Paint Color Pigments

The most important pigment is white lead. Every one has heard of Dutch process white lead; it was invented before the Dutch took Holland, but that is no matter. It is the standard white lead; it is made from metallic lead, corroding it by the aid of acetic acid, but it does not contain any acid; it is a basic carbonate of lead, a very white, impalpably fine powder; heavy, as all lead compounds are, and having a natural affinity for oil. It is sold as dry white lead, a powder, or as paste white lead, which usually contains ten per cent of linseed oil, and is a thick, heavy paste, to which oil may be added by stirring it in with a stick or paddle. A gallon of white lead paint weighs 21 or 22 pounds; if we add 5 gallons of oil to 100 pounds of paste lead we have 6½ gallons of paint weighing 21.3 pounds per gallon; a gallon of oil weighs 7.7 pounds, a gallon of turpentine 7.2 pounds, and a gallon of benzine a little over 6 pounds. 15 pounds paste lead and 6.3 pounds oil make a gallon of paint; 14 pounds dry lead and 7¼ pounds oil make a gallon. A gallon of anything measures 231 cubic inches, and a gallon of water weighs 8 1/3 pounds. But linseed oil is sold by the barrel on the conventional or trade agreement that 7 ½ pounds shall be sold as a gallon, or a 5O-gallon cask will measure nearly 3 gallons short; this is to cover the cost of the barrel. Similarly, turpentine when bought by the barrel is usually bought on Charleston or Savannah measure and is two or three per cent short, having evaporated through the staves of the barrel. White lead of similar composition is also made by a more rapid process than the Dutch process, and is probably not very different in its qualities. White lead of this composition turns black if exposed to sulphur, and white lead paint becomes yellowish after a time if not exposed to sunlight. In the sun it constantly bleaches out to a clear white.

Reducing paste paint to the consistency of paint by the addition of more oil is by painters called "breaking" it; this is done by putting the paste in a suitable vessel and stirring it with a paddle (or in a mechanical mixer) with enough oil to make a thick fluid; and when it is uniformly mixed add more oil until the necessary amount has been put in. Paint composed largely of white lead improves by standing in a tightly covered package; it is appreciably better after a year than when it was first made. This is probably not true of other pigments, but there seems to be a slow action on the oil which is advantageous. White lead acts as a drier on oil, and it is possible to make a paint of white lead and raw oil without any drier which will dry fairly well in hot, dry weather out of doors.

Sublimed white lead is a very different thing. It is made directly from ores containing lead and zinc by roasting them in a furnace; the lead is converted into a basic lead sulphate, and the zinc into oxide, and these are carried off in a current of air, and after cooling are collected as a white powder containing about five per cent of zinc oxide. This sublimed lead is a white powder, very fine, and is not much discolored by sulphurous gases. It is claimed by the makers to be superior to the other kind; but according to Professor Ladd, who has investigated this matter, many troubles have been encountered in its use. It is less expensive than standard white lead, and is used by some manufacturers of mixed paints. Probably it is a valuable pigment, if not quite so good as the other. Zinc-lead is a similar compound containing a very much larger proportion of oxide of zinc.

Next to white lead in importance is white zinc, or zinc oxide, made by burning zinc ore in a current of hot air, though French zinc is made by burning the metallic zinc. It is the most nearly pure white pigment we know, and is a most valuable ingredient of paint. It is less heavy than white lead; 9.5 pounds white zinc and 5.7 pounds oil make a gallon of paint, which weighs 15.2 pounds, or 1 gallon oil and 12 pounds zinc make 1.3 gallons; or 100 pounds zinc and 8 1/3 gallons oil make 10 5/8 gallons of paint.

Lithopone is a white pigment made by adding a solution of zinc sulphate to a solution of barium sulphide; the result is a precipitate containing barium sulphate and zinc sulphide. Lithopone, however, is seldom of the exact composition thus indicated, and usually contains some zinc oxide. It is a pigment of considerable value, but cannot be used with lead; even a lead drier will turn it dark. It is said that it will mix with shellac, which neither white lead nor white zinc will do.

There are other white powders, which are often put into white paint, to cheapen it (extend it), or to produce certain desirable qualities. These are barytes, which is natural barium sulphate, ground to a fine powder; blanc fixe, which is precipitated barium sulphate; terra alba, which is hydrated calcium sulphate, or gypsum; whiting, which is powdered chalk, or calcium carbonate, marble dust, silica, silicate of magnesia, and China clay or kaolin. None of these are of any value by themselves; when wet with oil they become nearly transparent. It is not unlikely that some of these, in aggregate less than 25 per cent of the pigment, may be valuable; but their exact standing has not been settled.

There are two principal blue pigments, ultramarine and Prussian blue; the most important yellow is chromate of lead, or chrome yellow; ocher is a yellow clay, dried and powdered, containing iron; it is dull in color; green is a mixture of chrome yellow and Prussian blue, called chrome green; and for bright red we usually use a pigment made by precipitating a coal-tar color on barium or lead sulphate or some other neutral base. English vermilion is too costly to be used much in house-painting; the dull reds are oxides of iron, such as Venetian red, Tuscan red, Indian red, and the like; the browns are likewise iron oxides. The blacks are either boneblack or lamp black, though graphite is used as a preservative paint on metal. There are, of course, many other more expensive pigments, which are used in carriage-painting and the like, which may sometimes be used in the house, but as a rule house-paints are made up of the foregoing.

To mix these pigments properly with oil they should first be mixed in a mechanical stirrer, and then ground through a burr-stone mill; but it is not unusual to mix them in a stirrer, or mixer as it is called, and use them directly.

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