Carbon Black, - While nearly all black pigments, and especially those most in use for paint making, owe their coloring principle to the element, carbon, when we at the present day speak of carbon black we mean the hydro-gas-carbon black produced by the combustion of natural gas, the soot of which is collected on revolving plates and removed therefrom by fixed scrapers. When this gas carbon black was first placed on the market it commanded a price fully six times higher than it can now be bought for in quantity and was then, on account of its great staining power, mostly used for printing inks of a high type. Some twenty-five years ago, when in the Middle West the flow of natural gas was at its height and the demand for this black was not as great as it now is, it was offered at prices ranging from 25 to 60 per cent, less than those asked for it at present. At that time the average color grinder did not understand the manipulation of this black as it is understood to-day, and the black itself was not as well prepared, being much more granular and containing quite a large percentage of moisture. The black was then packed in sugar barrels containing fifty pounds net, but when kept in unheated warehouses or sheds it absorbed anywhere from 6 to 10 per cent or more of moisture, which kept it from drying when ground in oil, even if certain percentages of siccative were added when mixing.
The present-day practice of packing this pigment in paper sacks not only keeps it from absorbing moisture in transit and storage, but prevents waste and facilitates handling and also cheapens the cost of packages. While carbon black has been ground and sold as lampblack or has been made use of in strengthening inferior grades of lampblack, that practice is pretty well abandoned, as the discriminating consumer of to-day is better posted than formerly, and can readily tell gas carbon black from lampblack by its hue and the tint it produces when mixed with white. Good lampblack burned from oil, when mixed with zinc white one part by weight of the black to 100 parts by weight of zinc white, produces a bluish-gray tint, while carbon black produces a dull gray tint that would be called a smoky gray by most painters. Carbon black, in comparison with the better grades of pure lampblack, has a brownish hue, but is really blacker than any other form of black pigment. It has a granular form, but mixes more readily with water than lampblack of the better grades, and this is also a guide to distinguish it from lampblack. Take a glass tumbler or beaker of clear water and sprinkle a small portion of the black to be tested on the surface of the water and if carbon black it will soon sink to the bottom of the vessel, while lampblack burnt from oil will remain on top, even if container is shaken. Carbon black requires from four to five times its weight of linseed oil, according to its condition of dryness and fineness, to form a paste, and is best run through iron or steel mills, although it can be ground on soft stone mills, but the output on the iron mills is greater. Carbon black is not found as such in color grinders' price lists or catalogues, but, nevertheless, large quantities are ground in oil and listed under other names, generally velvet black or marine black.
The specifications of many railroads and other corporations are so designed that it is necessary to use carbon black along with inert mineral bases to obtain the depth of the standard furnished for black paint for equipment, as ordinary lampblack will not produce the black required. Printing ink manufacturers use enormous quantities of carbon black, but only the finest selected grades are used for lithographers and plate printers' purposes. These are ground on roller mills in specially prepared oil, known as burnt oil or lithographers' varnish.
Carbon black, as a base for black varnishes, must be specially selected for fineness and depth, and any that leans toward the very brown or grayish hue should be rejected for this purpose. Small quantities going a great way in this line, it pays the grinder to have a drying apparatus or oven, heated by exhaust steam or other economical means, fitted up with iron racks, on which the black may be placed in iron pans, so as to have all possible moisture driven off, as the black must be ground in varnish and moisture is to be avoided if good results are desired. The varnish in which the black is to be mixed and ground should correspond in quality to that which is to be used in thinning the black base to the consistency desired for use, and it depends upon the body of the varnish how much black can be incorporated. The grinding, of course, must be done on a water-cooled mill that is in perfect balance and condition, and great care is required to keep the mill from becoming overheated. It may take anywhere from eighty-five to ninety pounds of varnish to from ten to fifteen pounds of the black, depending on the more-or-less volatile nature of the varnish. Varnish that is inclined to liver or jellify with the black must not be used, as the resulting black varnish would be a failure.