Chinese blue is really a fine grade of Prussian blue, having a much better luster than the ordinary grade, and when in the lump form its fracture must show a decided bronze cast or luster, by which Chinese blue is distinguished from the ordinary Prussian blue. It should have a somewhat greenish undertone in comparison with the violet of the Prussian blue. When used for tinting whites, Chinese blue must give a clear sky-blue tint, neither leaning to green nor to the lavender-gray. In selecting the dry blue for grinding in oil, the color grinder must examine the powder, if he buys in that form, for softness, as it is very difficult and expensive to grind a hard blue that has been dried too rapidly. It should also be very thoroughly examined to see that the powdered material has not been scorched in the mill, as it is very apt to catch fire from the accidental presence of iron. The utmost care is necessary in mixing and grinding the blue in oil, japan or varnish to not only exclude particles of iron, but also to prevent overheating. Well-dressed soft stone mills, running at fairly slow speed, are best adapted to grind this blue, as well as Prussian blue, as excessive heat will ruin the tone and luster of these pigments. Pure Chinese blue requires its own weight of linseed oil to produce a commercial paste, but if wanted of extraordinary tinting strength it may be ground at the rate of 56 pounds dry blue to 33 pounds linseed oil and 11 pounds turpentine. The result, however, will not be over 95 to 96 pounds paste, as a good portion of the turpentine will be lost through evaporation during the grinding process. This practice will hardly appeal to the manufacturer of the present day and the lesser cost of oil will induce grinders to use all the oil consistent with producing a marketable article.
When a color grinder desires to place an extended blue on the market he will scarcely use the higher-priced Chinese blue, but make use of an ordinary Prussian blue, hence Chinese blue, when so labeled, will usually be found strictly pure. While Chinese blue is preferable to Prussian blue in artists' tubes, it is not well adapted for grinding in japan or varnish for the use of the coach and car painter on account of its bronze luster, which, if the blue is used solid, floats up when the color is thinned with turpentine, and in drying the surface, instead of showing a deep blue effect, gives a brownish surface when under varnish. This is most noticeable when Chinese blue is being used to make a composite green for coach work instead of the bronze-less Prussian blue of the violet tone.