a horizontal mixer, containing bronze blades for beating the pulp and provided with a steam jacket to drive off the moisture. While the pulped blue is thus beaten, the so-called burnt oil or lithographer's varnish of the required consistency for the kind of ink wanted, is added, a portion at a time, which facilitates the evaporation of the moisture, at the same time keeping the pigment soft and in fine division. From this mixer the ink is placed on mills with three rollers of hardened steel, or, still better, of fine polished porphyry, until fine enough to pass the test. The usual proportions for mixing and grinding are from 30 to 33 pounds of blue, figured dry, and 67 to 70 pounds of lithographers' varnish No. 1 or equal parts by weight of No. 1 and No. 2.
Leather blue is also of the Prussian blue type and is called for by leather dressers, who require the blue ground fine in linseed oil, of which they add certain portions to the black dressing that they prepare for their use by heat. The paste for this purpose should consist of 45 parts by weight of bronzeless blue and 55 parts by weight of pure well settled linseed oil. It is added to the black for the purpose of giving the dressing greater density and greater depth.
Celestial blue of commerce is not in demand as an oil color by the trade, and what little is still being purchased is in the dry form and does not interest the color grinder, as it is simply a very much extended Prussian blue that may contain anywhere from 6 to 20 per cent by weight of blue, balance being natural barytes, as there is no standard for blue of that name.