ZINC, or Spelter, is a semi-metal, naturally obtained in a state of combination with different minerals, in England, Hungary, and other parts of the globe: it is of a whitish colour, nearly resembling that of lead, though it does not so speedily tarnish.
Zinc melts a short time before ignition; but, when heated to redness in the open air, it is liable to combustion, and burns with a dazzling blaze; so that a loose white oxyd is precipitated, which is known under the name of flowers of zinc.
This crude semi-metal is of great utility in the arts. Combined with gold, in equal portions, it forms a hard, white compound, that admits of a fine polish, and may be advantageously manufactured into specula, for optical instruments.
Zinc and tin, melted together, produce a kind of pewter; and, as the former spreads more uniformly, while it is much harder, and less fusible than tin, it has been proposed as a substitute for the latter, in tinning copper-vessels.
Spelter and copper readily unite in the fire; provided the combustion of the former be carefully prevented during the process: in this state, it forms a metal, distinguished by the general name of yellow copper; but which is divided into several sorts, according to the respective proportions contained in the alloy. Thus, three parts of copper and one of zinc, constitute Brass;—five or six of the former, and one of the latter, afford Pinchbeck.—Tombac is composed of a still larger proportion of copper to that of zinc: it is of a deeper red than pinchbeck, and bears the name of its inventor. Prince's Metal consists of a larger proportion of zinc than either of the preceding compositions.—Similor, or Manheim gold, resembles pinchbeck: it is manufactured into spurious leaf-gold, laces, and similar shewy articles.
Zinc is dissolved in all acids; though the sulphuric is most frequently employed; and, in the proportions stated, p. 275, of the present volume, it forms white Vitriol.
An useful substitute for white-lead, in painting houses, has lately been discovered in zinc, by M. de Morveau. He directs this mineral to be calcined in a crucible, placed horizontally in the cavity usually made for retorts, in reverberatory furnaces. The oxyd thus obtained, is then to be washed in water, with a view to separate such particles as may not have been perfectly calcined; and, when it is reduced to powder, a small portion of earth of alum, or chalk, must be added; in order to give it a body. When this pigment is to be used, it will be necessary to form the powder into a heap, leaving a small hole in the middle, into which oil must be gradually poured, till it be reduced to a proper consistence; when the paint should be laid on, with a soft brush. The whitest drying oil must be procured, such as that obtained from poppies, if a white paint be designed; because coloured oil imparts a tinge that impairs its whiteness; but, if a yellowish or other shade be intended, any drying oil will answer the purpose.—M. Morveau observes, that such paint is perfectly harmless, emitting no hurtful effluvia; and, though it does not dry so speedily as that prepared of white-lead, yet it is not only more wholesome, but also eventually cheaper; as a smaller portion of zinc will be required.
In March, 1796, a patent was granted to Mr. John Atkinson, for his invention of a white paint, prepared from zinc, which may serve as a substitute for that of white-lead. He directs the former mineral to be first submitted to a reverberatory furnace, for six hours; in order to disperse ail the ferruginous particles which it may contain. Next, the zinc is to be reduced to powder, by the action of a mill, and mixed with one-eighth part of pulverized charcoal, by weight; after which it must be removed to a close, or muffed furnace, provided with two apertures, one on each side, "and (as the patentee expresses himself), dilated at the end from the furnace, by a distance of about 20 feet;" the other end joining the body of the furnace: such apertures should each be furnished with a door at the farthest extremity, and which ought to be sufficiently large to admit a man to enter, for the purpose of collecting the colour. Thus, the zinc must be introduced into the furnace, through the top or upper part: when it becomes red hot throughout, a large dense, white cloud, with a bright blue flame, will pass into the receptacles or apertures above-mentioned, where it will collect in the form of a pure, white metallic calx.
The oxyd of zinc is now to be diluted with water, and ground or triturated in a proper mill; from this machine it is conducted, by means of gutters or spouts into fine sieves, whence it passes into several cisterns full of water, communicating with each other by similar gutters; so that the finest particles float into the farthest reservoirs. After standing about 24 hours, the water may be drawn off, and the colour collected into pans, receivers, or other vessels, capable of bearing heat, in which they are dried; and in this state, the paint will be ready for sale; but previously to its application, it ought to be properly levigated.
According to M. Rinman, a fine green colour for painters may be procured from the oxydes of cobalt and zinc. He directs any portion of cobalt-ore to be dissolved in the nitro-muviatic acid (aqua regia), and to be mixed with half that quantity of nitrat of zinc: a lixivium of pot-ash is then to be added; and, when the precipitate is ignited to whiteness, it will be tit tor use.
Beside its utility in the arts, zinc is of considerable service in medicine.—Its flowers are advantageously employed as an antispasmodic, particularly in epilepsy, and in the acute spasms of the stomach, to which persons of delicate constitutions are subject: they have also been given with success in some cases of dry asthma. The dose is from one to two grains, taken twice in the day, and gradually increased lo 8 or 10 grains; but, being a very powerful remedy, it ought never to be resorted to, without medical advice.
For an account of the affections in which the vitriol of zinc may be of service, the reader will consult the article Vitriol.