6. White lead is a carbonate of lead produced by several methods, the best being obtained by the Dutch process, which consists in the taking of gratings of pure lead and exposing them to the fumes of acetic acid. The gratings are, by this treatment, corroded and covered with a crust of carbonate, which is removed and ground to a fine powder.

7. Precipitated white lead is made by suspending rolls of thin sheet lead, or small bars, over malt vinegar or pyroligneous acid in closed vessels, the evaporation of the acid being kept up by heat applied to the vessels, while immersed in a steam bath. The white lead, produced by precipitation, is generally considered inferior to that prepared by corrosion, wanting, as it is, in density or body, and, when mixed with its vehicle, absorbing too much oil. It has, however, one advantage; it requires no grinding to prepare it for use.

8. The following is a simple method by which pure white lead may be prepared when not otherwise obtainable: Procure an earthen vessel, with a cover something like a colander, but having bars across instead of being perforated. This cover is made of strips of clay well burned. For temporary purposes, however, bars of wood will suffice. Vinegar is poured into the vessel, filling it almost to the bars, on which have been placed narrow strips of common lead rolled into scrolls. These, to remove any accumulated oxide or foreign matter, should be carefully scraped before use, and then so placed over the bars that they do hot touch each other. The pot is gently heated, and the fumes of the vinegar corroding the lead reduce it to a white powder ready for mixture with oil.

9. The white lead, used in oil painting, is sold already ground in oil-reduced to a thick paste, much too thick for use, and requiring dilution to bring it to a proper consistency. This is done by placing some of it in a pot, pouring on it a small quantity of oil and turpentine, then stirring the compound thus obtained with a stiff palette knife or flat stick until the particles of the pigment are all separated and a perfectly smooth mixture obtained. Should this, however, still prove of too pronounced a consistency for use, it should be passed through a tin strainer or a piece of canvas, after which driers are to be added, and the paint reduced to the proper consistency for work.

10. There are other white oxides of lead, the heaviest and whitest of which are the best, being, in point of color and body, superior to all other whites. When pure and properly applied in oil and varnishes, they are safe and durable, and dry well; but excess of oil discolors them, while in water painting they are very changeable, sometimes becoming almost black. They have, besides, a destructive effect on all vegetable lakes, except the madder carmines, and are also injurious to minium, the red and orange oxide of lead; to orpiment, or king's yellow, which is a trisulphide of arsenic; to massicot, the yellow oxide of lead; to patent yellow, a chloride of lead; and to gamboge, a pigment derived from a powdered yellow gum; but with ultramarine blue, vermilion, yellow, and orange chrome, all hereafter described, and with the lakes, ochers, and siennas, these whites compound with little or no injury.

11. In oil painting, white lead is essential in the ground for dead coloring, or the formation of tints of any color, as it is also in scumbling, either alone or mixed with other pigments. (Scumbling is the softening or blending of two or more adjacent colors, by rubbing them over with a brush, or the finger, either dry or charged with additional color.) White lead is, moreover, when neutralized with black, the best local white.

Lead pigments should not, however, be employed in water colors, distemper, crayon, or fresco painting; for, with all such, they occasion change of color, either by becoming dark themselves, or by causing the colors with which they might be mixed, to fade. Cleanliness in using these colors is to be specially enjoined; for, although not virulently poisonous, they are, when taken into or imbibed by the pores or otherwise, pernicious, as, indeed, are all pigments of which lead is the basis.

White lead improves with age. It should not be exposed to the air, or it will turn gray. Old white lead of good quality goes further and lasts longer than if used when fresh. Paint made with fresh lead has, moreover, a tendency to become yellow. Fresh white lead has often a yellowish tinge, caused by the presence of iron.

12. Adulteration

Adulteration. White lead maybe obtained either pure, or mixed with various substances, such as sulphate of baryta, sulphate of lead, sulphate of lime, whiting, chalk, zinc white, etc. These substances do not combine so well with oil as does white lead, nor do they so well protect the surfaces to which they are applied.

Sulphate of baryta, the most common adulterant, is a dense, heavy, white substance, very much like white lead in appearance. Absorbing very little oil, it may be easily detected by the gritty feeling it produces, when the paint is rubbed between the finger and thumb.