A metal of a bluish-white colour, and when recently cut, of considerable lustre. It is very soft and flexible; not very tenacious, and consequently incapable of being drawn into very fine wire; yet its malleability permits it to be extended, either under the hammer or the rollers, into very thin sheets. Its specific gravity is 11.35; it soils paper and the fingers by friction, imparting a slight taste and a peculiar smell: it is a good conductor of heat; melts at 612° Fahr., and when cooled slowly, crystallizes into quadrangular pyramids. Lead is brittle at the time of congelation, and may then be broken to pieces with a hammer. Although the brightness of fresh cut or scraped lead soon goes off, it does not alter much by exposure to the air; owing, it is supposed, to a thin film of oxide being formed upon its surface, which defends the metal from further corrosion; this property renders it peculiarly suitable for the gutters and coverings of buildings. Lead ore is found in most parts of the world.
In Britain, the principal lead mines are situated in Cornwall, Devonshire; in Northumberland, Westmoreland, Cumberland, Derbyshire, Durham, Lancashire, and Shropshire; in Flintshire, and various parts of Wales; also in several districts of Scotland. The smelting is performed either in a blast furnace, called an " ore hearth," or in a reverberator}' furnace. In the former method the ore and fuel are mixed together and exposed to the action of the blast, which quickly fuses the metal and causes it fall into the lower part of the hearth, where it is protected from the oxygen of the blast by the scoriae that floats upon its surface. When the fluid lead is tapped, a sufficient quantity of it is left in the furnace to float the liquid scoriae; but when the whole of the lead is to be drawn off, the blast is stopped, and some lime is thrown into the furnace to concrete the scoriae whilst the lead is run out. In smelting by the reverberatory, which is undoubtedly the best, the fire is made at one end, and the flame passes over the hearth and enters into an oblique chimney, which terminates in a perpendicular one, called a stock, of considerable height.
The length of the hearth, from the place where the fire enters to the-chimney, is about 11 feet, 2 feet of which constitute the throat of the furnace; the remainder forms a concave surface, 4 1/2 feet wide at the throat of the furnace, and rather more than 7 feet at the distance of 2 feet from the throat, about 7 feet in the middle of the hearth, and 6 feet at 2 feet distance from the chimney, and nearly 3 feet where the flame enters the chimney, which it does through two apertures, each 10 inches square. The throat of the furnace is 2 feet long, 4 feet wide, and 6 inches deep. The length of the fire-place is 4 feet, equal to the width of the throat; its width is 2 feet, and depth 3 feet from the grate to the throat of the furnace; the section of the oblique chimney is 16 inches square, and of the perpendicular 20 inches, supposing a straight horizontal line drawn from the lower plane of the throat of the chimney to the opposite side of the furnace; the lower part of the concave hearth, which is in the middle of this cavity, is 19 inches below this line, the roof of the furnace being 17 inches above the same line; the rest of the hearth is conformably concave.
The furnace on one side has three openings, about 10 inches square, at equal distances from each other, and provided with iron doors, which can be removed as occasion may require. Besides these apertures, which are for the purpose of raking and stirring the ore, etc, and consequently upon a level with the horizontal line before alluded to, there are two others of smaller dimensions, one of them for the discharge of the fluid metal, and the other for the scoriae. The ore is introduced at the roof of the furnace through a hollow shaped vessel.
The ores of lead, like those of most other metals, are combined with various kinds of earthy matter, which require them to be pulverized before they undergo the smelting process. The pounding is sometimes performed by hammers, but usually by a stamping mill, or by rollers. When thus reduced, the heavy metallic matter is separated from the lighter earthy matter by washing. The common mode of effecting this is to put the powdered metal into a riddle or sieve, immersed in a large tub of water, wherein it is agitated by a movement that washes away the small particles through the sieve, and ejects the lighter portion of the matter over the sides of the sieve; while the metallic portion, from its specific gravity, is less disturbed, and is collected at the bottom of the sieve. Some improved apparatus for this purpose was patented by Mr. Harsleben, in 1827, the description of which will be found under the article Mining. In some establishments in this country, and very generally abroad, the ores are washed upon inclined tables, which are shook by machinery, whilst water is made to flow over them to separate the metallic from the less ponderous matter; which apparatus is also described under the article Mining, as it is equally applicable to other ores as to the ore of lead.
An improvement in the furnaces for smelting lead ores was patented by Mr. Joseph Wass, of Ashover, Derbyshire, the main object of which was to obviate the injurious effects upon animal and vegetable life within the range of the metallic vapours emanating from furnaces of the usual construction. But in addition to this important desideratum, there results from the adoption of this improved arrangement a considerable profit, which arises from the product obtained by the condensation of those volatile and deleterious substances that are usually allowed to mix with the atmosphere. In the specification which is before us, the patentee states, - "By the employment of this improved apparatus, smelting and calcining furnaces are divested of their pernicious effects, and such works may in future be erected in any convenient situation, either near to dwelling-houses, or by the side of public roads, or on the banks of navigable rivers or canals; and thus, in many cases, produce a very great economy in the expense of carriage.