White Lead

Synonyms:

Flake-White, White Lead, Céruse, Blanc d' Argent, Blanc de Plomb, Bleiweiss, Kremserweiss

White lead was known to the ancients. A face-powder or cosmetic, found, in its original pottery-box of about 400 B.C., in the neighbourhood of Athens, proved to be a mixture of white lead and whitening. Theophrastus, Pliny, and Vitruvius describe its manufacture from lead and vinegar. It was designated by several names, such as cerusa, cerussa, cerosa, psimuthion. In the first half of the fourteenth century it is mentioned as 'minium album'. It has been called by divers names after the place or method of its manufacture, or after persons who have devised special processes for preparing it.

White lead still continues to be made for the most part by processes which are essentially identical with the old method, now generally known as the 'Dutch' process. This consists in attacking metallic lead, in the form of 'crates,' 'grids,' or spirals, simultaneously by acetic acid, carbonic acid, atmospheric oxygen, and water-vapour. The metal is gradually converted into a mixture or compound of lead carbonate and lead hydrate. Other processes, generally yielding an inferior product, containing more carbonate and less hydrate, have been used. One of these consists in passing a current of carbonic acid gas through a solution of lead subacetate; in another, 4 parts of litharge, 1 part of common salt, and 16 parts of water, are kept in contact for some hours with constant agitation, and then carbonic acid gas is led into the mixture until it becomes neutral to test-papers.

The best white lead contains two molecules of lead carbonate intimately associated with one molecule of lead hydrate, and is represented by the formula 2PbCO₃Pb(OH)₂. This formula corresponds to about 70 percent of lead carbonate, and 30 percent of lead hydrate. If the proportion of hydrate rise above this percentage, the opacity of the paint is lessened seriously; if it fall much below the above-named figure, the binding-power and working quality of the white lead are impaired. Although the normal lead hydrato-carbonate is probably constituted of one molecule of each of its components, the formula previously given may be taken to represent the highest quality of white lead as a paint. It has been recently stated that two molecules of lead hydrate associated with three molecules of barium carbonate constitute a pigment actually better than flake-white in some respects.

It is scarcely necessary to say that the metallic lead used in the manufacture should be as nearly pure as possible, such, for instance, as the lead from the Upper Hartz, which contains but 2 parts of foreign metals per 1,000. These foreign metals, the presence of any one of which in sensible quantity may cause a discoloration of the product, are copper, bismuth, silver, cadmium, antimony, nickel, and, more particularly, iron. But not only must the raw material be pure, but it is necessary to guard against the contamination of the white lead, during its manufacture, by dust or sulphuretted gases.

The impurities and defects of white lead are (1) accidental, (2) intentional.

(1) Of the accidental impurities and defects of white lead made from pure metal, the following are the chief:

• Metallic lead, imparting a grey hue to the product.
• Massicot or litharge, the yellow oxide of lead.
• minium or red lead, which gives a rosy hue.
• Excess of lead hydrate, which causes translucency.
• Excess of lead carbonate.
• lead acetate.

A simple experiment will suffice to show whether lead acetate be present in objectionable proportion in any sample. Some of the dry pigment is to be ground with distilled water into a paste, thrown on to a wetted filter and then washed with freshly-boiled distilled water. The clear filtered liquid should give nothing more than a slight cloudiness on the addition of a little dilute sulphuric acid. Some samples of flake-white which had been insufficiently washed contained from 2 to 11 percent of lead acetate removable by distilled water. In order to ascertain whether the lead carbonate and lead hydrate exist in due proportion in a sample of white lead, a weighed portion of the dry pigment, after having been dried at 212º F., should be carefully roasted in a current of dry air, and the water evolved (2 to 3 percent) intercepted by means of a weighed calcium chloride absorption tube. This operation, however, requires much manipulative experience, and, unless accurately performed, may lead to erroneous conclusions.

(2) Of the intentional adulterations of white lead the following are the most usual: a. Heavy spar, that is, native barium sulphate; or the same compound artificially prepared (permanent white, blanc fixe).

• Gypsum.

• China-clay.

• Whitening or chalk.

• Lead sulphate.

The first of these adulterations is by far the most usual. Barium sulphate, in the form of finely-ground barytes, or heavy spar, is the material employed on the large scale for cheapening the cost of production of ordinary white lead; precipitated, that is, artificially prepared, barium sulphate is used in the case of the finer makes of this pigment. In either case the sophistication is very readily recognised. Pure flake-white, for example, loses 14 1/2 percent of its weight when strongly heated so as to drive off its carbonic acid and water, but 'Venice' white, which is white lead and barium sulphate mixed in the proportion of equal parts, loses, under such treatment, no more than 7.3 grains per 100. 'Hamburg' white, with 33 percent only of white lead, loses 4.8 percent, and 'Dutch' white, of which three-fourths are barium sulphate, gives off no more than 3.8 percent 'Crems,' or 'Cremnitz' white, is, or ought to be, pure white lead. The complete solubility of pure white lead in dilute nitric acid may also be made use of to detect the presence of barium sulphate, which will remain undissolved as a dense white powder. The adulterations with gypsum, china-clay, whitening and lead sulphate, can be recognised only by further tests.

Gypsum, for instance, gives off water when heated, and 1 part of it dissolves in 420 parts of water. China-clay also gives off water when heated, but is insoluble in water, and only slightly soluble in nitric acid. Whitening dissolves in all mineral acids, but lead sulphate is practically insoluble. After all, the detection of barium sulphate is the only point with which the painter need concern himself. It will therefore suffice if he ascertain that a sample of white lead is of first-rate colour and body, contains no sensible quantity of lead acetate, loses when heated 14 1/2 percent of its weight, and dissolves perfectly in dilute nitric acid.

It has been observed that white lead is less liable to be blackened by sulphuretted hydrogen and by other sulphides when it contains a small quantity of baryta-white, or of lead sulphate thoroughly incorporated with it by grinding. This observation opens the door to adulteration, it is true: and it is perhaps wiser to rely upon the protection furnished by resinous mediums and a final coat of mastic varnish rather than upon any admixture with other white substances.

The drawbacks attendant upon the use of white lead as a paint are its poisonous character, its sickly and noxious smell when used with oil, and its liability to discolour when exposed to sulphuretted hydrogen or any sulphide soluble in water. On the other hand the quality of the whiteness of the best flake-white is unimpeachable: the paint works admirably in oil, and has great body; moreover, flake-white not only mixes perfectly and safely with the majority of permanent pigments, but it serves to impart to slow-drying colours its own strongly siccative character. Besides all these merits white lead possesses a valuable property, which has scarcely been clearly recognised or duly appreciated. For when an old oil-picture is carefully examined, it will generally be found that if any portion of its surface (of the paint, not the varnish) show decided contractions and cracks, these are precisely those portions into which white lead has entered in smallest proportion, if at all. The most translucent parts, the rich glazings and the deepest shadows may be fissured, but not the high lights: examples illustrative of this point are referred to in Chapter XXIV (The Study Of Old Paintings And Drawings). of the present volume.

This property of white lead seems to depend upon a combination taking place between a part of the oil with which it is ground and a part of the lead hydrate which it contains. A degree of toughness and elasticity is thus imparted to those films of oil paint into which lead-white enters to any considerable extent.

Flake-white becomes brown, grey or black when exposed to the action of sulphuretted hydrogen, ammonium sulphide, or any metallic sulphide soluble in water. This discoloration, which is due to the formation of lead sulphide, occurs more readily in the presence of moisture: it is favoured by darkness to such an extent that a piece of perforated cardboard laid upon a dry oil-painted surface of white lead will, after a few weeks' exposure, give a white pattern representing the perforations on a buff ground, which corresponds to the solid parts of the cardboard. But, after the removal of the perforated card and subsequent exposure of the painted surface to strong light, this pattern will disappear, the coloured sulphide of lead being oxidized into the white sulphate. The same change may be more speedily brought about by means of a solution of hydrogen peroxide. By laying a sheet of white filter-paper soaked in this liquid upon the discoloured lead-priming of a prepared canvas the original colour of the paint may be gradually brought back, especially by the aid of a moderate degree of warmth.

This method is not available in the case of drawings or water-colour paintings in which flake-white has blackened; but even these may often be successfully treated by exposure to moist ozone, or by light touches of a solution of hydrogen peroxide in ether. The latter treatment has been successfully applied to a series of architectural drawings in gouache by C. Clerisseau in the Soane Museum. In these the high lights had become black. Old silver-point drawings, in which the lights were heightened with lead-white, may sometimes be thus restored to their pristine state.

The specific gravity of the best flake-white is 6.6; 100 parts by weight of it require from 11 to 15 parts of linseed oil in order to form an oil-paint of suitable consistence. It is sometimes ground with poppy oil when a particularly pure white product is demanded. The yellowish tint of some makes of white lead is occasionally neutralized by the addition of a trace of indigo or of artificial ultramarine. Burnt or roasted white lead is sometimes used as a pigment. It is of a cream-colour, a buff, or a pale yellowish salmon, according to the temperature at which it has been prepared, or the length of time during which it has been heated.