1902 Encyclopedia > Lead

Lead




LEAD. This metal was known to the ancients, and is mentioned in the Old Testament. The Romans used it largely, as it is still used, for the making of water pipes, and soldered these with an alloy of lead and tin. Pliny treats of these two metals as plumbum nigrum and plumbum albiim respectively, which seems to show that at his time they were looked upon as being only two varieties of the same species. In regard to the ancients' knowledge of lead compounds, we may state that the substance de-scribed by Dioscorides as p,okvB8aiva was undoubtedly litharge, that Pliny uses the word minium in its present sense of red lead, and that white lead was well known to Geber in the 8th century.
Of the various plumbiferous minerals, galena (a com-pound of lead and sulphur, formula PbS, demanding 86'6

per cent, of metal) and white lead ore or cerusite, PbOC02 (77-5 per cent.), might almost be said to be the only ones which come into consideration as lead ores. Occa-sionally, however, the following also are utilized :—lead-vitriol or anglesite, PbOS03 (68'3 per cent.), and the pyro-morphite group, 3(P2 or As2)05.3PbO + PbCl2 (76 to 69 per cent.). Bournonite, CuPbSbS, may also be named, although, containing 13 per cent, of copper besides 42'3 per cent, of lead, it is rather a copper than a lead ore.
Galena, the principal lead ore of the Old World, is a dark-coloured metallic-looking compact solid of 7,3to7'7 specific gravity and 2°'5 hardness, crystallizing in cubes or other forms of the regular system, but often presenting itself in non-crystallized granular masses. All galena is contaminated with sulphide of silver,—the proportion of noble metal varying from about O'Ol or less to 0'3 per cent., and in rare cases coming up to | or 1 per cent. Galena occurs in veins in the Cambrian clay-slate, accom-panied by copper and iron pyrites, zinc-blende, quartz, calc-spar, iron-spar, ifcc.; also in beds or nests within sandstones and rudimentary limestones, and in a great many other geological formations. It is pretty widely diffused throughout the earth's crust. The principal English lead mines are in Derbyshire; but there are also mines at Allandale and other parts of western Northumberland, at Alston Moor and other parts of Cumberland, in the western parts of Durham, in Swaledale and Arkendale and other parts of Yorkshire, in Salop, in Cornwall, in the Mendip Hills in Somersetshire, and in the Isle of Man. The Welsh mines are chiefly in Flint, Cardigan, and Montgomery shires; the Scotch in Dumfries, Lanark, and Argyll; and the Irish in AVicklow, Waterford, and Down. Of Continental mines we may mention those in Saxony and in the Harz, Germany ; those of Carinthia, Austria; and especially those of the southern provinces of Spain, from which country large quantities of lead are now im-ported into Great Britain.
The native carbonate occasionally presents itself in the form of pure crystals of the compound PbC03 (" cerusite "), but more frequently in a state of intimate intermixture with clay (" Bleierde "), limestone, oxide of iron, &c. (as in the ores of Nevada and Colorado), and sometimes also with coal (" black lead ore "). All native carbonate of lead seems to be derived from what was originally galena, which, in fact, is always present in it as an admixture. This ore, metallurgically, was not reckoned of much value, until immense quantities of it were discovered in Nevada and in Colorado (U.S.). The Nevada mines are mostly grouped around the city of Eureka, about 200 leagues from San Francisco. The ore there occurs in " pockets " dissemin-ated at random through limestone. The dimensions of these pockets are very variable ; one is quoted measuring 300 by 60 by 180 feet. The crude ore contains about 30 per cent, of lead and 0'2 to 0 3 per cent, of silver. The Colorado lead district is situated pretty high up in the B,ocky Mountains, a few miles from the source of the Arkansas river. The ore was discovered as late as 1877 by a mining engineer, Stephens. It forms gigantic deposits of almost constant thickness, embedded between a floor of limestone and a roof of porphyry. Stephens's discovery was the making of the city of Leadville, which, in 1878, within a year of its birth, had over 10,000 inhabitants. The Leadville ore contains from 24 to 42 per cent, of lead and 0-l to 2 per cent, of silver. In Nevada and Colorado the ore is worked chiefly for the sake of the silver; but this industry, especially since 1878, has developed at such a rate as to seriously affect the price of lead even in Europe. Of other American lead districts the most im-portant are those of Utah, of Missouri, and of the Upper Mississippi, where the ore consists substantially of galena.
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The extraction of the metal from pure (or nearly pure) galena is the simplest of all metallurgical operations. The ore is roasted (i.e., heated in the presence of atmospheric oxygen) until all the sulphur is burned away and the lead left. This simple statement, however, correctly formulates only the final result. The first effect of the roasting is the elimination of sulphur as sulphurous acid, with formation of oxide and sulphate of lead. In practice this oxidation process is continued until the whole of the oxygen is as nearly as possible equal in weight to the sulphur present as sulphide or as sulphate. The heat is then raised in (rela-tive) absence of air, when the two elements named unite into sulphurous acid (S02), while a regulus of molten lead remains. In Wales and the south of England the process is conducted in reverberatory furnaces of the form shown in fig. 1. The sole of the furnace is paved with slags from


FIG. 1.—Reverberatory Furnace. C, chimney ; D, opening for feeding the fire.
previous operations, and has a depression in the middle where the metal formed collects to be let off by a tap-hole T. The dressed ore, 12 to 24 cwts., is introduced through the " hopper" H at the top, and exposed to a moderate oxidizing flame until a certain proportion of ore is oxidized, the openings O, O at the side enabling the workmen to stir up the ore so as to constantly renew the surface exposed to the air. At this stage as a rule some rich slags of a former operation are added and a quantity of quick-lime is incorporated, the chief object of which is to diminish the fluidity of the mass in the next stage, which consists in this, that, with closed air-holes, the heat is raised so as to cause the oxide and sulphate on the one hand and the sulphide on the other to reduce each other to metal. The lead produced runs into the hollow and is tapped off. The roasting process is then resumed, to be followed by another reduction, and so on.
A similar process is used in Carinthia ; only the furnaces are smaller (adapted to a charge of only 420 Vb) and of a somewhat different form. They are long and narrow ; the sole is plane, but slopes from the fire-bridge towards the flue, so that the metal runs to the latter end to collect in pots placed outside the furnace. In Carinthia the oxidizing process from the first is pushed on so far that metallic lead begins to show, and the oxygen introduced predomi-nates over the sulphur left. The mass is then stirred to liberate the lead, which is removed as " Riihrblei." Charcoal is now added, and the heat urged on to obtain "Pressblei," an inferior metal formed partly by the action of the charcoal on the oxide of lead. The fuel used is fir-wood.
In Cumberland, Northumberland, and Durham the reverberatory furnace is used only for roasting the ore, and the oxidized ore is then reduced by fusion in a low square blast furnace (a "Scottish hearth furnace") as depicted in figs. 2 and 3. The rectangular cavity C is lined with cast-iron, as is also the inclined sole-plate which is made to project beyond the furnace, the outside portion "W (the "work-stone") being provided with grooves g guiding any molten metal that may be placed on the " stone " into the east-iron pot P ; t is the "tuyere" for the introduction of the wind.
As a preliminary to the melting process, the " browse " left in the preceding operation (half-fused and imperfectly reduced ore) is intro-

duced with some peat and coal, and heated with the help of the blast. It is then raked out on the work-stone and divided into a very poor "grey" slag which is put aside and a richer portion which goes back into the furnace. Some of the roasted ore is strewed upon


it, and, after a quarter of an hour's working, the whole is taken out on



the work-stone, where the lead produced runs off. The "browse," after removal of the "grey" slag, is reintroduced, ore added, and, after a quarter of an hour's heating, the mass again placed on the work-stone, &c.





In any form of the lead-smelting process one of the con-ditions of complete success is the absence of silica, because this when present unites with a certain proportion of the oxide of lead into a fusible silicate (slag). Practically the formation of a plumbiferous slag cannot be altogether avoided in any case, and such slag accordingly must be worked up. At Alston Moor, Cumberland, this is effected by means of a hearth (blast) furnace similar to the one just described. The slags '(oxide, sulphate, and silicate of lead) are introduced with coal-ashes, furnace bottoms, and other residues, and melted down, this leading to the formation of lead and of a poorer slag. The lead is run off as much as possible; the slag is run into water, which disinte-grates it so that the particles of metal shut up within it are set free and become recoverable by elutriation.
Lead being very appreciably volatile at a red heat, lead-smelting generally, but more especially the Scottish-hearth process, and pre-eminently the slag-recovery process, in-volve the production of large quantities of "lead-smoke" (finely divided highly impure oxide and sulphate of lead), which, for sanitary and economic reasons, must be con-densed and recovered. At Alston Moor the smoke for this purpose is led through a very long succession of flues, ascending the slope of a hill, into a chamber at the top which communicates with a chimney. The chamber, by a number of screens going alternately from the floor to near the top, and vice versa, is divided into compartments charged with such a quantity of water that the smoke, which is propelled by means of a fan, is compelled to bubble repeatedly through the water, where most of what has failed to come down in the flues is precipitated. The smoke deposit is collected, dried, and worked up for lead.
Carbonate and oxide of lead are easily reduced by char-coal or coal. In Leadville and Eureka (U.S.) the carbon-ate is smelted with charcoal in small blast furnaces, about 8 feet high, and rectangular section of 31 by 47 inches, worked with charges of about three tons of ore. There are five tuyeres, two at each of the longer sides, and one at the end opposite the outlet-hole. The " crucible " is quite surrounded by hollow wrought-iron plates, kept cool by circulating water.
Complex lead ores of course demand a complex treat-ment. The famous Prankenscharner Hiitte near Klausthal in the Harz, where a very complex ore is worked up with a wonderful degree of exhaustiveness and precision, may serve as an example. The ore in this case consists of argentiferous galena associated with copper pyrites, fahl-ore, bournonite, zinc blende, and a gangue consisting of silica, limestone, and heavy spar. After the copper pyrites has been, as far as possible, picked out by hand, the remainder is assorted so as to produce an average of about 55 per cent, of lead. One hundred parts of such ore are mixed with 11 of hearth-mass and lith-arge, 90 parts of a variety of slags from previous opera-tions, and 11 parts of metallic iron (or the equivalent of some rich iron ore plus charcoal), and melted down in blast furnaces similar to those used for iron-smelting, but only 22 feet high. The furnace is charged with alter-nate layers of ore mixture and charcoal. The smelting takes fourteen hours, and yields per charge of 100 parts of ore (containing in all about 74 parts of lead) 25 parts of metallic lead, and 18'4 parts of a "stein" consisting of an alloy of sulphides of lead, iron, copper, zinc, silver, antimony, intimately mixed with particles of metallic and (1 subsulphide of) lead—apart from the slags formed, which contain 4 to 8 per cent, of lead and a trace of silver. The " stein'' is subjected to a protracted series of roast-ings, and then melted down with iron and selected slags. There result a ferruginous slag, a certain proportion of metallic lead, and a " stein " of the second order, which of course is richer in copper than the original one was. This "stein" is again roasted, melted down with iron, <fec, until the whole of the lead is extracted, and the copper concentrated in a mass sufficiently rich and pure to be wrought as a copper " stein."
Refining.—The lead obtained by any of the above processes is as a rule contaminated with more or less of iron, antimony, zinc, arsenic, and silver, which must be removed,—the base foreigu metals because they deteriorate the lead, the silver on account of its high commercial value. The base metals are easily eliminated by subject-ing the crude metal to oxidizing fusion in a shallow cast-iron dish inserted into a reverberatory furnace; the foreign metals, being more oxidizable than lead, go to the top as an oxide-scum, which is constantly removed until pure litharge, instead of the foreign oxides, makes its appearance.
The extraction of the silver is easily effected by means of the process of cupellation, one of the oldest metallurgical operations, which dates back to a time beyond that of Pliny. The metal is placed on a shallow kind of dish made of compressed bone-ash powder and forming iho sole of a reverberatory furnace, and therein kept at a red heat in the presence of an abundant supply of air. Tke lead (and with it the foreign base metals) is oxidized into "litharge" (PbO), which, at the temperature prevailing, melts into a thin liquid, and is made to run off through a slit or hole made in the side of the " cupel" (or " test"); the silver remains unchanged, so that the regulus becomes richer and richer as the process proceeds. The foreign base metals, as will readily be understood, go off as oxides along with the first portion of litharge, and accordingly can be removed without contaminating the bulk of the latter product. When the percentage of silver has in-creased to about 8 per cent., the regulus, as a rule, is transferred to a fresh cupel, and thereon treated in the same way as before, until the last trace of litharge is seen to go off as a thin film on the regulus, presenting, on account of its thinness, in the glow of the fire, the magnificent appearance of a soap-bubble in sunlight. The silver then is "fine," i.e., almost pure, and ready for the market. The lead, however, is all obtained in the shape of oxide, and consequently, if not saleable as such, must

by reduced with charcoal or coal. The process accordingly is expensive, and generally does not pay with a raw lead containing less than JQ- 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 " concentration," 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 " rich" 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 Oo to L5 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 " Pattinsonized " with a profit.
2. Karsten's Process is still more perfect. It has long beeu 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 J 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, the mixture allowed to rest, and the argentiferous scum removed by means of perforated ladles. The scum, when subjected to "liquation" (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 " refining" (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 Corclurie for France in 1866 (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), wdiich 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 0T kilo of silver, or up to twice the proportion for richer leads. The argentiferous lead—10 tons at a time—is melted down in the pan, and the paddle-shaft 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 floor, 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 neated 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 " poor " 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. Hamp6 analysed a " refined" lead produced in the " Lautenthaler Hiitte " 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 fit 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 y^th of an inch thick is ruptured by a charge of about 30 B>. The specific gravity was deter-mined exactly by Eeich, who found for ingot 11 '352, for sheet metal 11-354 to 1L365 (water of 4° C. = 1). The expansion of unit-length from 0° C. to 100° C. is -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 yuVo^1 °f lts 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 0° C. being taketi 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 " massicot" and " litharge." 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 " flake litharge." Litharge is much used in the arts for the preparation of lead salts, for the manufacture of oil varnishes, of certain

cements, and of lead plaster, and for other purposes. Massicot is important as being the raw material for the manufacture of " red lead " or " minium." Finely divided massicot, freed from admixed metal by elutriation, is spread out on the flat sole of a kind of baker's oven, or (better) of a " muffle " heated from the outside, and therein exposed for twenty-four hours or more to air at a temperature of about 300° C. or 600° Fahr. The massicot, at a gradually decreasing rate, absorbs oxygen, and as the latter increases the colour becomes more and more intensely red,—the point of saturation corresponding very nearly to the formula Pb405. A more highly oxygenated kind of minium (" orange lead ") can be produced by substituting white lead for massicot as a raw material. The composition of orange lead approxi-mates to Pb304. It is very singular that this higher oxide cannot be obtained from massicot, although the first effect of heat on white lead is its conversion into the oxide PbO. Besides the two named there is another red oxide, of the composition Pb203, but it is not much known. Bed lead is largely used as a pigment and as an ingredient for flint glass, also for the making of certain cements. Any of these red oxides when treated with dilute nitric acid is converted into the binoxide PbO,,, protoxide passing into solution as nitrate: e.g., Pb304 + 2H2ON205 = 2 PbON205 + Pb02+2H20. The binoxide is a brown powder, in-soluble in aqueous oxygenated acids, but converted by hot hydrochloric acid into chloride PbCl2 with evolution of chlorine. To obtain the binoxide in the state of purity, the best method is to pass chlorine into a solution of acetate of lead mixed with excess of carbonate of soda. The hypochlorite formed oxidizes the PbO into PbO.„ with formation of chloride of sodium and free acetic acid (Wohler).
Action of Aqueous Reagents.—Water when absolutely pure has no action on lead by itself. In the presence of free oxygen (air), however, the lead is quickly attacked, with formation of hydrated oxide (PbOH.jO), which is appreciably soluble in water forming an alkaline liquid. When carbonic acid is present the dissolved oxide is soon precipitated as basic carbonate, so that there is room made, so to say, for fresh hydrated oxide, and the corrosion of the lead progresses. Now, all soluble lead compounds are strong cumula-tive poisons, hence the danger involved in using lead cisterns or pipes in the distribution of pure waters. We emphasize the word "pure" because experience shows that the presence in a water of even small proportions of bicarbonate or sulphate of lime prevents its action on lead. All impurities do not act in a similar way. Nitrate and nitrite of ammonia, for instance, intensify the action of a water on lead. It is to be remarked, however, that even pure waters, such as that of Loch Katrine (which forms the Glasgow supply), act so slowly, at least ou such lead pipes as have already been in use for some time, that there is no danger in using short lead service pipes even for them, if the taps, as in any house-hold under normal circumstances, are being constantly used. Lead cisterns of course must be unhesitatingly condemned. G. Bischoff found that a water pipe made of a " composition " consisting of 17 per cent, of antimony and 98'3 of lead was rapidly corroded by a water which, in virtue of its composition, had no action on lead pipes.





Action of Acids.—The presence of carbonic acid in a water does not affect its action on lead (Pattison Muir). Aqueous non-oxidizing acids generally have little or no action on lead in the absence of air. Dilute sulphuric acid (say an acid of 20 per cent, of H2S04 or less) has no action on lead even when air is present, nor on boiling. Stronger acid (e.g., any acid strong enough to fairly fall within the meaning of " vitriol ") does act, slowly in general, but appreciably, the more so the greater its concentration and the higher its temperature. According to Hasenclever, whose experi-ments were subsequently confirmed by A. Bauer and by James Mactear, pure lead, ext. par., is far more readily corroded than a metal contaminated with 1 per cent, or even less of antimony or copper. Hasenclever treated an almost pure lead with pure vitriol of 54° Beaume (P55 sp. gr., or 64-65 per cent. H2S04) in a glass flask. At 40° C. an evolution of gas was observable, which at 80° C. became very distinct. The same lead, after having been alloyed with a little antimony, was not visibly attacked below 85° C. A decided gas-evolution commenced only at 140° C. Boiling concen-trated vitriol converts lead into sulphate, with evolution of sul-phurous acid. Dilute nitric acid readily dissolves the metal, with formation of nitrate Pb(N08),.
Lead Alloys.—Lead unites readily with almost all other metals; hence, and on account of its being used for the extraction of (for instance) silver, its alchemistic name of saturnus. Of the alloys the following may be named:—
With Antimony.—Lead contaminated with small proportions of antimony is more highly proof against vitriol than the pure metal. An alloy of 83 parts of lead and 17 of antimony is used as typo metal ; other proportions are used, however, and other metals added besides antimony (e.g., tin, bismuth) to give the alloy certain pro-perties.
Arsenic renders lead harder. An alloy made by addition of aboui -sVth of arsenic is used for making shot.
Bismuth and Antimony.—An alloy consisting of 9 parts of lead,
2 of antimony, and 2 of bismuth is used for stereotype plates. Bismuth and Tin.—These triple alloys are noted for their low
fusing points. An alloy of 5 of lead, 8 of bismuth, and 3 of tin fuses at 94°'4 C, i.e., below the boiling-point of water (Pose's metal). An alloy of 15 parts of bismuth, 8 of lead, 4 of tin, and
3 of cadmium (Wood's alloy) melts below 70° C.
Tin unites with lead in any proportion with slight expansion (Kuppfer), the alloy fusing at a lower temperature than either component. It is used largely for soldering. The following are the compositions and melting-points of frequently used compounds (Tomlinson):—

== TABLE ==

" Pewter " may be said to be substantially an alloy of the same two metals ; but small quantities of copper, antimony, and zinc are frequently added. Common pewter contains about 5 parts of tin for 1 of lead. In France a tin-lead alloy, containing not over 18 per cent, of lead, is recognized by law as being fit for measures for wine or vinegar. " Best pewter" is just tin alloyed with a mere trifle (J per cent, or less) of copper.
Lead Salts.—Of the oxides of lead the protoxide, PbO, is the only one which under ordinary conditions is capable of forming salts. Towards potash and soda it plays the part of a feeble acid, being readily soluble in solutions of either caustic alkali; wdiile with acids it behaves as a decided diacid base. By a " diacid base " is meant a base which can unite with two monovalent acids at the same time, and form a stable salt. Take, for instance, the case of chloride of lead, PbCl2, which is re-lated to HC1 and Pb(OH)2 exactly as KC1 is to HC1 and K(OH); but, while there is nothing between KC1 and K(OH), the two lead compounds readily unite into CI—Pb—(OH), oxychloride of lead. This property, common to all diacid bases, is developed in lead oxide to a characteristically high degree.
The nitrate, PbON205 or Pb(NO:i)2, easily obtained from the metal as explained above, or by dissolving the oxide in aqueous nitric acid, forms white eystals, difficultly soluble in cold, readily in hot water, almost insoluble in strong nitric acid. It is decomposed by heat into oxide, peroxide of nitrogen (N.204), and oxygen. It is used for the manufacture of fusees and other deflagrating com-pounds. The numerous basic nitrates must here be passed over.
The acetate, Pb(C2H302)„.3H20 (called "sugar" of lead, on account of its sweetish taste), is manufactured by dissolving massi-cot in aqueous acetic acid. It forms colourless transparent crystals, soluble in one and a half parts of cold water and in eight parts of alcohol, which on exposure to ordinary air become opaque through absorption of carbonic acid, which forms a crust of basic carbonate. An aqueous solution readily dissolves oxide of lead, with formation of a strongly alkaline solution containing basic acetates (Acetuia Plumbi or Saturni). When carbonic acid is passed into this solu-tion the whole of the added oxide, and even part of the oxide of tho normal salt, is precipitated as a basic carbonate chemically similar, but not quite equivalent as a pigment, to white lead.
The carbonate, PbC03, exists in nature as cerusito. It can be produced by addition of a solution of lead salt to an excess of carbonate of ammonia, as an almost insoluble white precipitate. Of greater practical importance is a basic carbonate, substantially 2PbC03.Pb(OH)2, which is largely used as a white pigment undei the name of "white lead." For the manufacture of this important substance two methods chiefly are used. In the Old Dutch method, pieces of sheet lead are suspended in stoneware pots so as to occupy the upper two-thirds of the vessels. A little vinegar is poured into each pot; they are then covered with plates of sheet

lead, buried in horse-dung or spent tanner's bark, and left to them-selves for a considerable time. The organic bath, through its fer-mentation, keeps up a suitable temperature and a constant supply of carbonic acid. By the conjoint action of the acetic acid and atmospheric oxygen, the lead is converted superlicially into a basic acetate, which is at once decomposed by the carbonic acid, with for-mation of white lead and acetic acid, which latter then acts cle novo. After a month or so the plates are converted to a more or less con-siderable depth into crusts of white lead. These are knocked off, ground up with water, freed from metal-particles by elutriation, and the paste of white lead is allowed to set and dry in small coni-cal forms. The coherent, snow-white cones are sent out into commerce. The German method differs from the Dutch in this that the lead is suspended in a large chamber heated by ordinary means, and there exposed to the simultaneous action of vapour of aqueous acetic acid and of carbonic acid. In the famous works at Klagenfurth and in the Lavantthal, Carinthia, the carbonic acid is produced by the fermentation of apple-must or infusion of raisins kept in tubs below the chambers. The inferior varieties of com-mercial "white lead" are produced by mixing the genuine article with more or less of finely powdered heavy spar or occasionally zinc-white (ZnO), which latter, we may state in passing, is the most important of the relatively non-poisonous substitutes for white lead.
The chloride, PbCl2, is obtained by adding hydrochloric acid to a solution of lead salt, as a white precipitate, little soluble in cold water, less so in dilute hydrochloric acid, more so in the strong acid, and readily soluble in hot water, from which, on cooling, the excess of dissolved salt separates out in acicular crystals. A basic chloride Pb.jOCL was introduced by Pattinson as a substitute for white lead. Powdered galena is dissolved in hot muriatic acid (PbS + 2H(Jl = PbCl„ + H2S), the solution allowed to cool, and the deposit of impure chloride of lead washed with cold water to remove iron and copper. The residue is then dissolved in hot water, the dregs are filtered off, and the clear solution is mixed with very thin milk of lime so adjusted that it takes out one-half of the chlorine of the PbCl2. The oxychloride comes down as an amorphous white precipitate. Another oxychloride, PbCl2.7PbO, known as "Cassel yellow," is produced by fusing pure oxide, PbO, with ^th of its weight of sal-ammoniac.
The sulphate, PbS04, is obtained, by addition of sulphuric acid to solutions of lead salts, as a white precipitate almost insoluble in water, less soluble still in dilute sulphuric acid, insoluble in alcohol. Sulphide of ammonium blackens it, and it is soluble in solution of alkaline acetate of ammonia, which distinguishes it from sulphate of baryta. It is often obtained industrially as a bye-product.
The chromato, PbOCr03, is prepared industrially as a yellow pig-ment, by precipitating sugar of lead solution with bichromate of potash. The beautiful yellow precipitate is little soluble in dilute nitric acid, but soluble in caustic potash ley. The vermilion-like pigment which occurs in commerce as "chrome-red" is a basic chromate, prepared by treating recently precipitated normal chromato with a properly adjusted proportion of caustic soda, or by boiling it with normal (yellow) chromate of potash. The approxi-mate composition is Cr03.2PbO.
The identification of lead compounds is easy. When mixed with carbonate of soda and heated on charcoal in the reducing flame they yield malleahje globules of metal and a yellow oxide-ring. Solutions of lead salts (colourless in the absence of coloured acids) are characterized by their behaviour to hydrochloric acid, sulphuric acid, and chromate of potash. But the most delicate precipitant for lead is sulphuretted hydrogen, which produces a black precipi-tate of sulphide of lead, insoluble in cold dilute nitric acid, less BO in cold hydrochloric, easily decomposed by hot hydrochloric acid with formation of the characteristic chloride.

Statistics.—The lead, pig or sheet, imported into Great Britain during the year 1880 amounted to 95,202 tons, and during 1881 to 93,400 tons. In 1881 there were 12,824 tons exported to China, 8355 to Russia, 4715 to Australia, 3390 to France, 3349 to British India, 1041 to Germany, and 8837 to other countries. The following table exhibits the production of lead during 1876:—

== TABLE ==

The importation and production of lead in the United States were in the years stated respectively as follows :—

== TABLE ==

(W. D.)



Footnotes

In England coal is employed everywhere, sometimes along with peat.




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