Lead, one of the imperfect metals, is of a dull white, inclining to a blue colour ; and, though the least ductile and sonorous, it is the heaviest of metallic bodies, excepting mercury, gold, and platina.
lead is found in various countries ; but it abounds in England, especially in the counties of Derby and Devon. When dug out of the earth, it is crushed in a mill, and amelted in a manner similar to iron-ore, of which we have already treated.
A patent was granted in 1779, to Mr. W. ROE, for his new-invented process of extracting sulphur from poor lead ores, and rendering these as valuable, and saleable, as any other ores of this metal.—As this patent is now expired, and the principle of the inventor is equally simple and ingenious, we trust it is, or will be, generally adopted in our smelting-houses : the inquisitive reader will find it fully specified in the 6th vol. of the " Repertory of Arts and Manufatures."
Lead is employed in making varions vessels, such as cisterns for water, large boilers for chemical purposes, etc. Considerable quantities are likewise used in the casting of shot, for which a patent was granted in 1782, to Mr. William W'atts, in consequence of his invention of gradulating lead solid throughout, without those imperfections which other kinds of shot usually present on their surface.— The patentee directs 20 cwt. of soft pig-lead to be melted in an iron pot, round the edge of which, a peck of coal-ashes is to be strewed upon the surface of the metal, so as to leave the middle of the latter exposed. Forty pounds of arsenic are next to be added to the uncovered lead, and the pot closely shut; the edges of the lid being carefully luted with mortar, clay, or other cement, in order to prevent the evaporation of the arsenic. A brisk fire is then kindled, so that the two substances may be properly incorporated ; when the metal ought to be skimmed and laded into moulds, that it may cool in the form of ingots or bars, which, when cold, are called slag, or poisoned metal.—20 cwt. of soft Pig-lead (according to the quantity of shot intended to be manufactured) are next to be melted in the manner above directed; and, when it is completely liquefied, one of the ingots or bars of slag must be added: as soon as the whole is combined, a small quantity of the liquid metal is to be tak en out with a ladle, and dropped from a height of about two feet into the water. It the shot be not perfectly round, it will be necessary to add more slag, till it drops in a globular form. The metal is next skimmed, and the scum poured into an iron or copper frame perforated with round holes, according to the size of the shot designed; the scum is then'to be squeezed while soft, through the frame, into which the liquid should be poured, and dropped through the holes. For the smallest shot, the frame must be at least ten feet above the water, and for the largest, about 150 feet; the height being increased or diminished, in proportion to the size of Che shot.
There are various other purposes to which lead is usefully applied : it unites with almost every metal, except iron; but, if both metals be exposed to the fire in a proper vessel, the formers scorifies the latter, and melts with the calx, into a dark coloured glass. On account of this property of vitrifying the imperfect metals, lead is often used in the purification of gold and silver, neither of which combine with it, but remain pure on the bottom of the cupel.—It is also frequently employed by unprincipled dealers, for correcting the rancidity of damaged rape-seed Oils, and those of almonds or olives. This dangerous abuse may be discovered, by mixing a small quantity of the suspected oil with a solution of orpiment, or liver of sulphur, in lime-water: as, on shaking the two liquids together, and suffering them to subside, the oil will, if it be adulterated with lead, acquire an orange-red colour; but, if it be pure, it will assume only a pale yellowish shade. A similar pernicious fraud is practised with acid wines, which dissolve a sufficient portion of lead, so as to acquire a sweetish taste : this may be detected by means of the same solution, which forms the chief ingredient of the different liquid tests sold for that purpose.
Lead, when taken or inhaled into the human body, is productive of various fatal disorders, to which miners, potters, and all other persons concerned in its* manufacture, are peculiarly subject. Hence culinary vessels, or other domestic utensils made of this metal, are highly objectionable, especially if they are intended to com-tain cyder or other acid liquor. To this cause the Devonshire colic is justly attributed; for great quantities of cyder are, in that county, kept in vessels, consisting either wholly of lead, or such as are soldered with this pernicious metal. The dry belly-ach of the West Indies is of the same origin, and is occasioned by distilling rum through leaden worms.
In these dreadful complaints, the patient is seized with an acute spasmodic pain in the stomach, which extends gradually to the whole intestinal canal : the bowels are frequently inverted and drawn towards the spine, so as to render the application of clysters impracticable. At the same time, a most obstinate costiveness prevails ; and the affection at length terminates in palsy, or in fixed contractions of the limbs.
For the cure of this painful malady, gentle clysters and laxatives may at first be administered ; but, it these are not attended with be-: neficial consequences, Dr. Perci-; val decidedly recommends the in ternal use of alum; which, inslight of the Devonshire code, has generally effected a cure, when used to the extent of 15 or 20 grains every fourth, fifth, or sixth hour. Balsam of Peru, in doses of 40 drops, to be taken two or three times in the course of the day, has also been advantageously prescribed; at. the same time, castor-oil, or other mild laxatives, conjoined with gentle opiates, have greatly contributed to afford relief.—The patient's diet ought to consist of nourishing broths, panada, and gruel, or similar light dishes.
In whatever form lead may be introduced into the human body, it is equally deleterious and fatal, whether its vapours be inhaled through the lungs, absorbed through the pores of the skin, or particles of the metal betaken into the stomach. The only effectual antidotes to this insidious poison are, antimonial emetics; and, after them, the internal use of liver of sulphur, together with vegetable oils, both externally and internally, should be liberally continued.
Red-lead, or Minium, is a calx of lead of a lively red colour, which it acquires by slow calcination and reverberation.- Its preparation is as follows : A quantity of lead is first burnt in a furnace, till it is converted into a kind of litharge, being stirred continually with an iron spatula, while it is melting : it is then ground in a mill to a fine powder, after which it is again put into the furnace and stirred as before, when it assumes first a blackish hue, then a yellow cast, and at length becomes of a deep red colour. While this operation' is performing, the greatest caution is requisite to keep the fire at a certain height, in order to prevent the matter from adhering, and running together
The bright colour of minium would render it a valuable pigment, if it could stand either in oil or in water : but, being apt to become black, it is seldom employed, except as a ground for vermillion.-The genuine quality of red-lead may be ascertained by the brightness of its colour ; and, as it is frequently adulterated, such fraud may be easily detected, by mixing equal quantities of minium and charcoal-dust in a crucible, and placing the whole over a fire sufficiently intense to melt lead. When it has continued tor some time over the flame, it must be removed; and, when cold, stricken against the ground. Thus, the red-lead will. be reduced to its metallic state ; and, when freed from the charcoal, its diminished weight will shew the proportion of adulterated matter.
In medicine, red-lead is only-employed externally : it obtunds the acrimony of humours; mitigates inflammations; and, if judiciously applied, is of excellent service in cleansing and healing old ulcers.
White-Lead, or Cerussa, is" prepared by placing a vessel c taining vinegar in a moderately warm place, and over which thin plates of lead are suspended, so that the vapour arising from the-acid may circulate freely round the plates. A white powder settles in the course of two or three weeks, on the surface of the metal, which. is now removed into another room, and passed beneath a screen, and. pair of rollers, for separating the corroded from the sound part; that is again suspended, till it is wholly converted into a white calx ; when it is called cerusse, or white-lead. During diisoperation, a considerable portion of fine dusty particles settles on the skin and lungs of the workmen, to whom it is attended with the most pernicious consequences. In order to counteract such injurious effects, Mr. Ward, in the year 1795, laid a machine before the Society for the Encouragement of Arts, etc. and was consequently rewarded with their gold medal. It consists of a vessel 12 feet in length, six feet wide, and three feet ten inches deep, in which a pair of brass rollers is fixed, one above the other: the centre of these is about ten inches beneath the top of the vessel; and, one inch lower, a covering of oak-boards or riddles, about an inch thick, is inserted in a groove on the inside of the vessel, so that it may be occasionally removed. These boards are perforated in the centre with several holes, each of which is about five-eighths of an inch in diameter.
Previously to the working of this machine, the vessel is filled with water, about three inches above the oak-board.s; when the whole of the lower brass-roller, and half the upper roller, are completely immersed: the lead, on being corroded, is passed through these cylinders; and, by stirring the metal with a copper rake, the ce-russe is forced through the riddles, and the blue or uncalcined metal remains above. Thus, white-lead is prepared; and, by such useful contrivance, the minutest particles are prevented from ascending, and consequently their pernicious influence on the health of the workmen is effectually obviated.
Among the different patents registered for the manufacture of white-lead, we shall mention only those granted to Mr. JameS TuR-ner, in 1780; to Mr. RiChard Fishwick, in 1787; and to the Earl of DunDonald; of which the reader will find ample specifications in the different volumes of the"Repertory," etc - A patent was also granted in 1799, to Mr. John Wilkinson, for a new method of making white-lead: he directs any quantity of litharge to be ground very fine in sea-water, or other saline mixture; and, by repeatedly triturating, washing, and bleaching it, the patentee asserts, that white-lead of the best quality may be obtained. No vinegar, or other acid mixture, is necessary in this process; as levigation, repeated ablutions, and drying, are amply sufficient, provided more time be allowed for the operation, by the medium of the common atmosphere.
The last patent we shall notice, was obtained at the commencement of ISOI, by Mr. Thomas Grace, for a contrivance of making an acid to corrode lead, and also for a new process of manufacturing white-lead.—In either re-sect, however, the usual method is but little varied : the acid required for the purpose is generally prepared by fermenting melasses and water, together with other materials, which' are well known to vinegar-makers, and which it would be needless to detail. Among other articles, he makes economical use of the sours, or water in which wheat has been steeped for converting it into starch ; as likewise of the water employed for distilling oil of turpentine; both these liquids possess a considerable portion of acidity, which has hitherto been generally wasted.
All the different methods of preparing white-lead, however, are extremely pernicious, as well to the manufacturer as to those who use vessels that are glazed with it. Hence we have already (vol. ii. P.877) pointed out proper substitutes for this destructive metal, which were invented by foreign chemists; and shall, therefore, conclude with an account of the patent granted in 1796 to Mr. James
Keeling, for his contrivance of a substitute, both for red and white-lead, in glazing earthen-wares glass, enamel, etc. He directs any quantity of lead-ore, to be put into a reverberatory or other furnace, and to be roasted till it become of a white-heat, during which process the metal will emit a considerable portion of fumes. The lire must be continued till about an hour after the vapour is dissipated ; and, when the mass grows cool, it is to be removed from the furnace, and ground with water to a fine liquid state, when the other ingredients, usually employed in making glazes, may be added ; and the preparation will be complete. Thus the injuries occasioned to the workmen, by the dusty particles which settle upon their skin and lungs, will be effectually prevented ; but this succedaneum does not remove the poisonous qualities of the metal in the glazing of earthen-ware.
White-lead is employed in painting, and furnishes a tolerable white. —See vol. 11. p. 36"; and also Paint.
Cerusse is likewise used in surgery; and, on account of its cooling, drying, and astringent properties, is of considerable service when sprinkled over running sores or ulcers.
Black-Lea d, or Plum logo, a genus of inflammable substances, found in various parts of the world; but most abundantly at Borrowdale, in Cumberland; whence Britain, as well as the greater part of Europe, are supplied with this article.
Pure black-lead is of a very deep colour: when newly cut, it presents a blueish-white cast, and shines like common lead. It is insoluble in acids ; but, if it be put into a vessel placed over a strong fire, and exposed at the same tirne to the air, it is almost entirely volatilized, depositing only a little iron, and a small portion of siliceous earth.
Black-lead is chiefly used in the manufacture of pencils for drawing ; and though paper can be marked with them for a time, yet every trace may afterwards be totally rubbed out by means of soft bread, or elastic gum. In forming such pencils, the lead is divided into long pieces, and fixed into square grooves, cut in cedar or r soft wood : another piece is then glued over, and the whole worked into thin cylinders. A coarser kind of pencils is manufactured, by mixing pulverized black-lead with sulphur ; Which, however, are calculated only for carpenters' marks, or very coarse drawings.—The powder of black-lead also serves to cover razor-straps ; and considerable quantities of it are used for imparting a brigt gloss to cast-iron grates or stoves.—It may also be advantageously applied to smoothen the inner surface of wooden-screws, packing- presses, and other wood-work that is subject to frequent friction, for which purpose it is far superior to greasy, soapy, or oily matters.