LEAD appears to have bean known in the earliest ages of the world. Its colour is bluish white; it baa much brilliancy, is remarkably flexible and soft, and leaves a black streak on paper: when handled it exhales a peculiar odour. It molts at about 612o, and by the united action of heat and air, is readily converted into an oxide. Its specific gravity, when pure, is 11.445; but the lead of commerce seldom exceeds 11.35. (Brandt, 833.)
Lead is used in a state of comparative purity for roofs, cisterns, pipes, vessels for sulphuric acid, etc. Ships were sheathed with lead and with wood, from before the Christian era to 1450, after which wood was more commonly employed, and in 1790 to 1800 copper sheathing became general; of late years, lead with a little antimony has likewise been used, also Muntz's sheathing, an alloy of copper and zinc and galvanized sheet iron. The most important alloys of lead are those employed for printers' type, namely, about
for the smallest, hardest, and most brittle types.
for small, hard, brittle types.
for types of medium size.
for large types.
for the largest and softest types.
The small types generally contain from 4 to 0 per cent of tin, and sometimes also 1 to 2 per cent of copper; but as old metal is always used with the new, the proportions are not exactly known. - See pp. 293,310, and 323.
Stereotype-plates contain about 4 to 8 parts of lead to 1 of antimony.
Baron Wetterstedt's patent sheathing for ships, consists of lead with from 2 to 8 per cent of antimony; about 3 per cent is the usual quantity. The alloy is rolled into sheets.
Similar alloys, and those of lead and tin in various preparations, are much used for emery wheels and grinding-tools of various forms by the lapidary, engineer and others. The latter also employs these readily-fused alloys for temporary bearings, guides, screw-nuts, etc. - See foot-note, p. 293.
Organ pipes consist of lead alloyed with about half its quantity of tin to harden it The mottled or crystalline appearance so much admired shows on abundance of tin.
Shot metal is said to consist of 4*0 lb. of arsenic to one ton of lead.
In casting sheet-lead, the metal was poured from a swing-trough upon a long and nearly horizontal tabic covered with a thin layer of coarse damp sand, previously levelled with a metal rule or strike. The thickness of the fluid metal was determined by running the strike along the table before the lead cooled, the excess being thus swept into a spill-trough at the lower end of the table; but the sheet-lead now more commonly used, is cast in a thick slab, and reduced between laminating rollers; it is known as " milled-lead."
The metal for organ-pipes is prepared by allowing the metal to escape through the slit in a trough, as it is slid along a horizontal table, so as to leave a trail of metal behind it; the thickness of the metal is regulated by the width of the slit through which it runs, and the rapidity of the traverse; a piece of cloth or ticken is stretched upon the casting table. The metal is planed to thickness, bent up, and soldered into the pipes.
Lead pipes are cast as hollow cylinders, and drawn out upon triblets; they are also cast of indefinite length without drawing. A patent was taken out for casting a sheath of tin within the lead, but it has been abandoned
Lead shot are cast by letting the metal run through a narrow slit, into a species of colander at the top of a lofty tower; the metal escapes in drops, which for the most part assume the spherical form before they reach the tank of water into which they fall at the foot of the tower, and this prevents their being bruised. The more lofty the tower, the larger the shot that can be produced; the good and bad shot are separated by throwing small quantities at a time upon a smooth board nearly horizontal, which is slightly wriggled, the true or round shot run to the bottom, the imperfect ones stop by the way, and are thrown aside to be re-melted, the shot are afterwards riddled or sifted for size, and churned in a barrel with black lead.
Mr. Joseph Manton took out a patent for amalgamating the surface of leaden shot with mercury. One pound of mercury was added to every cwt. of shot; they were churned together in a revolving barrel nearly full of water, until the shot assumed a silvery coat. These shot were stated to foul the barrel of the gun in a less degree than others, and also to be less injurious to the game after it had been killed.