Page:The American Cyclopædia (1879) Volume X.djvu/251

 LEAD 245 It has a very low tensile strength, amounting, in the form of wire, and between 15 and 20 0., to only 3,620 Ibs. per square inch when the strain is slowly applied, and 4,172 Ibs. 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 mush- room it is sonorous. Chemistry of Lead. The atomic weight of lead is 207 (O=16), its sym- bol Pb (plumbum). When a freshly cut sur- face is exposed to moist air, it becomes covered with a gray incrustation of oxide, which pro- tects 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, pro- toxide, sesquioxide, and peroxide; the pro- toxide 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 atmos- phere 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 cool- ing has been rapid or slow ; it is composed of minute scales which have a talc-like feel ; when molten, it conducts electricity. Pro- toxide of lead is slightly soluble in water, to which it communicates a decidedly alkaline re- action ; 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 carbon- ate ; 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 ex- pressed by the formula PbsO*, and may be re- garded as a compound either of the protoxide with the sesquioxide, PbO, Pb 2 O 3, or of the protoxide with the peroxide, 2PbO, Pb0 2. It is prepared on the large scale by first oxidizing metallic lead to massicot, which is ground and levigated, and the resulting fine powder ex- posed to an oxidizing atmosphere on the hearth of a furnace for about 48 hours, at a temper- ature 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 carbon- ate 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 like- wise 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 vary- ing 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, Eng- lish, and Dutch methods ; they all depend pri- marily 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 so- lution is forced carbonic acid gas, which pre- cipitates two thirds of the lead, and this after settling is collected and dried. The superna- tant 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 Eng- lish process, litharge, with about 1 per cent, of acetate (sugar of lead), is mixed with water to a moist mass, and exposed under constant stir- ring 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 car- bonic acid for a long period. In earthen ves- sels are placed sheets of lead rolled into the form of a spiral ; into the bottom of these ves- sels, but not in contact with the lead, is poured a mixture of weak vinegar and substances ca- pable of fermentation, as yeast ; a plate of lead serves as a cover. From 1,500 to 2,000 ves- sels 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 sub- stantially the same as in the other processes given ; the lead is first converted into acetate, and subsequently into carbonate by the car- bonic acid given off by the decomposing mat'