Drop blacks in japan, as noted above, may be made by mixing bone black alone or with an addition of gas carbon black or a very fine grade of well calcined lampblack, or a high grade of animal black, with what is known to the trade as color grinders' or coach japan, grinding until fine through water-cooled mills of twenty or twenty-four-inch diameter, taking care to keep the millstones from becoming overheated, also the product from skinning over by exposure to draught from open doors and windows. It is essential that the millstones are sharp and well balanced, because otherwise the material will gum up and make the addition of extra thinners necessary, which in turn will render the covering power of the black deficient. The less the number of runs a black in japan will have to pass through the mill the better for the opacity of the finished product. In some instances it has been found beneficial for the binding properties of these blacks to mix and grind them as stout as possible and when fine enough to pass the paste once more through the mill with the addition of a few pounds of good rubbing varnish to each fifty pounds of paste, thus obtaining a buttery product that breaks up readily in turpentine. Bone black is very seldom, if ever, ground in straight varnish, as it requires a pigment of lighter gravity for making the better grades of black varnishes for engine finishing and other work of that class.
Color grinders' or coach japan for grinding drop black must be of good body, free from turbidity or sediment and should weigh not less than eight pounds six ounces nor more than eight and one-half pounds per gallon. Must be free of benzine or heavy naphtha (now known as turpentine substitutes), containing no other solvent or volatile matter but pure gum spirits of turpentine. Kauri gum is preferred by some consumers as the base of the japan, while others prefer it made with gum shellac. The black that is ground with japan made from kauri gum will set more slowly, drying more thoroughly, however; while, when ground with gum shellac japan, it will bind more rapidly, permitting parts touched up with it to be varnished over quickly without rubbing up, which cannot be done with black ground in the kauri gum japan. The average quantity of pigment and vehicle for a drop black in japan is equal parts by weight when pure bone black is being used.
For a denser drop black in japan would suggest forty pounds bone black and four pounds high-grade gas carbon black, both mixed together dry and dried over night in a temperature of at least 140 degrees F., then mixed with say fifty-six pounds of color grinding japan, and when fine a last run should be given, adding four pounds either of japan or rubbing varnish, which will make up for loss in grinding, yielding 100 pounds finished black out of a mixing of 104 pounds. Or animal black that contains at least double the quantity of carbon that is contained in bone black (which may be determined by assaying it with white for tinting strength, in comparison with bone black) may be used, mixing forty-eight pounds of the dry powder with fifty-two pounds japan, grinding fine and adding a few pounds of japan or rubbing varnish on the last or finishing run.
Ordinary bone black or animal black has no place on the palette of the artists, and these pigments are not put up in artists' tubes under the name of drop black, Ivory black takes their place in that line. But they will be found in the distemper color list as drop black ground in water without size and are used by grainers when doing work in distemper and also by show card artists, who add glue or dextrine size for binding medium. When fifty pounds of dry bone black in powder are mixed with sixty-five pounds of water and the mixture is run through a color mill until fine, the result will be about 100 pounds of so-called drop black in pulp. The term is derived from the ancient custom of grinding the calcined bone black or animal black in water, throwing the resulting pulp on a filter, then forming the stiff pulp into drops of different sizes that were placed in drying rooms and so offered to the trade, that in those days, consisted mainly of painters who by the use of slab and muller, or small hand mills, manipulated what small quantity of color was required for their purpose. The writer, as late as twenty years ago, purchased a lot of English drop black in the form of cones that weighed from eight to twelve ounces apiece and were baked so hard in drying that it was necessary to run them through a powerful crusher and then several times through a buhr stone mill in order to reduce them to powder. This practice is now obsolete, and the so-called drop blacks are obtained in powder of varying fineness.