The London Journal of Arts, Sciences, and Manufactures, and Repertory of Patent Inventions.
Conducted by Mr. W. Newton, of the Office for Patents, Chancery Lane. (Assisted by several Scientific Gentlemen.)
VOL. XXXVI. (Conjoined Series.)
London: Published by W. Newton, at the office for patents, 66, Chancerylane, and Manchester; t. and W. Piper, Paternoster Row; Simpkin, Marshall, and Co., Stationers' Court; J. McCombe, Buchanan St., Glasgow; and Galinani's Library, Rue Vivienne,
Paris. 1850
No. CCXVIII. The compound commonly known as white-lead (chemically speaking, the carbonate of oxide of lead) is universally employed, not only as a pure white pigment, but also as the basis of almost every color of light tint when covering power and great body of coloring matter are necessary. In some respects, carbonate of lead possesses properties which render it more extensively useful to painters than any other substance known to the chemist. In the first place, when ground with either oil or water, it forms a mixture of extraordinary density and opacity; which, when laid upon a surface of metal or wood, endues that surface with a coating of matter completely impervious to light; so that the rays of the latter falling upon such a surface, are scarcely at all absorbed, but nearly all reflected back to the eye: the painted surface consequently appears of a strong opaque white, and the original color of the object, whether it be natural or arise from a previous coating of paint, is perfectly covered and hidden. The great opacity of carbonate of lead gives to it as a pigment the property technically termed its "covering power," a property which does not exist to an equal degree in any substance hitherto proposed as a substitute for it. Carbonate of lead is also perfectly neutral; it therefore exercises no action upon other pigments, neither changing their color nor otherwise affecting them; and from its physical character or texture, it is easily ground with water or oil, blending completely with them, and forming a smooth homogeneous mix ture, which can be laid on by the brush with great facility. These are points in which white lead must be considered superior to any compound that has been employed in its stead; but it possesses a great defect, which is sufficient to render it highly desirable that some substance may be discovered which may be substituted for it in the arts. Certain metallic oxides or compounds have accordingly been, for some time past, proposed as a substitute for white lead; but among them, the oxide of zinc alone seems entitled to attention. In order, however, to understand the comparative value of these proposed substitutes, quo ad their chemical qualities, it will be necessary to examine into the habitudes of lead itself. Carbonate of lead is a compound upon which the normal constituents of the atmosphere are incapable of exercising any chemical influence; but it seldom happens that near the dwellings of man, at least in crowded cities, the atmosphere is found in its normal state of purity. The emanations from the bodies of living animals, and the exhalations from decaying animal and vegetable matter, serve to contaminate the air with a multitude of gaseous matters foreign to it in its natural condition; among these is found a gaseous compound of sulphur termed sulphuretted hydrogen. One of the striking properties of lead, in any state, is its affinity for, or tendency to combine with, sulphur. It is impossible for lead and sulphur to be brought into contact with out this combination taking place. So predominant and searching, indeed, is this force, that even if a piece of paper be wetted with a solution of sugar of lead (acetate of lead) and placed between the leaves of a thick book, sulphuretted hydrogen will, if it exist in the air, find its way to the lead on the surface of the paper and there combine with it, forming a black compound, which chemists call the sulphuret of lead. Every compound of lead is subject to this action, no matter whether it be soluble in water or otherwise; the sulphur will seize upon the lead, and the black sulphuret will be formed. This is the secret of the blackening of white paint in certain localities in which sulphuretted compounds are freely evolved; and this change takes place more or less rapidly under all circumstances, for there is always sufficient sulphuretted hydrogen in the air to ultimately effect the formation of the sulphuret of lead. It may be said that, practically, this pro perty of lead to combine with sulphur, and blacken under its influence, amounts to no great defect,—that the lead is so completely enveloped and protected by the oil and varnish, that the action of the sulphuretted hydrogen is set at defiance. Such, however, is not the fact; and this may be easily proved by examining a surface of white paint in the neighbourhood of a drain or cesspool, where it will be found covered with a blackish-grey semi-metallic-looking film of sulphuret of lead. Chemically, then, we see at once the reason why white lead can never be regarded as a permanent color; this remark applies as well to the colored preparations of lead as to white lead; and it is an in disputable reason why, in all cases where permanence of color is a chief object, some metallic compound should be employed, in which sulphuretted hydrogen can produce no change, or, at all events, no blackening. There are but few substances, perhaps one only, which fulfil the necessary conditions; the one we refer to is zinc As a white pigment, more than one substitute for white-lead has been proposed, such as oxide of antimony and sulphate of baryta; these, however, from their want of that remarkable opacity so peculiar to white-lead, seem to be of but little value. * The value of oxide of zinc as a pigment has been long known, but it could not be procured in sufficient quantities for any practical purpose. At the present time, however, this difficulty is removed: within a few years several patents have been taken out for the preparation of this substance, which is now becoming an article of commerce.The oxide of zinc alone keeps the field. Oxide of zinc, although not possessing the "covering power" of white-lead, has still sufficient opacity to admit of its being substituted for the latter with great advantage: It works as well with the brush, grinds with facility,—forming a perfectly smooth mixture with oil, and, like white-lead, is a neutral substance, inert, in relation to other coloring compounds. With respect to its chemical habitudes, oxide of zinc is far superior to carbonate of lead, and why? Simply because, in the first place, the affinity of zinc for sulphur is much less than that of lead for the same substance; and, secondly, be cause the sulphuret of zinc is as white as the oxide: so that if, by any means, the whole of the oxide of zinc, laid upon an object as a white pigment, became converted into sulphuret, the whiteness would remain unsullied, for the oxide would merely have passed into another compound as white as itself. Oxide of zinc may, therefore, be regarded as a truly permanent color; and is invaluable, under circumstances where the blackening of the lead compounds would prove a serious disaster.*
The chromate of zinc, a beautiful yellow substance, is another most valuable pigment, which may be well substituted for chrome-yellow in all cases.
It is, however, chiefly with regard to the labors of the artist that these considerations are of value. The blackening of a surface of white paint can, under ordinary circumstances, be easily remedied; but such a change in a picture would be irremediable: how much, therefore, does it behove the artist to study the chemistry of his colors before he commits him self to their employment.
T. W. K.
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