Lead, an elementary substance belonging to the class of metals, and having when pure the following characteristics: color white with bluish gray tint; lustre highly metallic; specific gravity 11.370 at 0° C., compared with water at 4° C. (Reich.); specific heat 0.03140 between 10° and 100° C, 0.03065 between - 77.75° and 10° C. (Regnault), and 0.0402 between 350° and 450° C. (Person); coefficient for linear dilation for 1° C. between 0° and 100°, 0.00003005 (Calvert and Johnson), for cubic dilation 0.000089 (Kopp). Its melting point is 326°C.=619°F. (Rudberg, Person); latent heat of fusion, 5.369 (Person). The conductivity of lead for heat and electricity is expressed by the numbers 8.5 for heat at 12° C, and 10.7 for electricity, silver in both instances being 100 (Wiedermann). It crystallizes in octahedrons of the regular system. Lead is not sensibly volatile below a white heat if air be excluded, but even at this temperature it cannot be distilled like zinc. It is very soft, can be cut with a knife, and can be rolled or hammered into thin sheets. It is, however, but feebly ductile, and cannot be drawn into fine wire.
Two clean and bright surfaces of lead may be united by simple pressure; tin and lead can also be united in the same way. The process seems to be one of true welding at ordinary temperatures, due to the softness of the metal. Finely divided metallic lead can also be united into a compact mass by pressure. When a pig of lead is heated near to its melting point, and then struck a sharp blow, it breaks like tin into a number of pieces having a remarkably columnar structure; the purer the lead the more columnar is the fracture. On bending cast or rolled lead no sound is emitted, as in the case of tin. When solidified and cooled slowly, lead is so soft that it can be indented with the finger nail; when melted at a high temperature and cooled suddenly, it is much harder.
It has a very low tensile strength, amounting, in the form of wire, and between 15° and 20° 0., to only 3,620 lbs. per square inch when the strain is slowly applied, and 4,172 lbs. when rupture is effected suddenly. The presence of tin, antimony, and arsenic renders lead harder and diminishes its malleability; oxide of lead disseminated through the metallic lead has the same effect; copper alone does not render lead harder (Brigel). Lead gives a dull sound when struck, but if cast in the form of a mushroom it is sonorous. - Chemistry of Lead. The atomic weight of lead is 207 (0=16), its symbol Pb (plumbum). When a freshly cut surface is exposed to moist air, it becomes covered with a gray incrustation of oxide, which protects the lead from further oxidation. When very finely divided, metallic lead takes fire spontaneously on exposure to the air. At a red heat lead oxidizes readily. With oxygen it forms four compounds, the suboxide, protoxide, sesquioxide, and peroxide; the protoxide and the mixture of the protoxide and sesquioxide are of importance in the arts. The protoxide of lead (PbO) is made on the large scale by the direct oxidation of metallic lead.
When lead is exposed to an oxidizing atmosphere considerably above its melting point, but below the temperature of fusion of the protoxide, the latter is obtained in the form of a yellow powder called massicot. Litharge is made in the process of cupellation (presently to be described), in which lead is oxidized at a high temperature. The oxide thus obtained is melted, and flows into a receptacle where it solidifies. The color of the resulting litharge is yellow or reddish, according as the cooling has been rapid or slow; it is composed of minute scales which have a talc-like feel; when molten, it conducts electricity. Protoxide of lead is slightly soluble in water, to which it communicates a decidedly alkaline reaction; it is soluble in acids and in alkalies; its solution in lime water blackens hair, horn, and other organic matters containing sulphur. Litharge absorbs carbonic acid from the air, and becomes partially converted into carbonate; it is used in the arts as a pigment, in the manufacture of glass, in glazing porcelain and earthenware, in the manufacture of varnishes, and in the preparation of lead compounds.
Minium or red lead, as ordinarily made, is not of constant composition; it is generally expressed by the formula Pb3O4, and may be regarded as a compound either of the protoxide with the sesquioxide, PbO, Pb2O3, or of the protoxide with the peroxide, 2PbO, Pb02. It is prepared on the large scale by first oxidizing metallic lead to massicot, which is ground and levigated, and the resulting fine powder exposed to an oxidizing atmosphere on the hearth of a furnace for about 48 hours, at a temperature from 300° to 450° C, care being taken to prevent the temperature from rising sufficiently high to sinter the mass. The protoxide of lead absorbs about 1.5 to 2 Per cent. of oxygen, and is converted into a beautiful red or orange-red powder; when prepared from the carbonate or white lead it has generally an orange tint, owing to its fine division. Minium is used as a pigment and also in glass making; for the latter purpose it is preferred to litharge, owing to the larger amount of oxygen it contains, which serves to oxidize organic matters, or to peroxidize iron. Minium was known to the ancients; Pliny speaks of its being made by the calcination of white lead or ceruse, and mentions the adulteration of cinnabar with minium.
Protoxide of lead forms numerous compounds with acids; the most important in the arts are the carbonate, acetate, and chro-mate. The carbonate, or white lead, was likewise known and used by the ancients; it owes its superiority as a pigment to its great " body " or covering power, and its opacity; it is not a simple carbonate, but a compound of hydrate with carbonate of lead, in proportions varying from 2 to 4 of carbonate to 1 of hydrate. There are three methods by which it is made on the large scale, known as the French, English, and Dutch methods; they all depend primarily on the formation of the basic acetate of lead and its conversion into carbonate. In the French method, a solution of basic acetate of lead is prepared by the digestion of litharge with acetic acid or a solution of acetate of lead, or by the action of acetic acid on finely divided metallic lead with access of air. Into this solution is forced carbonic acid gas, which precipitates two thirds of the lead, and this after settling is collected and dried. The supernatant clear liquid, which is a neutral or slightly acid solution of acetate of lead, is boiled with litharge, and the basic acetate thus formed again treated with carbonic acid.
Experience has shown that it is not absolutely necessary to have the basic acetate of lead completely in solution as in the French process. In the English process, litharge, with about 1 Per cent. of acetate (sugar of lead), is mixed with water to a moist mass, and exposed under constant stirring to the action of carbonic acid, when the litharge is converted with great rapidity into white carbonate. The Dutch process, which is the oldest in use, consists in exposing thin sheets of lead to vapors of acetic acid and carbonic acid for a long period. In earthen vessels are placed sheets of lead rolled into the form of a spiral; into the bottom of these vessels, but not in contact with the lead, is poured a mixture of weak vinegar and substances capable of fermentation, as yeast; a plate of lead serves as a cover. From 1,500 to 2,000 vessels thus prepared are piled together in so-called loogen, and surrounded with spent bark or stable litter; after six weeks the lead will be found thickly coated or entirely converted into white carbonate.
The action is here substantially the same as in the other processes given; the lead is first converted into acetate, and subsequently into carbonate by the carbonic acid given off by the decomposing matters present, which also serve to maintain an elevated temperature. The Dutch white lead contains more oxide of lead and possesses more body than the French, but is said to have a greater tendency, when used as a paint with oil, to turn yellow on exposure. White lead is frequently adulterated with other substances, principally sulphate of baryta. As a pigment it has been to a considerable extent supplanted by oxide of zinc, which, though of less body, is cheaper and does not blacken when exposed to sulphuretted hydrogen. Protoxide of lead dissolves readily in acetic acid; four salts may thus be formed with varying amounts of oxide of lead, namely, the normal, sesquibasic, tri-basic, and sexbasic acetate. The normal and tribasic acetates are of importance in chemistry and in the arts. The former when crystallized is generally called sugar of lead; the solution of this is used in medicine under the name of Goulard's water.
The solutions of the acetate have a sweetish astringent taste, and absorb carbonic acid with avidity from the atmosphere; acetic acid also dissolves metallic lead when there is free access of air. The chromates of lead may be formed by precipitation of solutions of the acetate by a solution of bichromate of potassa; they are valuable pigments. The neutral chromate has a beautiful lemon-yellow color; the basic chromates are orange or red. Lead combines with great avidity with sulphur. Four sulphides are known to exist, of which only the protosul-phide is important; it will be described further on in treating of lead ores. The best solvent for metallic lead is dilute nitric acid; it is but feebly attacked by sulphuric or muriatic acids, owing to the formation of insoluble sulphate or chloride, which protects the metal from further action by the acid. Sulphuric acid is generated in leaden chambers, and concentrated to a certain extent in leaden pans; a small quantity of lead is always taken up in concentrating the acid, which may be precipitated by dilution of the acid by water. - Lead Poisoning. Nearly if not quite all the compounds of lead are poisonous.
In cases of acute poisoning, where a large quantity of a lead salt has been accidentally or otherwise taken, there is a metallic taste in the mouth, burning pain in the stomach, nausea and vomiting, followed by prostration and death or by chronic symptoms resulting in convalescence. Cases of chronic poisoning are very common among those engaged in smelting and handling lead, and in the manufacture and use of its compounds. Painters using lead pigments, workmen engaged in red and white lead works, in the manufacture of glazed cards, in the preparation of materials for making flint glass and glazing earthenware, and in bleaching Brussels lace, which is beaten with white lead to whiten the fibre, are especially liable to the chronic form of lead poisoning. Workers in metallic lead, as plumbers, are much less liable to the disease. It is a disputed point whether lead poisoning ever results from working in lead mines; it may be that the sulphide of lead is innocuous, while the more rarely occurring carbonate is occasionally the cause of the disease. The susceptibility to the effects of the poison varies greatly in different persons.
Some are able to follow their occupation as house painters or in manufactories of white lead without suffering materially, while an instance is on .record where, after careful inquiry, a severe case of lead colic in a fishing-tackle maker could only be traced to his chewing bits of metallic lead, which he chipped off while engaged for a few days in making sinkers. Among artisans who use it in their work, lead is introduced into the system either by the air passages or by the digestive organs; it is inhaled in line dust, or it is swallowed. In this way, carefully washing the hands previous to meals is of great importance as a preservative from its effects. Lead is often introduced into the system accidentally or as an adulteration in the manufacture of various liquors. The old name of lead colic, colica Pictonum, colic of Poitou, arose from the prevalence of colic there produced by its use in the manufacture of wine. Devonshire colic was traced by Sir George Baker to the use of lead in clarifying cider; the dry bellyache of the West Indies arose from its use in the distillation of rum. Many glazed articles of earthenware, when acted on by acids, give up the lead contained in their glazing.
In Nos. 10 and 11 of the Medicinische Zeitung, published by the medical society of Prussia (1859), a case is cited of lead poisoning produced by snuff, and upon an analysis of the snuff by Hockel it was found to contain 2 1/2 Per cent. of lead. Snuff packed in lead foil always contains lead. The wrappings of lead foil when the package of snuff is first opened often exhibit an incrustation resembling mould, which is carbonate of lead. The lead foil is frequently tinned on one side; but this proves to be a very insufficient protection, as it is often corroded through, and the workmen are sometimes so careless as to put the tinned surface outside. - There are four distinct affections produced by lead: colic, arthralgia, paralysis, and brain disease or encephalopathy. Of these, colic is by far the most frequent, it having occurred in 1,217 of the 2,171 cases of Tan-querel des Planches (Traite des maladies de plomu, Paris, 1839), while arthralgia occurred in 755 cases, paralysis in 127, and encephalopathy in about 72. In chronic poisoning by lead, the skin is dry and of an unhealthy earthy color, the pulse is slow, the secretions generally are diminished, the bowels constipated, and the patient loses flesh and strength.
When the gums are examined, a blue line is generally found on their free margin at its junction with the teeth, particularly the incisors; occasionally the mucous membrane lining the lips and palate has the same bluish color. If the patient be now attacked by lead colic, he is conscious of a sensation of pain and sinking, which he refers to the centre of the abdomen. The bowels are obstinately constipated; there is constant pain in the belly, aggravated in paroxysms, and relieved or not increased by pressure. The walls of the abdomen are hard and sometimes retracted; there is often nausea and vomiting; dysury is sometimes present, and the patient is exceedingly restless, sleepless, and anxious. Notwithstanding the gravity of the symptoms, the tongue is clean or but slightly coated, the skin cool, the pulse regular, and perhaps a little slower than natural. Left to itself, the disease is of uncertain duration, but under proper treatment it is ordinarily subdued in a few days; the patient, however, is subject to relapses, and when he remains exposed to the original cause of the complaint, the colic gradually becomes complicated with palsy, or perhaps it is terminated by a fatal affection of the brain.
Where death has taken place, post-mortem examination shows no special lesion; but in this as in other varieties of lead poisoning, chemical reagents detect the presence of the metal in the blood and the tissues of the body. The following treatment adopted at Guy's hospital, London, where numerous cases of the disease are received, was contributed to Percy's "Metallurgy of Lead " by Dr. Owen Rees: "In the majority of cases an ounce of castor oil mixed with 10 drops of laudanum is given twice daily, and large hot poultices of linseed meal are applied to the belly. At night a draught, consisting of 40 drops of Battley's sedative solution (liquor opii sedativus) in camphor mixture, is prescribed, if the bowels have been acted upon by the oil taken during the day. In severer cases, accompanied with tenderness of the belly and retching, bleeding from the arm to the amount of 1G or 20 fluid ounces may be practised with advantage when the patient is young and strong. Pain and spasm are thereby very quickly allayed; and the stomach often loses its irritability and immediately afterward retains food and medicine. Where the patient is less robust, the application of 15 or 20 leeches to the belly followed by a warm poultice has been found to produce much relief.
Castor oil with Battley's sedative solution is also given as in other cases. Clysters of this oil may be resorted to when the stomach is too irritable to retain medicine; and in all cases a pint of warm gruel injected into the rectum greatly soothes the patient." Mr. Williams states that according to his experience "no relief is obtained unless the patient is kept under the influence of opium for a few hours before giving purgatives and clysters; that while the spasm lasts the use of purgatives is futile, even clysters of castor oil being expelled without being soiled by faecal matter or tainted by faecal odor; that he begins his treatment by administering a dose of three or four grains of opium, and that in severe cases he has not seen any soothing effect from the injection of warm gruel into the rectum." - In lead arthralgia, besides the general symptoms of chronic poisoning, the patient suffers from paroxysms of sharp, darting pains, commonly in the limbs, but sometimes in the trunk; these pains do not follow the course of the nervous cords, and they are increased by motion and diminished by pressure. In the intervals of the paroxysms the patient suffers from a sense of fatigue and constriction in the affected parts.
Sulphur baths given daily for seven or eight days form the most efficient method of treatment. It would seem, according to the experiments of M. Melsens, that iodide of potassium administered internally has the power of eliminating lead from the system in cases in which it exists; and Valleix has found this remedy particularly efficacious in lead arthralgia. It may be given in doses of from 6 to 10 grains three times a day. - Lead paralysis is very rarely general; commonly it is confined to either the upper or lower extremities, and in these to one system of muscles. In five cases out of six the upper extremities alone are affected, and the paralysis is limited to the extensor muscles of the hand and wrist, the hand remaining permanently flexed, giving rise to the "dropped wrist." The muscles have lost their contractility, be-come wasted, and in bad cases after death look pale and as if converted into fibrous tissues. Often the paralyzed parts have more or less lost their sensibility; sometimes this loss of sensibility (lead anaesthesia) occurs independent of paralysis of movement. Lead palsy is rarely a primary affection, lead colic or arthralgia commonly preceding it. Its progress is slow, and in well marked cases the results of treatment are somewhat uncertain.
The internal use of iodide of potassium for the purpose of eliminating the poison, the employment of small doses of strychnia, of electricity, and of friction to stimulate the injured muscles, are the means most to be relied upon. In all cases the disease is chronic, and the treatment requires to be persevered in for a long time. - Lead encephalopathy, brain affection produced by lead, is happily the rarest of the forms of lead poisoning. In a few cases the patient is attacked suddenly and without warning; but in the majority of instances, after the symptoms characteristic of the effect of lead upon the system are already well marked, and often after colic has supervened, the patient is attacked with headache, vertigo, sleeplessness or somnolence, frequency of pulse, and stiffness or pains in the limbs. The disease may now assume one of three forms,He may be suddenly attacked by a delirium, which in some cases is mild and tranquil, in others serious, the patient being dangerous to himself and others. In other cases he is seized with epileptiform or epileptic convulsions, in the intervals between which he only partially recovers the use of his intellect, remaining stupid and confused. Both these forms are apt to terminate in coma.
In a third the patient is comatose or deeply somnolent from the commencement, neither delirium nor convulsions being present. The mortality in encephalopathy is very high, more than one half of all the cases proving fatal. - Of more importance than the treatment of lead disease is the adoption of means for its prevention, and in this regard there is often much to contend with in the indifference of operatives to sanitary measures. Thorough ventilation and scrupulous cleanliness would in most instances prevent the contraction of the disease. Prophylactic remedies have been much used, those proving the most useful being drinks acidulated with sulphuric acid and milk. Cases of poisoning of animals in the vicinity of lead works are not uncommon. This subject was investigated by Dr. George Wilson in 1852, and the results were communicated to the royal society. He found the herbage in the neighborhood of a lead-smelting establishment to be impregnated with carbonate of lead, as well as the water in a stream used for washing ore. Fourteen horses and some cows died, it was believed, from the effects of the lead. Examination of the tissues of the animals showed the presence of lead in two cases, but not in the others. The lead found was largely present in the spleen.
Instances have been recorded of the death of cows from swallowing the "bullet spray" scattered in target practice. - The frequent occurrence of »cases of poisoning resulting from the very general use of lead pipe in the conveyance of water into dwellings, has led to investigations by many chemists into the action which different waters exert on this metal. Bright lead remains unchanged in perfectly dry air or in pure water deprived of air and protected from contact with it; but in a moist atmosphere, or in rain water, its brilliancy is soon dulled, and its surface is covered with a thin film of oxide, which adheres closely to the metal and protects it from further oxidation. The oxide, however, is partially soluble in water, and is no sooner taken up by this than it combines with any carbonic acid gas present or absorbed from the air, forming with it a film made up of silky scales of hydrated oxycarbonate of lead. More lead is then oxidized, dissolved, and converted into carbonate, and so the process of corrosion goes on.
The oxycarbonate is almost insoluble in pure water, this taking up of it only about 1/60 of a grain to the gallon; and so perfectly does this separate from water, that if distilled water holding four or five grains of oxide of lead to the gallon be exposed to the air, the carbonic acid soon imbibed will cause the precipitation of silky crystals of the hydrated oxycarbonate, leaving in solution not more than one part of the metal to 4,000,000 of the liquid, or 1/60 of a grain to the gallon. But an excess of carbonic acid gives to the water the property of dissolving this carbonate of lead in the same way that it acquires also the property of dissolving carbonate of lime or limestone. So far it seems therefore that carbonate of lead is as likely to be found dissolved in water that comes in contact with the metal, as carbonate of lime in water flowing over limestone. But the presence of certain salts in the water, even in very minute quantity, modifies materially this action. The sulphates, phosphates, and carbonates of the alkalies, and the sulphates, carbonates, etc, of the alkaline earths, or indeed any neutral salt, the acid of which can produce with lead or its oxide an insoluble compound, greatly diminish this action, even if present in the water to the amount of only four or five grains in the gallon.
Bicarbonate of lime, which is almost always present in spring water, is especially remarkable for its protecting influence. The action of these salts is to form insoluble precipitates, which accumulate upon the surface of the lead, and cover it with a protecting lining. The protection is not, however, uniformly efficient, for there are certain other salts and acids which exert a contrary influence, and frequently completely overpower the beneficial effects of the first class named. Such are the chlorides and nitrates, and especially nitric and nitrous acids, in solution. These acids are generated in all waters containing decomposing animal matter, and therefore must be almost universally present to some extent. Dr. Medlock, who has given much attention to this subject, goes beyond other chemists in the importance he ascribes to the influence of these acids. The action of any water on lead, he states, is entirely due to the presence of nitrous and nitric acid, resulting primarily from the decomposition of organic matters and of ammonia contained in the water; and further, that water deprived of these acids, and of substances capable of producing them, has no action on lead, and may be conveyed with perfect safety through leaden pipes or stored in leaden cisterns.
He devised a method of removing the nitrates and also organic matters from water, unless the latter be present in great excess. It was by suspending coils of iron wire or pieces of sheet iron in the water, and after a time filtering off the deposit. The iron decomposes the nitric acid, being itself peroxidized, and nitrous gas is liberated, which oxidizes the carbonaceous matters, so that they are resolved into carbonic acid and a lower oxide of nitrogen. The original paper of Henry Medlock, Esq., " On the Reciprocal Action of Metals and the Constituents of Well and River Waters," is the 24th article of vol. xiv. (4th series) of the "London, Edinburgh, and Dublin Philosophical Magazine " (1857). - The quantity of lead in solution in the gallon of water which will suffice to produce injurious effects, depends very much on the individual constitution and on the length of time that the water continues to be used. Dr. Penny, professor of chemistry at Glasgow, cites an instance of the health of a whole community being deranged by water containing only one ninth of a grain of lead to the gallon; and also quotes the conclusion of Dr. John Smith of Aberdeen, that the limit of manifestly deleterious action would seem to be somewhere between one tenth and one twentieth of a grain.
An interesting case is reported of the lead disease attacking a large number of the household of the ex-royal family of France in 1848, while they resided at Claremont, Surrey, England. The spring that supplied the palace had been selected for the purity of its water, and lead pipes had been laid 30 years previously to the palace, two miles distant. Four members of the family manifested some symptoms of poisoning after five months' use of the water, and in seven months 13 persons were alarmingly affected. The water on examination was found to contain one grain of lead per gallon. In experiments on the effect of Dantzic water on lead pipes, Dr. Lissauer found that the maximum amount of lead was present after one week's use. After the third week there was still a small amount, while after four weeks and during the six months succeeding the water was free from lead. He found further, that when the water contained 3.8 grains of carbonate of lime to the gallon the water was without effect on lead. In contact with a less oxidizable metal, lead is more readily attacked than when alone. Lead pipe is consequently found to be more corroded in the vicinity of the soldered joints than elsewhere.
A notable quantity of lead has been found by Dr. Hayes in water contained in metallic ice pitchers where the sides were soldered to the bottom. A safe substitute for lead pipe for water supply has been found in the tin-lined lead pipe or lead-encased tin pipe. Careful investigation has shown that these pipes when properly made and jointed effectually resist corrosion by ordinary potable waters, although the waters of some springs and wells have been known to attack tin. As it is not probable that the use of lead pipe will be soon abandoned in cities, it is important to understand how it may be used with the least risk. The greater danger is in general to be apprehended the longer the water is allowed to stand in it before using, and the more the pipes are exposed to the alternate action of air and water as they are filled and emptied. Pipes in the upper parts of buildings are frequently left empty of water by this being drawn off below, and for this reason are more exposed to chemical action than those constantly filled. The first flow of water through any lead pipes that have been left some time without use will wash out the dissolved salts of lead.
If this water is allowed to run waste to the amount of several times the contents of the pipes, that which follows is not likely to contain any injurious quantity of lead. By thus drawing off every morning the water that has stood in the pipes, and then washing them out by the continued flow for a short time, all risk of lead poisoning may be avoided. - The published information on the subject of lead poisoning and the effects of water upon lead is scattered through a vast number of medical and chemical works and reports of sanitary committees. The most important works to consult are Christison on poisons, and L. Tanquerel des Planches on lead diseases, translated from the French by Dr. Samuel L. Dana (Lowell, 1848). Convenient reference may be had to the opinions of a great number of chemists in the "Collection of Reports (condensed)," prepared and published in 1859 by Mr. James P. Kirk wood, engineer of the Brooklyn water works; also to the "Report of the Metropolitan Board of Health" (New York, 1869), and an article by Prof. William Ripley Nichols on the action of Cochituate water upon lead pipes, in the " Second Annual Report of the Massachusetts State Board of Health" (1871). Appended to this latter article is a list of books and monographs on the subject. - Ores of Lead. Lead has been found in a few instances native, but such occurrences are extremely rare.
The principal ore of lead is the sulphide or galena. When pure it consists of 86.6 Per cent. of lead and 13.4 of sulphur. It crystallizes in the isometric system, the prevailing form being the cube; and it has a perfect cubic cleavage. Hardness 2.5 to 2.75; specific gravity 7.25 to 7.7; lustre metallic; color and streak pure lead gray. Heated in an open tube, it gives fumes of sulphurous acid; before the blowpipe on charcoal it fuses, emits sulphurous fumes, coats the coal yellow, and yields a globule of metallic lead. It is soluble in nitric acid. It occasionally contains, as impurities, antimony, arsenic, copper, zinc, and cadmium. All galena is more or less argentiferous, and also probably auriferous, but the physical characters give no indication of the amount of silver present. Generally galena occurring in true veins in the older rocks contains more silver than that occurring in deposits in the more recent formations. The following list shows the amount of silver in galenas from a few localities:
Percentage of silver.
Equal to troy ounces in a ton of
0.03 to 0.05
9.8 to 16.3
0.05 to 0.75
16.3 to 245
002 to 003
6.5 to 9.8
Lead hills, Scotland.........
0.03 to 0.06
9.8 to l9.6
Eaton, N. H..........
Shelburne, N. H..........
0.0012 to 0.0027
0.4 to 0.9
0.03 to 0.05
9.8 to 16.3
0.15 to 0.06
49.0 to 65.0
0.05 to 0.06
16.3 to 19.6
Antimonial galena from Tuscany .....................
0.325 to 0.72
106 to 235
Peru, undressed ore, but exceptionally rich...........
0.52 to 3.20
170 to 1133
According to E. J. Chapman, galena is seldom highly argentiferous except when associated with mispickel or other arsenical ore. Associated with galena, and generally resulting from its decomposition, are frequently found oxidized compounds of lead, as the carbonate, sulphate, phosphate, and arseniate. In some I localities these compounds are abundant, and form a considerable proportion of the ore | raised. The carbonate of lead or cerussite crystallizes in the orthorhombic system, and contains when pure 77.52 Per cent. of lead. It occurs both crystallized and earthy; in the latter condition it is white, if not contaminated by copper or other metals; it is the most abundant of the oxidized lead ores. The sulphate of lead, or anglesite, occurs frequently in orthorhombic crystals of great size and beauty; it contains 68.31 Per cent. of lead. The phosphate or pyromorphite is found frequently in the upper part of lead veins; it occurs in beautiful green hexagonal crystals, which are composed of three molecules of phosphate and one of chloride of lead. The arseniate or mimetesite corresponds in composition to the phosphate, containing three molecules of the arseniate to one of the chloride; it occurs in yellowish crystals of the hexagonal system.
All of the above oxidized compounds of lead, with the exception of the carbonate, have more of a mineralogical than metallurgical interest. Some mines are noted for the occurrence of line crystallized specimens of these and other lead compounds. The Wheatley mine, near Phoenixville, Pa., has been one of the most celebrated mines in the world in this respect. The number of minerals occurring there is referred to subsequently. There are, further, a number of minerals containing, besides lead, other metals, as antimony, copper, and silver, which are subject to metallurgical treatment, but which are not properly speaking lead ores. - Galena is widely disseminated in nature; it is found in crystalline and stratified rocks, and occurs in veins, beds, and irregularly distributed masses. Two classes of deposits are to be distinguished: 1, those in which the galena is associated with other metallic sulphides, as silver, copper, iron, and zinc, and often combinations of these sulphides with antimony and arsenic; and 2, those in which the galena is nearly or quite free from associations with other metals.
To the first class belong many of the most celebrated silver mines, as those of Freiberg in Saxony, Glaus- thai in the Harz, Przibrara in Bohemia, and . many of those in the United States. The veins worked in these mines are generally in the I older crystalline or metamorphic rocks, and usually belong to the class of true veins continuous in depth containing the metallic deposits and gangue minerals in regular bands or layers. Although the lead products of these mines may be relatively large compared with that of the other metals present, yet they would not be profitable were they worked for lead alone. The presence of lead is, however, advantageous in the extraction of the other metals, especially silver. The workable deposits of galena free from other metals are not very numerous; they are chiefly confined to England, Spain, and the United States. They occur mainly in or are associated with limestone and dolomite, in pockets, layers, and gash veins, but rarely in true veins. The metals gold, silver, and tin are seldom found in workable veins except in the older and crystalline rocks, while lead is mostly found in unmetamorphosed and more recent rocks.
In this respect it resembles zinc, with which it is often associated. " In Great Britain galena occurs in veins in palaeozoic rocks, namely: in the carboniferous or mountain limestone in Cumberland, Durham, Northumberland, Yorkshire, Derbyshire, and Flintshire; in the Devonian in Devon and Cornwall; and in the lower Silurian in Shropshire." (Percy.) The mountain limestone series is made up of beds of limestone which alternate with sandstones and shales, having a thickness altogether in the northern counties of 2,000 ft., and in Derbyshire of 1,500 ft. The miners distinguish three classes of deposits: rake veins, pipe veins, and flat veins. The rake veins correspond nearly with what are called true or transverse veins, showing a comby structure and "slickensides," or polished walls; they do not, however, always descend through the strata in a regular manner, but go down by a series of vertical and oblique portions, the change of inclination being coincident with a change in the character of the rock through which the vein passes. The pipe veins are quite irregular deposits of no great length, more like what have been designated as gash veins.
The flat veins are deposits formed between the layers of two adjacent beds. "As a general rule, the lodes of the limestone districts have a comparatively soft matrix; large bodies of clay, locally called 'flucan' and 'dowk,' often occupy a great portion of the original vein fissure, and these in many localities are found to alternate with portions of exceedingly pure and solid galena; while even the superficial clay, when a thick cap of that material overlies the back of the lodes, has often yielded large amounts of ore, as in the 'hushes' of the north and in Flintshire, and as most remarkably exemplified in the 'diggings' of Missouri. When the lodes, on the other hand, exhibit less of mechanical detrital matter, and are filled chiefly by crys-talline deposits, the matrix consists most abundantly of calc spar, with which fluor spar and barytes are variously associated; while zinc blende, or in its absence calamine, is a general concomitant, iron pyrites comparatively infrequent, and spathic iron ore, quartz, and pearl spar, or occasionally witherite and calamine, are confined to certain districts and zones of depth.
The general direction of the lodes is more or less east and west, although cases occur where some of the great 'cross courses,1 or north and south veins, have in particular parts, as notably near Holywell in Flintshire, yielded large amounts of ore. The Devonian rocks (' killas') of Cornwall and Devonshire have for many years past been noted for the production of lead ores yielding a very large proportion of silver, and the structure of the lodes themselves offers many points of high interest. They are invariably found at a greater distance from the granite bosses which form so striking a feature of these counties than the tin and copper-bearing veins, and usually in killas of a softer character. The majority of the lodes are cross courses, traceable in some instances for miles, although it seldom happens that the richly lead-bearing part has been found to extend for more than a few hundred feet, or at most fathoms, in length. The associated minerals are principally fluor spar, quartz, sometimes in a loosely granular state, often as a white hornstone, bitter-spar, zinc blende, and more rarely spathic iron ore, fahlerz, and bournonite, while the intersections with east and west veins have been marked by the occurrence of various ores of silver." (Smyth.) Other localities are the Silurian slates of Cardiganshire and Montgomeryshire, and near Shelve on the south of Shrewsbury, where the veins run east and west, and in the isle of Man, which yields a highly argentiferous galena. - In the southern part of Spain, in the mountain range of the Sierra Nevada, there are extensive deposits of lead ores, which have been mined by the Phoenicians, Carthaginians, Bo-mans, and Moors, and are still productive at the present day; and the smelting of the immense masses of rich slags left by the Romans has been a profitable industry in recent times.
The rocks of the Sierra Nevada are mainly mica, talcose, and clay schists, enclosing strata of dolomite and sometimes gypsum. The principal lead mines occur in the spurs of the Sierra Nevada, namely, the Sierra Cartagena, Sierra Almagrera, and the Sierra de Gador. " The former is composed of sedimentary rocks of the ancient transition strata, argillaceous schists, slate, mica schists forming the base of the fissured black crystalline limestones, sandstones, and conglomerates. The erupted rocks are greenish and porphyric, and they are par excellence the rocks accompanying nearly all the metalliferous deposits of this locality, which for the most part are in contact with schists and limestones, and extend over a large area. Although numerous veins of ore occur, vet more generally the ore exists in intercalated beds and irregular pockets in the limestones. In the districts of Almazarron the erupted rocks are traversed at many points by veins of galena, which would indicate that the ore is of later date than those rocks. There are two classes of ore, carbonates and galenas, of which the former predominate. The galenas delivered to the smelters vary in produce from 15 to 60 Per cent. of lead, containing from 32 to 121 oz. of silver per ton of lead produced.
The deficiency of water throughout the entire metalliferous district of the south of Spain, added to the absence of all motive power, will ever render the dressing of the ores difficult if not impossible." (Petitgand, quoted by Percy.) The Sierra Almagrera consists of argillaceous schists, passing into mica schists, without any eruptive rocks, but traversed by a number of ore veins, the largest of which is called the Jaroso. This vein is about 18 ft. wide at a depth of 180 ft., narrowing to 4 1/2 ft. at a depth of 420 ft. It is filled principally with brown hematite, throughout which is disseminated argentiferous galena, with some blende and copper pyrites. The Sierra de Gador is the most famous of all the Spanish lead-producing localities. The plateau on its summit is literally riddled with shafts of greater or less depth, and shallow excavations. The ore, mainly galena unassociated with other metallic sulphides, occurs in pockets of variable size and extent, sometimes isolated, sometimes continuous and corresponding to the stratification of the rocks. The ore is occasionally found in dolomite, but is then not very abundant.
It is most productive in a yellowish clayey mass which contains small pieces of dolomite ore, and associated with masses of galena; or the whole deposit may consist of fragments of ore and rock enveloped in clay. The Sierra de Gador was the scene of extraordinary ac-tivitv during the third decade of this century, the production amounting in 1827 to 42,000 tons. This enormous production overstocked the market, and the price of lead fell to a point below the cost of production. The ore was here largely obtained from shallow excavations which were soon exhausted. Mining is still carried on, but the production is at present small. 'These mines seem not to have been worked by the Romans, owing to the small amount of silver in the galena. - In the United States the most important lead deposits are found in the Mississippi valley. There are two prominent localities, known as the upper and lower mines, the latter in Missouri, the former included within the bordering states of Wisconsin, Illinois, and Iowa. The upper mines have been thoroughly investigated by Prof. J. D. Whitney in connection with the geological surveys of Iowa, Wisconsin, and Illinois, and the results of his studies are to be found in the published geological reports of these states.
The following description is taken from these reports. The extent of this lead-producing region is about 4,000 sq. m., of which about 2,200 sq. m. is in the state of Wisconsin. The most productive portion of the region is that which lies between Dubuque, Galena, and Shullsburg, so that Iowa and Illinois raise more lead in proportion to the area over which mining operations have been conducted than Wisconsin does. A circle of 4 m. radius, with its centre a little N. E. of Galena, would include nearly all the productive diggings, with the exception of those at Apple river and in the vicinity of Elizabeth. It is probable that these mines were not worked by the aborigines. Fragments of galena have been found in the ancient mounds of the northwest, but no metallic lead. The discovery of lead ore in this region is attributed to Le Sueur, who made a voyage up the Mississippi in 1700 and 1701, for the purpose of discovering ores. The first mining seems to have been done by Julien Dubuque, a half-breed Indian, who in 1788 settled on the site of the city which bears his name, and continued until his death in 1809. The land occupied by him was relinquished to the United States by the Indians in 1832. Leases were first issued by the government in 1822 (under the act of March 3, 1807), but mining did not become general till 1826-7. Owing to the difficulty of collecting rents, a resolution was adopted in the house of representatives in February, 1839, providing for the survey and valuation of these lands, with a view to their sale.
This survey was made by Dr. D. D. Owen, with the aid of 139 assistants, in the autumn of the same year, and the report was transmitted to the land office in April, 1840. In 1847 the mineral lands were thrown open for entry and purchase. From records kept at Galena by Capt. Beebe and others, it appears that the amount of lead annually produced by the mines of this region increased from 5,000 to 10,000 tons (of 2,240 lbs.) between 1829 and 1839; after that it rose rapidly, and attained its maximum from 1845 to 1847, when it reached nearly 25,000 tons. Since that time the production has materially declined, and no trustworthy record of its amount is obtainable. A notable feature of this lead region is the entire absence of drift, although the country surrounding it on all sides is covered by gravel, pebbles, and bowlders, showing that the surface has not been covered by water since the earlier geological period, and certainly not during the quarternary peri-od or that of the drift. Geologically, this district belongs to the lower Silurian. Resting upon a floor of crystalline slaty quartzose and granite rocks is the lower sandstone, the equivalent of the Potsdam of New York, from 300 to 500 ft. thick.
Above this sandstone, and passing into it by alternating beds of silicious and dolomitic material, is the lower magnesian limestone, the equivalent of the calciferous sandstone of New York. It is nearly pure dolomite, heavy and compact, with a thickness of from 250 to 300 ft. It is slightly metalliferous, but no profitable mining for galena has ever been carried on in it. The next layer is the upper or St. Peter's sandstone, a purely silicious rock of but slight coherence; its thickness is 80 to 100 ft. Next in order come the buff and blue limestones. The former, allied to the birds-eye and Black river limestones, is a dolomite from 15 to 25 ft. in thickness, containing 10 to 25 Per cent. of insoluble matter. The latter, the equivalent of the Trenton limestone, occurs in beds 20 ft. thick, and is the first group of strata in the series, as developed in the northwest, in which there are any purely calcareous beds, one of the divisions being a pure limestone with but a trace of magnesia; this blue limestone is decidedly metalliferous.
The next formation containing the deposits of galena was formerly called the upper magnesian limestone; but as the term originated in a misapprehension of the geological structure of the district, it has been superseded by the more appropriate name of the Galena limestone. This formation is a thick-bedded, light gray or yellowish gray dolomite, nearly chemically pure, distinctly crystalline in its texture, and usually rather granular, although occasionally quite compact; its maximum thickness is from 250 to 275 ft.; the middle portion is marked by an abundance of flints arranged in parallel layers. Next above the Galena limestone is the Cincinnati group, composed chiefly of argillaceous and silico-argil-laceous shales, with a small amount of calcareous and magnesian carbonates; its thickness is from 60 to 100 ft. Above the Cincinnati group follows everywhere in the lead region and its vicinity the Niagara limestone, a heavy mass of pure dolomite, the third in order, containing like the Galena limestone silicious nodules or flints, especially in its middle and lower portion, arranged in layers parallel to the stratification. The thickness of the formation is undetermined; 350 ft. is given as an approximation.
The outcrop of the Niagara limestone forms a marked feature in the topography of the lead region, since, owing to the peculiar denudation which has taken place in the district occupied by these rocks, it is left in abrupt and picturesque bluffs or cliffs along the principal streams. It also caps the mounds or outliers of rock which, severed from their original connection, stand like sentinels posted for observation. The lead occurs in the Galena limestone in the form of sulphuret or galena, filling either partially or entirely fissures and cavities in the rock. The fissures are confined to the one formation, and are hence not true veins, but are distinguished by the name of gash veins. The principal forms of occurrence are the vertical and flat crevices or sheets, and the expansion of these crevices into pockets, openings, and caves. The crevice openings and cave openings are mostly confined to the upper portion of the Galena limestone, and the flat sheets and flat openings to the lower portion. The latter is the only form of deposit which is found to any extent in the blue limestone. The vertical sheets are generally solid masses of ore, unaccompanied by gangue or veinstone, while the flat sheets are generally associated with blende, calamine, and pyrites.
The longitudinal extension of a vertical sheet of ore varies from a few yards to 100 yards, and its vertical extension from 20 to 40 ft. The greater the length and width of the sheet, the more likely it is to pass into some other form, and lose its simple sheet character. One instance has been noticed of a continuous sheet of ore 140 ft. in depth. It is from the openings or caves that by far the greater part of the ore is raised. These are not generally filled with solid ore, but are partially filled with a mass of loose material, consisting of ore and dolomite in fine particles, the result of the disintegration of the rock, and infiltrated clay. The sides of these openings are often covered with large and beautiful crystals of galena. An exceptionally large cave in the vicinity of Dubuque was 123 ft. long, 40 to 50 ft. wide, and 20 to 30 ft. deep, and when first opened was found half filled with detritus. The decomposition and disintegration of the upper layer of the Galena limestone gives rise to surface ore, the galena being distributed in the clayey loam of the prairies. It is called float mineral, and generally indicates proximity of mineral-bearing crevices. Some openings are noteworthy for their extraordinary productiveness.
The Longworthy crevice, which has been worked in different places along a line of nearly three fourths of a mile, has produced, it has been stated, about 10,000,000 lbs. of ore. Several other cavities have produced from 2,000,000 to 4,000,000 lbs. The origin of these crevices seems to be due to the same cause by which what are called joints by geologists have been formed in almost every variety of rock occurring in large homogeneous masses, and especially where they have a decidedly crystalline texture. In the dolomitic rocks of the lead region we have all the conditions which usually occur in the formation of a well developed jointed structure. It is a marked feature of the district that the fissures have approximately an E. and W. and N". and S. direction, a fact which is everywhere recognized by miners, and which is of great practical importance. All through the mining district the heaviest diggings, with but few exceptions, will be found in crevices varying but little from E. and W. in their general direction. The N. and S. fissures, on the other hand, are usually much less important, as they are generally in sheet form or their bodies wedged in close between walls of solid rocks, and do not extend into openings.
It is probable that the E. and W. course of the principal crevices has been determined by the fact that this is the direction of the axis of upheaval by which the whole lead region has been slightly elevated along the N. boundary of the district, and which will be seen to have determined the draining of the region. The axis of upheaval may have determined the course of the main set of fissures, while the tendency of all masses of rock thus situated to the formation of a subordinate set nearly at right angles to the principal ones may not unreasonably be looked on as the origin of the norths and souths. The disposition of ore in the fissures is due, according to Prof. Whitney, to precipitation of the lead from the oceanic waters, from which the rocks themselves were thrown down, by means of organic matter, either directly, by the reduction of the sulphates to sulphides, or by the generation of sulphuretted hydrogen by the decomposition of the organic matter, and the subsequent conversion of the metal into sulphide. The theory of the injection of the mineral masses from below is untenable, owing to the entire absence of continuation of the crevices into the underlying strata.
The cause of the abundant mineral deposition in the Galena limestone, while in the lower strata it is very slight, is doubtless due to the increase of animal and vegetable life during this period, as the fossiliferous character of the blue limestone abundantly proves. The galena from this region is of great purity, and contains but a trace of silver. - The lead deposits of Missouri may be divided into three districts, the southwest, the middle, and the southeast. The latter, by far the most important, are embraced in an area 5 m. in width and 100 m. in length. They were discovered and first worked in 1720 by Renault and his mineralogist La Motte, who came out with a large party under authority of a patent granted by the French government to John Law's famous company. Mine La Motte and the Potosi lead mines were discovered and opened by them; little however had been done up to Renault's return to France in 1742. The only smelting of the lead ores appears to have been done on log heaps, a wasteful process, much practised even of late years.
In 1798, as stated by Schoolcraft in his "View of the Lead Mines of Missouri," p. 19, Moses Austin of Virginia, having obtained a grant of land from the Spanish government near Potosi, sunk the first regular shaft, and erected a re-verberatory furnace, and also a shot tower. According to the same authority, there were 45 mines in operation in Missouri in 1819, giving employment to 1,100 persons; in 1811 Mine Shibboleth produced 3,125,000 lbs. of lead from 5,000,000 lbs. of ore. From 1798 to 1816 Mine a Burton and the Potosi diggings were estimated to have produced over 500,000 lbs. annually; and from 1834 to 1837 the production of Mine La Motte is rated at an average of 1,035,820 lbs. of lead per annum. For 14 years succeeding 1840 Dr. Litton in his state geological report makes the annual average of all the mines over 3,833,121 lbs. The lead ore is mainly confined to the third magnesian limestone, which is nearly a pure dolomite. No workable lead deposits have been found either in the overlying or underlying strata.
Beneath the limestone, throughout this region, porphyries are found which are older than the Silurian limestones, and belong, according to Pumpelly, to the azoic formation, of which they may be the youngest member in Missouri. They are the near equivalents, in point of age, to the great iron-bearing rocks of Lake Superior, New Jersey, and Sweden. Various other ores are found associated with the galena, as the carbonate of lead, sulplmret and silicate of zinc, iron and copper pyrites, and at some of the mines, as Mine La Motte, ores of manganese, nickel, and cobalt. The surface of the country in the lead region is strewed with crystallized quartz derived from the lead-bearing rocks, and called by the miners "mineral blossom." The modes of occurrence of the lead ore are generally the same as those already described as common to the northern mines. The openings on the vertical fissures vary from the capacity of a cubic foot to 10 or 12 ft. square, and when very small are called pockets. They do not preserve a uniform course, but connect one with another by passages filled with material different from the walls, and extending toward every point of the compass.
Valle's mine in St. Francois co. and Perry's on its extension S. present a remarkable network of veins spread over an area of about 1,500 ft. in length by 500 in breadth, ranging N. W. and S. E. They are also examples of mines of a more permanent character than are found in the northern lead region. Valle's mine was discovered in 1824, and it is believed has been worked ever since without interruption. Three series of caves are found, the second set 18 or 20 ft. below the first, and the third about 8 ft. below the second. The middle set has been most worked. They run out in every direction, and in some instances communicate by chimneys with the series above or below. They are filled with clay, loose rock, and ore, the last often an intermixture of galena and silicate and carbonate of zinc, which requires roasting and washing to prepare it for the furnace. Dr. Litton was of opinion that three fourths of all the lead obtained in Missouri had been from clay diggings overlying the rock. These operations have often been highly productive, but were carried on without system and without capital by men who had no interest or ability to prosecute the work in the rock, and thus the more permanent deposits have been passed over.
In some localities in Missouri lead has been found in the coal measures, and has been worked in the sub-carboniferous limestone. In Moniteau co. a bed of dense cannel coal 40 ft. thick is traversed by a network of veins crossing at all angles filled with galena, zinc blende, and calc spar. - There are numerous deposits of lead ore in the Atlantic states, but none of them have as vet proved to be of economic importance. (See Whitney's "Mineral Wealth of the United States.") In the belt of metamorphic rocks which extends along the S. E. flank of the Appalachian chain, there are a number of lead veins, many of which have been worked, but subsequently abandoned, owing to the small quantity of the ore, or the difficulty of separating it from the associated minerals, or the expense of mining in hard rock. As some of the deposits are highly argentiferous, it is not improbable that work on them may be resumed with profit at some future time. Among the localities best known in the New England states may be mentioned Shelburne, Eaton, and Warren in New Hampshire, Northampton and vicinity in Massachusetts, and Middletown and Plymouth in Connecticut. Some of the ore from the Shelburne mine gave 84 oz. of silver to the ton of lead, and from the Warren mine 60 to 70 oz. to the ton.
The mines in the neighborhood of Northampton, Mass., were worked as long ago as 1765. The vein here is extensive and well defined, but is not very productive, and owing to the expense of mining work on it, though often resumed, has been as often abandoned. At Middletown, Conn., the existence of lead ore was probably known in 1651, when a license was granted to Gov. John Winthrop to work mines of this and other specified mineral productions, with particular reference to any he might discover near Middletown. There is no tradition of the time when the mine was first worked. The ore is highly argentiferous galena, but not abundant. Shipments made to England yielded 25 to 75 oz. of silver to 21 cwt. of lead; and what was remarkable, a peculiar fine-grained variety of the ore, such as is usually found to be most argentiferous, proved to be only one third as rich in silver as the coarsely cubical ore. The vein consists chiefly of quartz, often in crystallized plates or combs, with some calc spar, sulphate of baryta, and fluor spar, also blende and iron and copper pyrites. It is from 10 in. to 3 ft. in thickness, and is included in silicious and micaceous slates, with the dip and direction of which it appears at the surface to coincide. Active operations at this mine have long since ceased.
In Dutchess co., N. Y., explorations were made for lead in 1740, and during the revolutionary war the committee of public safety sought to obtain supplies there. Veins of argentiferous galena are found also in Columbia, Washington, and Rensselaer cos., but have never proved productive. They traverse the strata near the junction of the metamorphic slates and limestones. The principal one is the Ancram or Livingston mine in Columbia co. On the other side of the Hudson river lead mines have been worked at various localities in the unaltered Silurian limestones and sandstones; but these, too, have all been abandoned as unprofitable. The most productive among them were in the Shawangunk grit of Ulster co., which overlies the Hudson river slate group. On the W. slope of the Shawangunk mountain, at Ellenville, several nearly vertical veins have been followed into the hard sandstone, the strata of which and the direction of the mountain ridge they cross nearly at right angles. The principal one of these yielded in 1853 galena which produced about 459,000 lbs. of lead, and 60 to 70 tons of pyritous copper, 50 tons of which produced 24'3 Per cent. of metal.
The vein was unlike the true veins of the metamorphic rocks, having no gangue or veinstones, but wherever productive filled between the walls with rich galena and pyritous copper, the former sometimes being 5 ft. thick unmixed with other matters. In places it contracted to a knife-edge seam in the hard sandstone, and again opened out in hollow fissures, one of which, extending to more than 100 ft. in depth, with an equal horizontal range, has never been completely explored. It was partially filled with tough yellow clay, in which were imbedded loose fragments of sandstone, magnificent bunches of quartz crystals, and lumps of lead and copper ores; and its walls were also lined in places with the same ores. In these features a striking resemblance is exhibited to the " openings " in the western lead mines, although found there only in limestone. The vein is moreover like those of the western mines in lacking veinstones, and probably also in being limited to certain rock formations, beneath which it will not prove productive. - In Chester and Montgomery cos., Pa., near Phoenixville, is a group of lead and copper mines, in a small district of only 5 or 6 m. in length by 2 or 3 in breadth, some of the remarkable productions of which have already been noticed.
They occur in gneiss and the red shale and sandstone of the middle secondary, cutting the strata in direction and dip. Nearly all the veins, of which there are 12 or more, are parallel to each other, directed N. 32°-35° E. and dipping steeply S. E. When confined chiefly to the gneiss, they produce as a general rule lead ores; when included in the red shale, their principal product is copper ores. Quartz and iron pyrites make up the larger part of the lodes, the latter in the upper portions of the mine decomposed to a soft brown gossan. This material sometimes yields 10 oz. of silver to the ton. Prof. H. D. Rogers, from whose "Geology of Pennsylvania" these data are obtained, enumerates the following large variety of metallic constituents of the Wheatley lode, besides the gangue of quartz and sulphate of baryta: of lead - sulphate, carbonate, phosphate, arseniate, molybdate, chromate, tung-state, chromo-molybdate, arsenio-phosphate, sulphuret, and antimonial argentiferous; of zinc - sulphuret, carbonate, and silicate; of copper - native metal, sulphuret, black oxide, malachites green and blue; of iron - the oxide containing silver, pyrites, brown spar, and hematite; native silver; black oxide of manganese, and native sulphur.
Gersdorfite, or sulph-arsenide of nickel, has also been found. This mine was opened in 1851, and up to September, 1854, had produced, according to the manager's report, 1,800 tons of lead ore, principally phosphate, estimated to yield 60 per cent. lead. The Chester county mining company commenced operations in 1850 in the same vicinity, and up to November, 1851, had smelted 190,400 lbs. of dressed ore, almost exclusively phosphate, which produced about 47 per cent. lead. Dr. Genth found this kind of ore to contain 1.6 oz. of silver in 2,000 lbs.; the coarsely granular galena gave 16.2 oz., and the radiated and finely granular galena 11.9 oz. Operations ceased at these and the other adjacent mines in 1854 and 1855. - In S. W. Virginia and E. Tennessee many lead mines, not in the metamorphic belt, but in the great Silurian limestone formation of the valley of Virginia, have been worked with greater or less success for many years past, their ores being compact and crystallized carbonate of lead as well as galena. Those belonging to the Wythe union lead company on New river, Wythe co., Va., are known to have been productive in 1754, and are still worked.
All the mines of this region resemble in their geological associations and metallic products those of the western lead region. - The Washington mine, Davidson co., N. C, has attracted interest on account of its being the only mine in the United States that has produced much silver. It was opened in 1836 in the silicious and talcose slates of the gold region, and, like most other veins of the metamorphic rocks of this region, has the strike and dip of the strata. There are two parallel veins worked together, which dip steeply toward the north. They are underlaid by a granitic rock, and above is talcose slate. The mine was commenced for the carbonates of lead, which were found in considerable quantity in a heavy dull ore of earthy appearance, and also in glassy crystals. Some galena and phosphate were also met with. It was not until after smelting large quantities of these ores that native silver was discovered, and the argentiferous character of the lead ores. In 1840 the display of native silver in arborescent forms and disseminated through the magnesian veinstones was very striking, and excited expectations of great richness at lower depths; but the rich and easily reducible carbonate of lead was soon exhausted, the ore in greater depths consisting of zinc blende and argentiferous galena which presented difficulties in treatment.
Till 1844 the mine continued to produce largely rich argentiferous ores, after which the ores gradually diminished in quantity; the yield of that year is stated to have been $24,009 of silver and $7,253 of gold, separated from 160,000 lbs. of lead, an average of 240 oz. of auriferous silver to 2,000 lbs. In 1851 the production was 56,896 lbs. of lead and 7,942.16 oz. auriferous silver, equal to 279 oz. to the ton of lead. Dr. Genth found the proportion of silver in the sulphurets very variable, running from 2.5 to 19.5 oz. to 2,000 lbs. An average sample taken from 2,000 to 3,000 tons of ore (1849) contained 45 Per cent. of zinc, 21 Per cent. of lead, and about 8 oz. of silver to the ton, with minute quantities of copper and gold. In 1852 mining operations were stopped as unprofitable, but were soon after renewed, and have since been continued intermittently. The mine was actively worked during the civil war by the confederates for lead. In 1871 it was 650 ft. deep, and produced about 400 to 500 tons of ore per month, part of which was slaty and required dressing. - There are numerous well developed lead veins in the azoic of northern New York. The most important are those of Rossie in St. Lawrence co., which are particularly famous for the magnificent crystallizations of calcite and galena.
Although occurring in the older rocks, the galena is almost entirely free from silver. The Coal Hill vein, 2 ft. wide, in gneiss, was worked with great activity in 1837 and 1838; during these years 3,250,000 lbs. of lead were smelted* The mining was however so recklessly conducted, being directed solely to the richest masses, most easily reached, that the abandonment of the mine became necessary in the following year. Argentiferous galena occurs in the sub-carboniferous limestone of Kentucky, but mining operations, which have hitherto been very imperfect and superficial, have not been productive. According to Owen ("Kentucky Geological Survey"), the most favorable localities for development of lead mines in this formation, judging by analogy with English mines, are in Crittenden and Livingston cos. Lead also occurs in the blue limestone formation of central Kentucky. A vein 4 to 6 in. wide in Franklin and Woodford cos. has been somewhat worked, but did not prove profitable. - Metallurgical Treatment. Ordinarily the ores as they are raised from the mine have to undergo a process of preparation or dressing, to free them from adhering gangue, or to separate the different metallic minerals before they are ready for smelting.
The character of this dressing will differ according to the amount and nature of the associated minerals. Simple breaking and hand sorting often suffices to separate masses of nearly pure galena, while the fine ore and that composed of an intimate mixture of a number of minerals is submitted to various mechanical processes, by which the different minerals are separated according to their specific gravities. The processes employed for the extraction of lead from galena are three in number: 1, the roasting-reaction or air-reduction process; 2, the roasting and deoxidizing process; and 3, the iron-reduction or precipitation process. In cases where oxidized ores are smelted, the second process is employed, with the omission, of course, of the roasting. The first process depends on the interesting reaction which takes place when sulphide of lead is heated either with sulphate or oxide of lead, resulting, when the oxygen and sulphur are present in the mixture in the proportion of two molecules of the former to one of the latter, in the production of metallic lead and sulphurous acid gas. Where this proportion does not exist, a residue will remain, consisting of the excess of oxide or sulphide, as the case may be.
The reaction is shown in the following equations: PbS + 2PbO=Pb3 + S02, and PbS + Pb O,SO3=Pb2 + 2SO2. When therefore galena is partially roasted so as to form a certain amount of sulphate or oxide of lead, and the oxide thus formed is heated in intimate mixture with the unaltered sulphide, metallic lead at once separates. This reaction takes place to some extent when galena is placed on the top of an ordinary fire; it is in this way that the backwoodsman often obtains his lead for bullets. Galena has indeed been smelted on the large scale, in localities remote from civilization, by simply throwing it upon a fire of logs. In the second process the galena is roasted, either completely or nearly so, and the resulting oxide, with some sulphate, reduced in a shaft furnace by the carbon of the fuel. In the third process advantage is taken of the superior affinity of sulphur for iron at high temperatures, so that galena heated with metallic iron is reduced to metal with the formation of sulphide of iron. The selection of the process for the treatment of lead ores depends largely on the presence or absence of other metals, and on the richness of the ores.
Where galena alone is treated with but small amount of earthy matters, the roasting-reaction process is generally employed, and the smelting is performed either on an ore hearth or in a rever-beratory furnace. During the early part of the last century the ore hearth was almost exclusively used in England; it is now mainly confined to the north of England, Scotland, and the United States. The American ore hearth, a modified and improved Scotch hearth, is used in the western lead regions, and was formerly extensively used in the state of New York. It consists of a working plate of cast iron about 3 ft. wide and 2 ft. from front to back, sloping downward and forward about 1 inch in 12. It has a diagonal groove on its upper surface for the flow of lead from the hearth to the cast-iron pot. Enclosing the back half of this working plate on three sides is a hollow casting 14 in. high, forming an air chest, through which the blast passes before it readies the hearth; this arrangement serves the double purpose of keeping the cast-iron sides cool and heating the blast. The tuyere is on the back plate about 2 in. above the level of the hearth bottom. The operation is an extremely simple one.
The fuel used is light wood or charcoal, and the mass of ore and fuel fills up the hearth and slopes down to the front, which is open. Fuel and ore are added in small quantities at short intervals, and the operation is continued uninterruptedly. The lead accumulates at the bottom of the hearth, and flows out in the groove in the front of the working plate. The yield is dependent to a considerable extent on the skill of the smelter in charging and stirring the mass. At the Rossie works in New York, now abandoned, the average yield of each hearth for 24 hours was about 7,500 lbs., with a consumption of wood per day of three fourths of a cord; the direct yield of lead amounted to 70 to 80 Per cent. of that in the ore. There is also a considerable quantity of slag formed, which is subsequently treated in a low blast furnace. The great loss of lead, both mechanically and by volatilization in the ore hearth, has led to the general substitution of the reverberatory furnace in the roasting-reaction process. The chief advantage of the ore hearth is the small expense for plant and the simplicity of the manipulation.
In the reverberatory furnace process, called also the English process, the same reactions are involved, but they are effected on a much larger scale, and are much more completely under control. The process as conducted in Flintshire is as follows: The hearth of the reverberatory, which is about 11 ft. long with an average width of 9 ft., is generally concave, and slopes toward the "well" about the middle of the front side. The charge, 21 cwts. of ore, is evenly spread over the furnace, and stirred for two hours with an ample supply of air at as high a temperature as possible without causing sintering. The fire is then increased until the charge becomes semi-liquid, and any portion of it which may have run down toward the well is raked back to that which remains on the upper part of the hearth. The temperature is then lowered until the charge becomes thick, when it is pushed back toward the bridge and back part of the furnace. The fire is again raised, and the charge melted down as quickly as possible into the well, when slaked lime is thrown in and raked over the surface of the melted mass. The slag and reduced portions, being thus rendered sufficiently stiff, are again thrown up or "set up" on the sloping sides of the bed, there left to cool a little, and again remelted.
Lime is again added, and the slag is pushed back from the surface of the lead, and left to drain a little, when the lead is tapped off from the well into a receptacle below. The slag is run or drawn out of the furnace in pasty lumps, and is termed " gray slag." The quantity of fuel per charge varies, according to the ore used, from 12 to 16 cwts. The ores treated contain from 75 to 80 Per cent. of lead, and the yield is about 14 1/2 cwts. of lead from a charge of 21 cwts. Of this yield 91 Per cent. is obtained direct from the furnace, and 9 Per cent. from the slag and fume. There are numerous variations in the manipulations practised at different works, but in its essential features the process is everywhere the same. It is necessary that the ores should be rich and not contain more than 5 Per cent. of silica. It is evident that where oxidized ores are available in sufficient amount to mix with the galena, the preliminary roasting may be much abridged, or even omitted. The gray slags from this process are rich in lead (over 50 per cent.), and are treated by themselves in low blast furnaces, where reduction is effected by the fuel alone; the lead thus produced is known as slag lead.
The roasting-reduction process and the precipitation process are never employed for pure rich lead ores, but are confined to ores poor in lead or those containing other metals, as copper, antimony, and nickel. The roasting-reduction process may be conducted in a great variety of ways. The roasting, which is generally effected in reverberatory furnaces, may be partial or complete. Partial roasting is resorted to where the ore contains, besides lead and silver, the metals mentioned above. In the subsequent smelting of this partially roasted ore there is produced, besides lead containing the greater part of the silver, a regulus containing sulphur, iron, lead, and copper. Where arsenic, nickel, and cobalt are present, there is also formed a Speise, which contains these metals combined with iron. But where other metals than lead and silver are not present (iron and zinc excepted), the ore may be roasted completely ("sweet"), the heat being carried sufficiently high to sinter the mass and convert the oxide of lead into silicate. This agglomerated roasted mass is smelted in a shaft furnace, with the addition of iron, either metallic or as oxide, in order to effect the complete decomposition of the silicate of lead.
In case the roasting has been effectual and all the sulphur driven off, no regulus is formed. Examples of this latter mode of treating lead ores (i. e., complete roasting) are found at Pontgibaud, Vialas, and La Pise in France, and at Bleiberg, Rhenish Prussia; of the former (i. e., partial roasting), at Sala in feweden, and Freiberg in Saxony. - As the smelting process at Freiberg is a good example of an intelligent metallurgical practice, involving the extraction of a number of metals, a brief outline of the process will be given. The ores smelted there are divided into five classes: A, Glanze, or bright lead ores, containing about 30 Per cent. of lead; B, Mei-ische Erze, or leady ores, containing from 15 to 29 Per cent. of lead; 0, Durrerze, or dry ores, consisting chiefly of veinstuff with small quantities of pyrites and galena, containing from 0.05 to 0.1 Per cent. of silver; D, Kup-fererze, or copper ores, containing from 1 to 10 Per cent. of copper, averaging 3 per cent.; and E, Zuschlagserze, or ores containing less than 0.03 Per cent. of silver, and chiefly composed of pyrites, mispickel, and zinc blende, with some lead and copper, mixed with quartz and calc spar.
The mixture for smelting consists of about 60 Per cent. of A, 20 of B, and 20 of a mixture of C and D. The lead in this mixture amounts to from 34 to 38 per cent., and the silver from 0.15 to 0.18 per cent. This ore mixture is roasted in charges of 10 cwts. in a double-hearth reverberatory for 16 hours, until all but 5 Per cent. of sulphur has been expelled. It is then smelted in shaft furnaces with roasted reverberatory regulus (Rohstein), presently to be described, lime or fluor spar, slags produced in the same process, and certain lead products, such as furnace bottoms. Silver ores of class C, when containing over 0.1 Per cent. of silver, are added to the mixture without roasting. The products of this smelting are Werkblei, work or furnace lead, containing 0.5 to 0.6 Per cent. of silver, from 0.20 to 0.60 of copper, and about 1.5 of antimony and arsenic; Bleistcin, lead matte or regulus, or blast-furnace regulus, consisting of the sulphides of iron, lead, and copper, and containing on an average 20 Per cent. of lead, 10 of copper, and 0.20 of silver, besides small amounts of nickel, zinc, arsenic, and antimony; Speise, a compound of arsenic and iron, containing the greater part of the nickel and cobalt of the ore mixture; and Schlacken or slag, consisting of silicate of protoxide of iron, containing about 5 to 6 Per cent. of lead and 0.02 of silver.
The lead is refined and then enriched by Pat-tinson's process, and cupelled for silver. The silver is dissolved in boiling sulphuric acid and a little gold obtained. The regulus is roasted and smelted with copper ores of class D and slags rich in lead, producing a second regulus containing 30 Per cent. of copper and 0.18 of silver. This operation is thrice repeated, resulting in the production of two enriched reguluses containing respectively 54 and 73 Per cent. of copper. This latter is roasted " sweet " in a re-verberatory furnace, and treated with sulphuric acid of specific gravity 1.5, diluted with its bulk of water, and the oxide of copper dissolved out, leaving a residue containing lead and silver, and some copper, which is added in the course of the lead smelting. The solution of the copper is crystallized to blue vitriol, and sold as such. In each of the three or more smeltings of the regulus there are also produced lead, speise, and slag, which are the subject of further treatment. The slags from the ore smelting are treated with ores of the classes C and E, in a reverberatory furnace, producing a regulus (Rohstein) and slag.
The former, containing from 7 to 10 Per cent. of lead, 4 to 5 of copper, and 0.15 to 0.20 of silver, is added after the roasting to the ore-smelting mixture, as stated above; while the slag which contains but 1 Per cent. of lead and 0.0028 of silver is thrown away. The speise obtained in the various smeltings of ore and regulus is, after concentration of the nickel and cobalt, treated for the extraction of these metals. There is further produced at Freiberg a considerable quantity of arsenical preparations from the fumes of the furnace, and a small quantity of bismuth is extracted from the test in which the silver is refined. The third of the smelting processes enumerated above, viz., the iron-reduction or precipitation process, finds its best example in the upper Hartz. Here sulphuretted ores are smelted, which contain on an average from 54 to 50 Per cent. of lead and 0.10 of silver at the Clausthal and Altenau works, and 62 to 64 Per cent. of lead and from 0.09 to 0.10 of silver at the Lautenthal works; they contain also some copper, zinc, and antimony.
They are smelted with the addition of cast iron, in such proportions that the resulting lead shall bear the proportion in weight to the lead regulus of 4 to 3 or 5 to 4. The result of this smelting is practically the same as that at Freiberg; furnace lead, lead regulus, slag, and occasionally speise, are produced, which are treated substantially in the same way as the corresponding products in Freiberg. The use of cast iron has been in a great measure superseded by that of iron-finery slags, or in the upper Hartz by slags from copper smelting in lower Hartz, which are silicates of protoxide of iron containing between 1 and 2 Per cent. of copper and a small amount of silver. The reaction in this case depends on the reduction of the iron in the slag and the subsequent action of the metallic iron thus formed on the sulphide of lead. The four products of lead smelting, viz., lead, speise, regulus, and slag, separate on standing according to their relative specific gravities, in the order named. The shaft furnaces used for lead smelting have been until comparatively a late period of small trapezoidal section, narrowing from the hearth to the throat, and blown with one or two tuyeres.
In modern practice the furnace is generally circular in section, widening toward the mouth, and blown with eight or ten tuyeres. A marked improvement in smelting has been the result of this change in furnace construction. The yield and purity of product have been increased, and the consumption of fuel and loss by volatilization decreased. The Raschette furnace, of rectangular section and expanding walls, was introduced into the Hartz for lead smelting, with results much more favorable than the old style of furnace, but inferior to those obtained in the Pilz furnace, which differs from the Raschette chiefly in its circular section. A modified Pilz furnace has been introduced for silver smelting in Nevada, by Mr. Arendts of the Eureka consolidated works, with good results. A vertical section through the fore hearth and horizontal section through the tuyeres are seen in the accompanying figures. The hearth of the furnace is 3 ft. wide and 4 1/2 ft. deep; the breast is open; the tuyeres, 12 in number, are on the sides and back of the furnace. The height of the furnace may be from 8 to 20 ft. above the tuyeres, according to the nature of the ore to be treated. Ordinarily the height is 10 ft. from tuyeres to charging opening.
Mr. Arendts has also devised an automatic tap for the lead, consisting of a three-inch wrought-iron pipe in the furnace walls, one end communicating with the lowest part of the hearth inside the furnace, and the other emptying into a basin on the outside. It is said that by the use of this tap the furnace runs more regularly, the lead obtained is purer, iron sows are prevented, and the work of the smelters is lightened. (For fuller details in regard to smelting argentiferous lead in the western states, see the reports of R. W. Raymond, United States commissioner of mining statistics, and vols. i. and ii. of the "Transactions of the American Institute of Mining Engineers.") - Owing to the volatility of lead and its compounds, and to the strong draught of the furnaces used for roasting and smelting the ore, there is considerable loss of metal, as is abundantly shown by the white fumes in which lead works are often enveloped. The interception of this lead fume is of great importance both in an economical and sanitary point of view. The methods devised for this purpose may be enumerated as follows: 1, long horizontal flues; 2, condensation chambers; 3, exposure to artificial rain; 4, forcing through water; 5, intermixture with steam and its subsequent condensation.
Experience seems to have proved that the most simple and effective method is the long horizontal flue, terminating in a chimney sufficient to give the requisite draught. In the north of England there are flues nearly a mile in length. But this method is not always applicable, owing to unfavorable situation of the works. The use of steam, although theoretically the most promising of all, has not been found to give good results. The composition of the fume differs with the ores used, and also with the distance from the furnace, where the flue system is employed. It is mainly sulphate of lead mixed with a small amount of the earthy and metallic ingredients of the ore, and varies in color from gray to white. The loss of lead in the fume, when condensing arrangements are not used, has never been accurately determined. Fallize estimates it on an average as 10 Per cent. of the assay value of the ore in lead, which is doubtless too high. The amount of lead obtained from the fume in proportion to that obtained from the ore and slag, at the Keld Head smelting works in England, for one year's work, was 7.63 to 100. - The character of the furnace lead depends on the nature of the ores from which it is produced.
It is very frequently contaminated with other metals, notably antimony and copper, to a degree to make it unfit for use in the arts. It must then undergo a process of purification. This process, called indifferently refining, softening, improving, or calcining, is effected by melting the lead in the hearth of a reverberatory furnace, or in a large shallow cast-iron pan in the place of the hearth, and exposing it at a dull red heat to free access of the atmospheric air, or to the action of a blast of air. The foreign metals present oxidize and rise to the surface in the form of a scum or dross, which also contains a large amount of oxide of lead. This is skimmed off from time to time, and a fresh surface of lead exposed. The operation is continued until a test sample shows that the lead has acquired the proper degree of softness. The length of time required for this process depends on the amount of impurities present and on the extent of surface exposed. Three or more days are often required where the amount of antimony is considerable; and if present in very large amount, the lead may be completely oxidized before it is softened.
The dross or scum, consisting mainly of oxides of lead and antimony, is reduced by coal and a second hard lead obtained, which is much richer in antimony than the first. If this cannot be profitably softened, it is sold as hard lead, to be used in the manufacture of bullets, shot, type metal, or pigments. The lead is frequently "poled " after tapping; this operation consists in the immersion of a piece of green wood under the surface of the molten lead; the escape of moisture and the carbonizing of the wood cause a lively ebullition, exposing a considerable surface to the oxidized influence of the air, and thereby assisting in the removal of the last traces of impurities. The following are analyses of a few varieties of furnace and refined lead:
Lead Furnace. - Fig. 1. Vertical section on line H Y of fig. 2. Fig. 2. Horizontal section on plane V T of fig. 1.
A, shaft of the furnace; B, chimney; C, hearth; D, foundation; E, bottom stone; a, dam plate; b, hearth plates of cast iron; c, cast-iron pillars on which the flange d rests; e, dam; f, fore hearth: g, bridge; h, tymp stone; i, breast; k. slag sprout; I, matte spout; m,"syphon tap; p, p1, p2, etc., tuyeres; q, nozzles; r, wind bags; s, main blast pipe; t, charging door.
. . .
I. Freiberg furnace lead. II. Bleiberg (Carinthia) furnace lead. III. Upper Hartz furnace lead. IV. English hard lead. V. Lead from dross of calcination of IV. VI. Lead from dross of calcination of V. VII. Refined English lead. "W. Black -ett, best selected.1' VIII. Renned Upper Hartz lead (Clausthal), desilverized by zinc.
- Silver is extracted from lead by exposing the melted argentiferous lead to the action of a blast of air at a temperature above the melting point of litharge. The lead is converted into oxide, while the less oxidizable silver remains unaffected. This process, called cupella-tion, is of great antiquity. In the opinion of Percy it is plainly indicated in Jer. vi. 29, 30: "The bellows are burned, the lead is consumed of the fire; the founder melteth in vain; for the wicked are not plucked away. Reprobate [refuse] silver shall men call them, because the Lord hath rejected them." " In this passage all the essential points are mentioned: the artificial blast, the oxidation or consumption of the lead, and the 'reprobate silver,' silver dross, or litharge. There is either no residue of precious metal, or what remains is contaminated with impurities, so tenaciously adherent as not to admit of being separated by the oxidizing and solvent action of molten litharge." The operation may be performed in a variety of ways.
The two prominent systems are the German and the English. The German cupelling furnace consists of a large shallow hearth, somewhat elliptical in shape, about 8 or 9 ft. in diameter, made of marl or a mixture of limestone and clay, firmly stamped down, but retaining a certain degree of porosity and absorbent power for molten litharge. On one side is a fireplace for either wood or coal, and at right angles to it are the nozzles, generally two, for supplying the blast. Nearly opposite to the blast, and near the fireplace, is an opening for the removal of the litharge. The top or head of the furnace is movable, and is let down and luted on when the operation is ready to begin. From 35,000 to 37,000 lbs. of lead are treated at one operation, of which 13,-000 are introduced in the form of pigs at the beginning, and the rest after the litharge has begun to flow. When the pigs first introduced are melted, there remains a sand-like scum (Abzug) on the surface, composed of impurities in the lead, some metallic lead and oxide, and particles from the hearth. This is removed, and the surface of the metal exposed.
The first litharge which forms is pasty, and contains largely the impurities in the lead, principally antimony, and is called black litharge (Abstrich)', it gradually becomes lighter in color, and passes into pure litharge. This is allowed to flow from the hearth as fast as formed by cutting notches in the marl of which the hearth is composed, and is caught in iron moulds. The interior of the masses thus formed cools slowly, and has generally a red color, and is then sold as such. The rest of the yellow litharge is reduced to metal in furnaces by itself. That which forms during the latter part of the operation contains considerable silver, and the lead obtained from it is again cupelled, after concentration of the silver, by one of the processes presently to be described. When the operation is nearly completed the thin layer of molten litharge covering the silver breaks away, leaving the bright surface of the silver exposed. The silver is then said to "blick," and the mass is called blick silver. This still contains from 8 to 10 Per cent. of impurities, and must be further refined.
This could be effected on the hearth of the cupellation furnace, by simply continuing the operation at an increased heat; but as the loss of silver would be large under these circumstances, the silver is removed and placed on a small hearth especially prepared for it, and exposed at a high temperature to a strong blast, with occasional stirring, until it is "fine." In this operation it absorbs oxygen, which on cooling is often violently expelled, giving rise to the phenomenon of " spitting." In some cases a large amount of lead is cupelled until the silver is concentrated to the extent of 10 Per cent. of the lead, when it is tapped off and further cupelled on another hearth. The cupellation of large quantities of lead at one operation is especially adapted to those cases where the lead is impure, as the foreign matters are then all removed in the abstrich, the litharge subsequently formed being pure. In Freiberg, Saxony, where the amounts mentioned above are cupelled at one operation, there are formed 400 to 000 lbs. of abzug, but no abstrich, as the lead is previously refined, from 32.000 to 33,000 lbs. of litharge, and 520 to 530 lbs. of silver; the loss of lead is from 8 to 10 per cent.
Ore very rich in silver is often directly submitted to cupellation by placing it under the pigs of lead in charging the furnace. In the English system an elliptical frame of wrought iron, about 4 ft. long and 2 1/2 ft. wide, is filled with moistened bone ash, firmly packed down, and then scooped out to the depth of about 3 in. This " test" is placed in a furnace, having a large fireplace on one side of the ellipse; at one end is the nozzle supplying the blast, and at the other is the opening for the removal of the litharge. The test having been previously heated, melted lead is poured into the cavity in the bone ash, and oxidation at once commences. A constant supply of lead is kept up by gradually protruding pigs of lead through openings at the back of the furnace. When the lead in the test has become enriched, say from 200 or 250 oz. to 3,000 oz. per ton, it is tapped off, by drilling a hole through the bottom of the test, into an iron pot below, and cast into pigs. After tapping, the hole is plugged up with the same material of which the test is composed, and the process resumed. The enriched lead is placed on a fresh test, and the operation continued until the silver is fine. The loss of lead in the English process is said to be 5 per cent.
The tests and hearth in both processes which have become soaked with litharge are broken up and added in the ore smelting. - The amount of silver in lead that will repay extraction by cupellation was formerly considered to be about 8 oz. per ton. Within the last 50 years two processes of concentrating silver in lead have been invented, by means of which silver to the amount of 2 oz. or less can be profitably extracted. The first of these processes, introduced by Pattinson about 1833, was universally employed until replaced in a great measure by Parkes's process, which was perfected about 1866. Pattinson's process depends on the fact that when molten lead containing silver is allowed to cool slowly, crystals of lead nearly free from silver separate, which can be removed by means of a perforated ladle, leaving a lead much enriched in silver. Lead to be submitted to this process should be refined, and contain only traces of antimony or other metals which impede the formation of crystals of poor lead. The following will serve to give a general idea of the manner of carrying out this process in practice. A battery of seven large iron pots, capable of holding 6 1/2 tons of melted lead each, is arranged in a straight line, each pot being provided with a separate fireplace.
At one end is a smaller pot holding 3 tons for the "poor" or marketable lead. Pot No. 4 we will suppose to contain 126 cwts. of molten lead, containing from 7 to 8 oz. of silver per ton of lead. As fast as crystals of lead form on cooling, they are taken out on a perforated ladle, allowed to drain thoroughly, and placed in pot No. 5; 90 cwts. of lead, containing from 3 1/2 to 4 oz. of silver, are thus transferred, and the remaining 36 cwts., containing 16 to 8 oz., are ladled into No. 3. To No. 5 is added 36 cwts. of lead, with the same amount of silver as that already in it (16 to 18 oz.); and of this 90 cwts. of crystals, containing from 2 to 2 1/2 oz., are ladled into No. 6, and the 36 cwts. remaining, holding 7 to 8 oz., transferred to No. 4. No. 6 is treated in the same way as No. 5; 90 cwts. of lead crystals, containing 1 to 1 1/4 oz., being ladled into No. 7, and 36 cwts. of 3 1/2 to 4 oz. into No. 5. From No, 7 crystals containing 9 to 10 dwts. of silver are ladled into the market pot, and then cast into pigs ready for sale. The nature of the operation in the other direction toward pot No. 1 is sufficiently indicated by the above. The operation goes on continuously, market lead being produced at one end, and at the other rich lead which is cupelled.
It is found that concentration of the silver beyond 200 to 300 oz. to the ton of lead is not advantageous. From 846 cwts. of lead, of 7 to 8 oz. silver to the ton, there is obtained (in an example given by Pattinson) 36 cwts. of rich lead, containing 160 to 170 oz., and 810 cwts. of poor lead, with 7 to 8 dwts.; ratio of rich to poor, 1: 22.5. There are numerous systems of conducting the process, but the above two-thirds system is perhaps the one most generally employed. - Parkes's process is founded on the fact that when zinc is thoroughly mixed with melted lead containing silver, and the mixture allowed to cool tranquilly, nearly all the zinc will rise to the surface and bring the greater part of the silver with it. Parkes first introduced his process in England in 1850, but owing to some practical difficulties it was abandoned. Experiments were made on it in Germany under the direction of Karsten in 1851, which did not result favorably. In 1866 the subject was again investigated in Germany, the difficulties previously encountered were overcome, and the process was made a practical success; it is now rapidly replacing Pattinson's process. The argentiferous lead is melted in a large pot, and zinc added in a perforated iron box and thoroughly mixed by stirring.
The amount of zinc used is from 1 to 1 1/2, and sometimes as high as 2 Per cent. of the weight of the lead; it is generally added in three portions. After the removal of the zinc crusts, the lead contains only traces of silver, but retains about 0.75 Per cent. of zinc. This must be removed before the lead is marketable. It may be effected by heating in a reverberatory furnace, as in the usual process for softening, by prolonged poling, or by steam. The use of steam was introduced by Cordurie, and is found to be prompt and efficacious in removing the zinc. The steam is forced through the molten lead in a large kettle provided with a cover. When a sufficient quantity of zinc crusts have accumulated, they are liquated at a gentle heat, giving lead, which is returned to the desilvering pots, and a residue containing, according to the amount of lead removed by liquating, from 1 to 3 Per cent. of silver. The treatment of this enriched product without loss of silver has been the greatest difficulty in the practical working of the zinc process. It is at present either heated in retorts, by which the zinc is distilled off and a rich lead left, or treated by Cordurie's process by steam, by which the zinc and part of the lead is oxidized, and a lead rich in silver left as before.
In Havre, where Cor-durie's process was first introduced, the steam in the treatment both of the desilverized lead and the zinc crusts is employed at a pressure of 60 lbs. or more, while in the Hartz a pressure of 15 lbs. is found to be ample. The oxides of lead and zinc produced by the action of the steam on the zinc crusts contain considerable silver. In the Hartz they are added directly to the rich lead in cupelling; in Havre they are treated with muriatic acid to dissolve the oxide of zinc, and the residue, consisting mostly of chlorides of lead and silver, is melted in an iron pot, and a lead rich in silver obtained. The oxides produced by the dezincification of the poor lead are generally, after removal of shots of metal by washing, used for pigments. It has been found experimentally in the Hartz that when zinc is added in quantity much too small for complete desilverization, as for instance when 40 lbs. of zinc are used for 12 1/2 tons of lead, a greater part of the copper and all of the gold is removed in the first skimmings. In this way small quantities of gold in lead may be concentrated.
It is stated that formerly at Rothschild's works in Havre 250 tons of lead were treated every month by Pat-tinson's process, or 10 tons in 24 hours; 50 to 52 men were employed, and coal was used equal in amount to 40 to 50 Per cent. of the weight of the lead. The loss on Cartagena lead was 6 per cent., on pure lead 4 per cent. In 1868, by the zinc process, with two mixing and two receiving pots containing 16 tons each, 20 tons were treated in 24 hours, or 500 tons per week; 23 men only were employed; the loss upon pure lead was 1 per cent., and the consumption of coal 10 Per cent. of the weight of the lead; the cost was 25 francs per ton, against 55 francs in Pattinson's process. Later results from the Hartz show a loss of lead in the zinc process of about 1.5 Per cent. of lead, not including the lead in the various large products which are either sold as such or worked over. (For a full discussion of the metallurgy of lead, see Percy's work on that subject, London, 1870, from which much of the above has been taken.) - Uses and Manufactures of Lead. As metal, lead is principally employed in sheets for sulphuric acid chambers and concentrating pans, and for linings of tanks, cisterns, etc, in pipes for water and gas supply, and in shot.
It further enters into the composition of many useful alloys. The compounds of the metal are mainly used as pigments and in the manufacture of flint glass. Sheet lead, formerly made by casting, is now generally made by rolling, or milling, as it is usually termed, the product bearing the name of milled lead. The Chinese prepare their well known tea lead by casting, notwithstanding its thinness. The operation is performed by two men, one of whom pours the molten lead from a crucible upon a large flat slab, when the other quickly places a large stone on the fluid lead, and presses it out to a thin flat plate, which is then removed and trimmed. English milled lead has largely replaced the tea lead of home manufacture in China. In the process of milling, slabs of lead, formed by casting in open moulds of cast iron, are passed between cylindrical iron rolls until the lead has attained the desired dimensions. The rolls used are provided with suitable mechanism by which the distance between them can be regulated, and with reversing gear so that the slabs and sheets can be passed backward and forward. When the slab has attained unwieldy dimensions by extension, it is divided into pieces of suitable size, which are rolled separately.
The same slab may be passed through the rolls 200 to 300 times, and become thereby elongated from 6 or 7 to 400 ft. If in rolling any depressions are observed on the upper surface of the slab, little pieces of sheet lead are placed in front of these depressions so as to force the subjacent lead into them and fill them up; when this is accomplished the pieces are taken off. The rolling may be conducted immediately after casting, while the slabs are still hot, or they may be allowed to become cold before rolling. - Lead pipe was formerly made by casting a short thick cylinder with the required bore, and rolling this cylinder out over a mandrel. Tubes made in this way were limited to 20 or 30 ft. in length. By the improved method a hydrostatic press is employed to force the melted lead through dies of the required sizes. The press is under the floor, through which the piston passes, entering a strong upright metallic cylinder. This can be filled with lead as required by a spout in the top, and the spout can be then closely shut. The cylinder is kept to the temperature of melting lead by an annular fireplace or receptacle for live coals by which it is surrounded.
Connected with the top of the cylinder is a steel die of the diameter required for the outside of the pipe, and through its centre passes from the centre of the piston below the mandrel which determines the diameter of the bore. As the piston is driven upward, the lead in the cylinder is forced through the annular space between the fixed collar or die and the mandrel, and emerging above cools in the form of a finished pipe, and is immediately coiled upon a drum suspended above the apparatus. - Lead shot might with propriety be classed among the alloys of lead, for though sometimes made of simple lead of inferior quality, the metal is very commonly combined with arsenic, introduced in the form of white arsenic (arsenious acid) or of orpiment (the sulphuret). The effect of the arsenic is to render the hard, brittle qualities of lead, which are contaminated by antimony and iron, softer and more ductile, and of the proper consistency, when melted and subjected to the usual process in shot making, for taking the globular form. The more ductile the lead the less arsenic is required, but hard lead requires 1 Per cent. or more of arsenic; usually from 0.3 to 0.8 Per cent. is added.
When the lead to the amount of two or three tons in a pot is melted, a circle of ashes or powdered charcoal is laid around the edge of the metal, and the arsenical compound wrapped in coarse paper is introduced in the centre by means of a wire basket and stirred in. The pot is then covered and the lid luted down and left for some hours, during which time the arsenical compound is decomposed, the greater part of the arsenic combining with the lead, while a portion mixes with the litharge produced by the reaction of the white arsenic on the lead. The mixture is then tested by dropping a portion of it through a colander into water. If the particles assume a lenticular form, the arsenic is in excess; if they are flattened on one side, hollowed in the middle, or elongated, too little arsenic was used. When properly dosed it is run into bars, which are raised to the top of the shot tower, to be there melted and poured through the colanders. These are either hollow hemispherical iron disks or rectangular flat sheets, each one perforated with a set of holes of uniform size, made perfectly smooth and exact.
The lead when poured must be of the proper temperature for the special size to be made, and the workmen are careful to keep a film of the oxide as a lining to the colander, which is thought to have the effect of increasing the rotundity of the shot, possibly by expediting its cooling as it passes through. The holes vary from 1/50 to 1/360 of an inch, but the shot are of larger diameter than the holes. In falling to the base of the tower the particles of semi-fluid lead, acted upon alike over their whole surface by the current of air, are made to assume the globular form, and by the time they reach the bottom they are sufficiently hardened by cooling to bear the shock of striking the surface of the vessel of water placed to receive them. Large-sized shot require a greater height than small-sized, and while 100 ft. is sufficient for the latter, the former will require 150 ft. Dr. Ure notices a shot tower at Villach in Carinthia, 249 ft. high, as the highest erection of this kind. Taken from the cistern of water, the shot are dried, then assorted according to their sizes by sifting them in a revolving copper cylinder set slightly inclined and perforated with holes, which increase in size toward the lower end.
The smaller sizes thus drop through above and the larger lower down, and each size is received in its own box. The shot receive their superficial finish by being left for some time in a rotating cylinder with some pulverized graphite. Imperfect shot are separated from the truly spherical by allowing them to roll down an inclined plane, so arranged that the latter run straight down the middle and the former work off to one or the other side. Shot are also made by pouring lead upon a revolving table on which is placed a cylinder of perforated sheet brass. The table is revolved with a velocity of 1,000 ft. per minute on the periphery, and the lead is thrown by centrifugal action through the perforations in the sides, forming round brilliant shot, which strike against a linen screen placed so as to intercept them. A method has been patented in the United States of manufacturing shot without the high towers, substituting for them a low elevation up which a powerful current of air is blown, thus producing the effect of a long continued fall. - The most important alloys of lead used in the arts are those with antimony and tin. The alloys with antimony are more fusible than lead alone, but are much harder and more readily oxidized.
Type metal is of variable composition, but in general may be said to consist of 4 or 5 parts of lead to 1 of antimony; sometimes tin is added. The alloy of tin and lead is used for soldering. Three varieties, known as fine, common, and coarse solder, are composed respectively of 2 parts tin to 1 of lead, equal parts of tin and lead, and 1 of tin to 2 of lead. Pewter is also composed of lead and tin; but other metals, as copper, antimony, and zinc, are often added. Common pewters contain 80 parts of lead to 20 of tin, others equal parts of the two metals, while the finer kinds contain but 16 to 20 Per cent. of lead. The French government sanctions the use of vessels of 18 Per cent. lead and 82 tin as quite harmless for containing wine or vinegar. Although so soft in itself, lead has the property of hardening tin. These alloys are distinguished by the facility with which they ignite and burn. The alloy of 4 or 5 parts of lead and 1 of tin burns like charcoal at a red heat, the combustion continuing like that of an inferior peat with the formation of cauliflower excrescences. This action appears to be due to the affinity which exists between the two oxides, which when fused, either alone or with silica or an alkali, produce a white opaque enamel, used for dial plates and also in earthenware.
Bismuth unites readily with lead in all proportions, forming alloys which have no application in the arts, but are used to adulterate mercury. An alloy of 1 part of lead, 1 part of bismuth, and 3 parts of mercury is sufficiently fluid to pass through chamois leather. With bismuth and antimony lead forms an alloy which expands on cooling, and is used in casting stereotype plates. They are composed, according to Mackenzie, of 70 Per cent. of lead, 15 of antimony, and 15 of bismuth. The triple alloys of lead and bismuth with tin or zinc are remarkable for their low melting point, which lies in many instances below the temperature of boiling water. Many metals, as zinc and copper, have but* little tendency to form alloys with lead. When mixtures of lead and zinc are kept in a molten condition for some time at a comparatively low temperature, the zinc rises and forms a scum on the surface, leaving the lead nearly free from zinc. When a mixture of lead and copper, which has been quickly solidified, is heated to a point but little above the melting point of lead, a large proportion of the lead containing a little copper liquates out, leaving a residue of copper containing lead. This behavior of these metals is of practical value in the desilverization of lead and copper.
In the first mentioned case, the zinc in separating from the lead carries the greater part of the silver with it; and in the second, the lead which liquates from the copper likewise contains nearly all the silver. For an extended account of the alloys of lead the reader is referred to an article by Dr. Richardson in Watts's Dictionary of Chemistry."