THIS is the most important of all paint forming dry material, from the standpoint of general, indeed universal use, it answering most of the requirements demanded.

Its production cost and selling price is moderate, it can be used as a base to combine with other pigments, has great opacity, so-called covering power, mixes readily with linseed oil and turpentine, aids the drying properties of the paint of which it forms a part, works easily under the brush when properly mixed, and has reasonable durability. The last-mentioned property is governed by the quality of the oil used and the conditions to which the painted surface is exposed. White lead or basic carbonate of lead of average quality contains approximately:

As met with in commerce, it is a heavy pigment of varying degrees of whiteness, depending upon the method of manufacture and the care exercised in its production.

Sometimes a trace of Prussian Blue is added to increase its appearance of whiteness.

The processes of manufacture in practical operation may be divided into the slow or so-called Old Dutch Process, with its modifications, and the various quick process methods whose name is legion, few of which, however, are in practical use.

The Old Dutch Process consists in casting the metallic lead, free from impurities, in the form of buckles or thin strips of spiral formation; the lead thus formed is placed in earthern ware pots so that it is held or suspended above vinegar or acetic acid which covers the bottom of the vessel. These pots are arranged in rows and covered with planks and a layer of dung or spent tan bark. Row upon row is built up in this manner to form what is termed a stack. The fermentation of the dung or tan bark produces sufficient heat to volatalize the acetic acid, the fumes of which, with the assistance of the oxygen of the air, converts the lead into basic acetate of lead, which in its turn is converted into basic lead carbonate by the carbonic acid resulting from the fermenting manure or tan bark.

The process of conversion requires about three months. The resulting product is removed, separated from the metallic lead which may still remain to some extent, washed, ground and dried, if it is to be sold in that form. When to be ground in oil at the place of manufacture, the drying process is usually eliminated. The wet or pulp lead in this case being passed, after screening, into a pulper. Linseed oil, in proper quantity, is introduced and the mechanical action is such, that the oil displaces the water contained in the pulp lead and the now finished product is run into kegs.

This is pulp lead in contradistinction to mill ground lead made in the old way by grinding dry lead with linseed oil in stone mills of various types.

Mill ground lead appears to have some favorable points to those unacquainted with the pulp process, but the latter method is so perfect as to produce lead practically free from water and in such condition as to "break up" in oil or turpentine much more quickly and easily than will the mill ground product. Excess of moisture is the essential point to guard against.

The English method of making white lead is to convert the metallic lead into litharge, which is done by oxidizing the metal in a furnace. The litharge in a finely divided state is moistened with lead acetate solution, placed in closed troughs into which carbonic acid gas is driven, the litharge being continually agitated by suitable machinery; white lead is the resulting product. This lead covers well, and is considered good. Of the other quick process methods, the one most largely used in this country, subjects the metallic lead in a finely divided state to the action of carbonic acid gas and acetic acid, in large revolving cylinders.

Another method is to dissolve the lead in acetic acid, forming a solution of basic acetate of lead and precipitating this lead in the form of white lead by introducing carbonic acid gas into the solution.

The physical properties of white lead produced by the various processes differs in opacity, fineness, density, and color.

Quick process leads have, as a rule, a crystalline formation, giving a tendency to transparency, while slow process lead (Dutch Process), on account of the amorphous condition of the substance, is denser, more opaque, and masks or covers better. This opacity is heightened by the pigment being coarser in grain than the precipitated leads.

Slow process leads are apt to be more or less off* color and to contain impurities unless very carefully prepared. The quick process leads, in which the methods are under perfect control, produce a product, when complete, excelling in whiteness and fineness of texture. This latter property, the fineness of the particles, makes the lead bulkier and hence it requires more oil in producing a workable paint and is another reason why it seems to cover poorly, The proportion of lead being less and oil greater.

Of all dry materials used as pigments, white lead has probably the most action on linseed oil, the hydroxide of lead combining with a portion of the oil, to form lead soap, which helps to hold the carbonate of lead in suspension and accounts for the smooth working qualities and the tough elastic coat it produces. Too much action between the particles of lead and oil produces the chalking tendency of white lead. This is sure to occur eventually, because the natural tendency of the lead is to aid the oxygen in the air to oxidize the oil until the binding properties are entirely destroyed. When white lead is used, however, the surface left is in better condition for repainting than that left in the use of most other pigments.

White lead is sensitive to the action of alkalies, acids, and many substances containing sulphur, such as sulphuretted hydrogen, which occurs in coal gases. The sulphur in coal gases causes white lead to turn black, due to the formation of lead sulphide. The drip of rain water from trees or other foliage also tends to make lead blacken in spots and mildew, particularly where the shade is dense and little sunlight penetrates. Tinting colors containing sulphides, or linseed oil which is impure, or which has been bleached and clarified with certain chemical agents also exercises a bad effect on white lead, under certain conditions. What appears to be a dirty blackened condition of the paint may also be due to dust particles adhering to the surface of the paint. In this case, washing with a very weak solution of sal soda will renovate the surface, the paint underneath being found to be in good condition. In a small way, the blackening can be removed by the use of a strong solution of Hydrogen Peroxide with 5 per cent. of ammonia water added, applied with a sponge. This chemical converts the blackened lead into lead sulphate which is white. Sunlight also tends to bleach darkened white lead paint.

White lead is adulterated with lead sulphate, (a poorly covering white, which mixes badly with oil), chalk, clay, barytes, gypsum and silex.

These adulterations, in most cases, being neutral pigments, rather increase the durability of the paint. The objection to these substances is that they detract from the covering power of the paint, give it a tendency to crack, and are very much cheaper than the white lead you are paying for.

Indeed, in the writer's experience, a case was noted where pure lead and oil were used and mildew and blackening occurred, whereas, on part of the same work, the same lead and oil, with 50 per cent. paris white added, was applied at the same time the pure lead was used. The latter work showed a white, glistening and perfect surface when the pure lead paint was in a deplorable condition. The only favoring condition given the adulterated paint, was its exposure to the direct sunlight, while the other painted work was shaded by surrounding trees.

It is likely that the neutralizing action of these cheap pigments do reduce the chemical action of the white lead on the oil, preventing the binder of the pigment from perishing so soon as it would without them, but producing brittleness and lack of elasticity.

Tests

Subject the suspected sample of lead to the blow pipe flame on charcoal. Pure white lead is readily reconverted into metallic lead without residue. Any residue present in the form of white powder is likely to be sulphate of lead or barytes. Pure white lead is perfectly soluble in dilute nitric acid and the addition of caustic potash solution should not form a precipitate. A residue in the nitric acid solution indicates gypsum, barytes, or lead suphate.