Chemistry of Paints

White Pigments.

White lead, of which Flake White and Kremnitz White are varieties, approximates in composition to the basic lead carbonate and possesses greater covering power than most white pigments. Used as an oil colour, its films are prone to yellowing in the dark; this discoloration, which is attributed to the basic character of the pigment and is less marked with the normal carbonate, disappears on re-exposure to sunlight. Though readily blackened by sulphuretted hydrogen, it has great atmospheric durability and is unaffected by light and sulphur dioxide. Its resistance to sulphides is greatly influenced by the presence of lead acetate as impurity; pure white lead in oil is stated to be compatible with ultramarine, cadmium yellow and vermilion without discoloration. On account of its poisonous nature many attempts have been made to replace it, but always with a sacrifice of covering power or durability when used for external painting. Of the more notable substitutes, the following may be mentioned :—Mulhaus White (lead sulphate) ; Freeman's non-poisonous White Lead, a mixture of lead sulphate, zinc oxide, magnesium carbonate and barium sulphate ; and Sublimed White Lead, a basic lead sulphate containing zinc oxide. Lead sulphate being deficient in body is used mainly in admixture with other pigments, e.g., lead chromes, to produce paler shades.

Zinc White (Chinese White, Zinc Oxide) was first proposed as a white-lead substitute by Courtois of Dijon in 1780 and became generally adopted as an artistic and industrial colour in 184o. It is a brilliant white possessing considerable covering power though lacking in opacity, is non-poisonous, and is unaffected by light or sulphuretted hydrogen. As a water colour it is incompatible with most pigments, this being more marked when gum arabic is used as a medium. In oil the paint dries slowly to a hard film having a tendency to scale; it is extensively used in white enamels. As an artists' colour it is sometimes preferred to white lead, but owing to its higher oil absorption it is more readily lowered in tone by the physical changes in the oil. Lithopone is a white pig ment obtained by precipitation of zinc sulphide and barium sul phate simultaneously in approximately molecular proportions and subsequent calcination. The properties of Lithopone depend largely on its purity and method of manufacture ; thus in particu lar may be mentioned its tendency to darken in sunlight. This discoloration is reversible and the white colour is restored in the dark. Owing to its non-poisonous nature, stability to sulphuretted hydrogen and atmospheric influences, and its good covering power, it is used in considerable quantities as a water, size and oil colour; in textile printing; in the manufacture of linoleum and in many other industries. Titanium White (Titanox, Titanium Oxide) is a white pigment of exceptional body and covering power, far exceeding that of white lead or zinc oxide, in admixture with which latter pigment it is often employed. Owing to its chemical inert ness, it is unaffected by atmospheric influences and is non-poison ous. As a paint its films do not crack or peel but show a strong tendency to "chalk," a defect arising from the gradual destruction of the film in moist air. Antimony TV kite (Timanox, Antimony Oxide) was introduced as a white-lead substitute during the World War, and has since found extended use in the manufacture of paint and linoleum. It is non-poisonous and is less prone to discoloration than white lead.

Among the white pigments are also found most of the bases used in colour-making as extenders and adulterants: Barytes (Bar ium Sulphate), which occurs naturally as "Heavy Spar" and is produced artificially as "Blanc Fixe" (Permanent White) ; Hy drate of Alumina (White Lake) ; Whiting (Calcium Carbonate) ; Terra Alba (Calcium Sulphate) ; China Clay; and others.

Yellow Pigments.

Lead Chromes form by far the most important class of yellow pigments. These cover a variety of bright shades of yellow ranging from a pale primrose (in which the chrome is intimately admixed with lead sulphate or arises direct as a precipitation of a lead sulpho-chromate) to a deep orange-red. The orange and deeper shades owe their colour more properly to the formation of basic lead chromates; the physical state of the pigment, however, plays an important part and is largely dependent upon the condition under which the chrome is made. The lighter varieties of lemon (neutral) lead chromates, Chrome Yellows, are produced at a low temperature, by using very dilute solutions or otherwise controlling the process so that the precipitate is maintained in a very fine state; such chromes possess good covering power, a property diminishing as the shade deepens, owing to the consequent increase in the specific gravity of the colour. The raw materials employed in the manufacture are lead nitrate or acetate and the precipitant is the dichromate of sodium or potassium; for the deeper shades, basic salts of lead are used, or the chrome receives subsequent treatment with caustic alkali.

Zinc Yellow (Zinc Chromate) is inferior to Chrome Yellow in body and covering power but is not blackened by sulphuretted hydrogen. It is used mainly as an oil and water colour in the pro duction of lime yellows, and in the manufacture of the non poisonous Zinc Greens by admixture with Chinese Blue. Cad mium Yellow (Cadmium Sulphide) possesses excellent covering power and is largely used as an oil and water colour. The shades of pure cadmium sulphides depend essentially upon the method of preparation and vary from bright lemon yellow to orange. In the manufacture of cheaper varieties of the paler shades, the cadmium sulphide is often admixed with cadmium carbonate, giving a product of good body but prone to fade in moist air. A cheap pigment fast to light is obtained by the simultaneous pre cipitation of cadmium and zinc sulphides, providing no carbonate is present. Compounds of cadmium sulphide with cadmium selenide in varying proportions form a further series of colours covering a range from orange to maroon-crimson according to their composition ; such pigments are largely applied in the cera mic industries owing to their heat-resisting properties. A new series of compounds have been recently introduced in which cad mium sulphide (with or without selenide) is precipitated together with barium sulphate to form Cadmium Lithopones (cf. Litho pone, above), which on account of their comparatively low cost lend themselves to more extended industrial applications. Cad mium yellows form a stable series of bright greens with Ultra marine. Aureoline (Cobalt Yellow), possesses only moderate stability to light, but is unaffected by sulphuretted hydrogen and is used mainly as an artists' oil and water colour and for the pro duction of pure blue shades in glass and porcelain painting. It is transparent in oil and turns brown in excess of this medium.