INGREDIENTS OF PAINT White-Base Pigments. The principal white base pigments are white lead and zinc white. Besides these there are several white pigments, which are variously regarded as inert pigments or as extenders. Among these are barytes, whiting, and gypsum.
White-lead, one of the oldest known pig ments, is the hydrate-carbonate of lead, and is of varying composition, no absolute formula being recognized; but the best white lead contains about 25 per cent of hydrate to 75 per cent of the carbonate. White lead may be made by the stack (or so-called "Old Dutch" process), which requires about 100 days for corrosion, or by one of several so-called "quick" processes, wherein the principles of the Dutch process are applied to finely powdered or thin sheet lead, contained either in revolving vats or chambers or in some other suitable form of apparatus by means of which acetic acid, water, and carbonic acid gas may be brought in contact with the metallic lead.
White lead unites with linseed oil to form an inelastic, insoluble lead soap, and this prop erty gives it one of its greatest values as a pig ment. It perishes by gradually powdering away on the surface, termed "chalking," so that a house painted with white lead may always be repainted without scraping or burning off. If the lead is poorly made, especially if it contains acetic acid which has not been washed away after corrosion, the "chalking" will be quite rapid and will be a serious objection.
Sublimed white lead, which is used to a con siderable extent in mixed paint making, is not a true white lead, but is a lead sulphate that is white in color and is produced by a fire process. It is chemically inert, and does not unite with linseed oil to form a soap. It has a tendency to correct the "chalking" of the ordi nary white lead when mixed with it.
Zinc white, or oxide of zinc, is made by blow ing a current of air through molten zinc. It is comparatively inert chemically, although it unites with oil to a certain extent to form a zinc soap. It is free from the poisonous qualities which render white lead objectionable for an indoor paint, and is not acted on by sulphur gases, which darken white lead. Zinc white spreads much more rapidly than white lead, but does not cover so well, so that it will require four coats or even more of zinc white to hide a surface as well as three coats of white lead.
Zinc white dries to form a hard and brittle paint film; and perishes, by cracking, peeling, and scaling, which are very objectionable unless neutralized by the addition of some other pig ment. It is, therefore, very seldom used alone on exterior work, but almost invariably in com bination with lead or some other white base.
Barytes, or sulphate of barium, is a native rock, which is very finely ground and is used as an inert extender of paint. It has no affinity for linseed oil and absolutely no covering power. Four coats of barytes mixed with linseed oil will not hide the surface to which it is applied and the film is practically transparent. The same is true of silica, which is another so-called inert pigment.
Whiting is lighter in weight than barytes and possesses a decided affinity for linseed oil. Mixed with oil in paste form, it becomes putty. It has very little covering power in itself.
Linseed oil, the oil expressed from the seed of the flax, is remarkable from the fact that it unites with oxygen to dry to a waterproof film. It is this fact that it is a "drying oil" that gives it its great value in paint. There are some other drying oils, such as walnut oil and poppy-seed oil, but they are not produced in sufficient quan tities, and are therefore too expensive to be of any interest to the house painter. Linseed oil is adulterated with various mineral oils (petro leum products), with rosin oil, with corn oil, and with fish oil. The first named are the most common adulterants.
Turpentine is the distilled sap of the long leafed pine. It is used to make paint more fluid and hence to make it spread easier. It dries by evaporation, leaving a slight gummy residue. Owing to its high price, it is apt to be adulterated with the lighter mineral oils. The source of supply of turpentine is decreasing, and many efforts are being made to find something that will take its place. A so-called "wood turpen tine" has been introduced, which is a product of the destructive distillation of pine wood. As it is a wood distillate, somewhat akin to wood alcohol, it may possibly possess the poisonous property of this material, which seriously affects the nerves of the eyes when inhaled to any great extent. It is therefore well to be cautious about wood turpentine.