Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/395

 LEAD 377 be reduced with charcoal or coal. The process accordingly is expensive, and generally does not pay with a raw lead containing less than -^ per cent, of the noble metal. The process, in its direct application to the lead, is now almost extinct, being superseded by the following two methods of &quot; concentration,&quot; which offer the advantage of desilverizing at least the bulk of the lead without depriv ing it of its metallicity. 1. Pattinson s Process (invented about forty years ago) is founded upon the fact that, when molten argentiferous lead is allowed to cool slowly, a relatively silver-free lead crystallizes out while a richer metal remains as a mother- liquor. It will be readily understood that, by a persistent systematic application of this method of partial separation to the primary products and again to their derivatives, it is possible to, so to say, split the original material into a very poor portion containing most of the lead, and a &quot; rich &quot; one containing almost all the silver. Practical smelters are generally satisfied when the proportion of silver in the former is reduced to from the one to the three millionth of the weight of the lead, and the latter enriched to the extent of O5 to 1 5 per cent, of silver, although it is possible to bring up the percentage to 2 5. A lead containing as little as half an ounce of silver per ton can be &quot; Pattinsonized &quot; with a profit. 2. Karsten s Process is still more perfect. It has long been known that lead refuses to alloy itself with more than traces of zinc. In 1842 the eminent metallurgist Karsten made the important discovery that, when argentiferous lead is mixed with 1 per cent, or more of zinc (at a temperature insuring liquidity to even the latter metal), about ^ per cent, of zinc remains dissolved in the lead, while the rest rises to the top as a scum, and, besides a deal of lead, takes almost the whole of the silver with it. Parkes subse quently brought the process into a workable form, for which he took a patent in England in 1850. The argen tiferous lead is molten in large cast-iron pots, intimately mixed with about 30 parts of zinc per unit of silver present, tli3 mixture allowed to rest, and the argentiferous scum removed by means of perforated ladles. The scum, when subjected to &quot;liquation&quot; (partial fusion) on an inclined sole, lets off a quantity of rich lead, which goes to the cupel. From the residue the bulk of the zinc can be withdrawn by distillation, the non-volatile part being fit for cupella- tion. The desilverized lead is freed from its zinc and the other base impurities it may contain by &quot;refining&quot; (see above). The Parkes process seems to be on a fair way of being superseded by a far more perfect form of the Karsten method which was patented by Corduri6 for France in 18G6 (October 18, No. 73,167), and of which the most characteristic feature is that the removal of the zinc from the scum and the refining of the desilverized lead are both effected by means of superheated steam. The treatment with zinc is effected in a deep upright half-egg-shaped cast- iron pan (standing on an upper floor), which is provided with a vertical shaft bearing horizontal paddles, and at its lowest point a perforated cast-iron box, which serves to accommodate the zinc ; 1 kilogramme per 100 kilos of crude lead containing O l kilo of silver, or up to twice the prop &amp;gt;rtion for richer leads. The argentiferous lead 10 tons at a time is melted down in the pan, and the paddle- shift with the zinc introduced and made to revolve until all the zinc has become incorporated with the mass. The shaft is then withdrawn, the mixture allowed to rest for a time at a lower temperature, the scum removed, and the zinc treatment repeated once or twice to eliminate the whole of the silver. The desilverized lead runs direct from the pan into another pan standing on the ground iloor, which has no tap-hole, but is provided with a, wrought-iron hood communicating by means of a pipe with a condensation chamber. In this pan the metal is heated to redness, and a current of superheated steam is blown through it for two or three hours. The zinc and the rest of the impurities are thereby converted into oxides which mostly remain on the surface of the metal, the rest being carried into the chamber and deposited there. The silver scums, after extraction from them of argentiferous lead by liquation, are collected, and, when a sufficient quantity has accumulated, worked with superheated steam like the zinciferous lead, to produce a richly argentiferous regulus, adapted for cupel ling, and an oxide-mixture intimately intermixed with particles of the former and containing even some silver oxide. The working of this bye-product seems to have given the inventor a deal of trouble. Passing over his method, we will mention the one introduced in Lautenthal since 1869. There they dispose of the argentiferous oxides by adding them to the rich lead during its cupellation ; the silver is sucked in by the regulus, the base oxides amalgamate with the litharge. The &quot; poor &quot; lead resulting from this form of the Karsten process contains only 5 or 6 grammes of silver per metric ton (i.e., per million grammes). The loss of lead with a pure material is only 1 per cent, as against the 4 per cent, involved in the Pattinson process. It is worth stating that the zinc removes, besides the silver, all the copper that may be present, and no doubt also part of the other foreign base metals. At any rate the purity of commercial lead, since the introduction of Cordurie s process, has undergone a marked increase. Hampe&quot; analysed a &quot;refined&quot; lead produced in the &quot; Lautenthaler Hiitte &quot; in 1870, and found it to contain only 016 per cent, of impurities. This to all intents and purposes means chemical purity ; yet even such lead is not lit for silver assaying, on account of the trace of silver contained in it. To obtain silver-free lead, we must prepare silver-free acetate of lead by digesting its solution in a lead vessel with lead shavings and filtering and reduce the dried salt with black flux in a crucible lined with charcoal. Properties of Lead and its Oxides. Pure lead is a feebly lustrous bluish-white metal, endowed with a characteristi cally high degree of softness and plasticity, and almost entirely devoid of elasticity. Its breaking strain is very small : a wire jV^h ^ an ^ nc ^ thick is ruptured by a charge of about 30 R), The specific gravity was deter mined exactly by Pieich, who found for ingot 11 352, for sheet metal 11 354 to 11-365 (water of 4 C. = 1). The expansion of unit-length from C. to 100 C. is &quot;002948 (Fizeau). The conductivity for heat (Wiedemann and Franz) or electricity is 8 5, that of silver being taken as unity. It melts at 334 C. = 633 Fahr. (Personne) ; at a bright red heat it emits vapours, at the rate, according to A. de Riemsdyk, of about y^Vryth of its weight per hour ; but he does not specify the surface. At a white heat it boils. The specific heat is 0314 (Regnault), that of water near C. being taken as unity. Lead exposed to ordi nary air is rapidly tarnished, but the thin dark film (of suboxide 1) formed is very slow in increasing. When kept in fusion in the presence of air lead readily takes up oxygen, with formation first of a dark-coloured scum (of suboxide ?), then of monoxide PbO, the rate of oxidation increasing with the temperature. This oxide is produced industrially in two forms, known as &quot; massicot &quot; and &quot; litharge.&quot; The former is produced at temperatures below, the latter at temperatures above the fusing-point of the oxide. The liquid litharge when allowed to cool solidifies into a hard stone-like mass, which, however, when left to itself, soon crumbles up spontaneously into a heap of resplendent dark-yellow scales known as &quot;flake litharge.&quot; Litharge is much used in the arts for the preparation of lead salts, for the manufacture of oil varnishes, of certain XIV. 48