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Black Pigments
Gasblack.-A product derived from the incomplete combustion of
natural gas abundant in certain parts of America.It is an intense black of great staining power, giving a slightly brownish tinge when reduced with white.
It is not so greasy as blacks made from the combustion of oils and thus mixes readily with water.
It is, however, more employed in the preparation of printing ink than in the manufacture of paint.
Lampblack.-This pigment owes its name to the fact that it was originally prepared from the sooty deposits from oil lamps ; to-day it is manufactured on more scientific principles, mainly from coal-tar oil.
It is not so intense a black as gasblack, though it is sufficiently so for ordinary purposes.
Owing to its greater oil content, it does not mix so well with water, and since the oil is of the non-drying type, lampblack tends to slow down the drying of an oil paint.
Bone Black.- This black is made by the calcination of bones and other residues, after the removal of the fat and glutinous matter.
It has a fine texture but is not so strong as gasblack or lampblack in tinting strength, though it mixes rather more readily with oil and also with water.
Like lampblack, it tends to retard the drying of oil.
Ivory black was originally prepared by calcining the parings and chips left by ivory turners and sculptors and produced a rather better black than that made from bones, though it was considerably more expensive.
Ivory is seldom used for this purpose nowadays and the name" ivory black" now applies to a high grade of bone black.
Bone black is also commonly referred to as " drop black"; the name has survived from the time when it was customary, in accelerating the drying of pigments, to squeeze them in paste form through perforated plates whence they emerged in drop-shaped pieces.
Vine Black is prepared from vine twigs, wine lees, or grape husks by a process similar to that used in the making of bone blacks.
It gives a rich, dense black with a faint bluish tone. It mixes readily with water but is not suitable for use in oil paints.
Graphite, known also as Blacklead or Plumbago, is a compound consisting mainly of carbon with small amounts of alumina, silica, lime, and iron.
It is not so black as gasblack or lampblack but possesses exceptional opacity and spreading power.
It is completely inert and is unaffected by acids or alkalis or by heat.
It is used for many other purposes than for paint, notably in the manufacture of pencils: a considerable proportion of the graphite employed in paints is used for coatings for ferrous metals.
It was formerly fairly extensively employed in primers for iron and steelwork but it has no anti-corrosive properties, its value in this connection lying chiefly in the fact that, owing to the structure of its particles, it has good moisture resistance.
It would thus seem to be more suitable in a paint intended for application over a good rust-resisting primer. Mineral Black.-A term given to a low grade of pigment prepared from crushed waste coal, slate, and other minerals, with or without the addition of lampblack or other carbon blacks to supply the required density.
Mineral black is suitable only for the manufacture of cheap black paints.