LEAD MANUFACTURE. Lead (Freoch,plomb; Italian, Fields° ; Spanish, plorno; Portuguese, thumb() (all from the Latin ptunibum); German, Mei ; Dutch, loot ; Russ., steinetz) was known and used by the Greeks and Romans for various purposes ; among others, it was employed for pipes to convey water, just as it is now. The lead-mines of Britain were worked by the Romans, of which we have evidence in the pigs of lead preserved in the British Museum, and stamped with the names of the emperors Domitiau and Hadrian. The early writers in this country, when speaking of the metals, are so confused, that it is by no means certain of which of them they are treating. This con fusion is so great, that Sir George Harrison, when writing in exposition of the atannary laws of England, says, "in a liberal construction, copper is tin." The framers and early expounders of those laws fell into some strange mistakes regarding even the nature of particular metals : these mistakes affected lead as well as other metals.
The principal lead-mines in Great Britain are in Cornwall, Devonshire, Somersetshire, Derbyshire, Durham, Lancashire, Cumberland, West moreland, Shropahire, Flintahire, Denbighshire, Merionethshire, and ,Montgomeryshire; in Scotland, at the Lead Hills on the borders of Dumfriesshire and Lanarkshire, in Ayrshire, and in Argyleshire. Ireland, lead is found in the counties of Armagh, Wexford, Wicklow, Waterford, Clare, and Down. No certain account of the produce has ever been obtained : the proprietors or occupiers of the principal mines declining, from prudential motives, to give detailed statements to that effect. In recent years, however, approximate estimates have been made, which we shall notice in a later paragraph.
The ore of lead, when extracted from the mine, is called galena, and is combined with various earthy matters. This galena, an impure sul phide or sulphuret, is the most important among the ores of lead ; it contains 86 parts of lead to 14 of sulphur. Of other ores of lead, very few are used as sources of the metal. The mining operations need not be specially described here. [lieleo.] The first procea-es subsequent to its extraction are those of crushing or pounding and washing the ore, in order to separate as far aa possible by mechanical means the impurities from the metal. The ore is then smelted, sometimes in a common smelting-furnace and sometimes in a reverberatory furnace, both of which are very similar in form and con struction to the furnaces reed for melting and puddling iron. [TROY 3sleurrecru en] When the fusion has been continued long enough to I cause the expulsion of the sulphur contained in the ore, and the sepa of the earthy matter in the form of scoria, the latter, which from its smaller apeciflo gravity floats on the melted metal, is removed from the furnace through an aperture provided for the purpose ; and the lead is allowed to run into a large iron pan, from which it is ladled into cast-iron moulds. It then constitutes what is called pig-lead. The
scoria still contains a portion of lead, and is subjected to the heat of another furnace, called a slag-hearth, for its separation, which occurs upon its fusion ; the metal then falls into a cavity, whence it is run and also cast into pigs.
Before tracing the lead to its employment in manufactures, we must notice a very remarkable reward which ingenuity has reaped. The pig-leul always contains more or lees of silver. The proportion is sometimes exceedingly minute, being not more than one ounce per ton In the metal raised in Derbyshire and Shropshire, while in every ton of the lead from Devon and Cornwall there is found from 20 to 40 ounces of silver : in some instances very much more. The produce of other mines contains the more precious metal in various proportions between these two extremes. The extraction of the silver is always performed when it exists in a proportion sufficient to pay the expense of the pro cess, which varies in different localities according to the cost of fuel. The process of extraction, which is called refining, depends upon the well-known circumstance that lead, when heated to redness, absorbs a large portion of oxygen from the air, and is converted into an oxide ; while silver does not undergo any such change, but retains its metallic form at almost any temperature. A cued, which is a shallow dish of adequate dimension, is filled with a mixture of burnt bones and fern ashes pressed down, upon which the lead to be refined is placed in the furnace. As soon as the lead is melted, a blast of air, introduced by the usual means, is made to play forcibly upon the surface. In a short time a crust of yellow oxide is formed. which is driven away as fast as it appears to the opposite side of the furnace, until all or nearly all the lead has been thus converted to an oxide. The silver, which remains behind, is still combined with some portion of lead, and must he sub jected to a second process similar to that here described, in order to liberate it in sufficient purity. The litharge, into which the lead has been thus converted, is easily restored to its metallic state by again heating it in a furnace in combination with carbonaceous matter, to which it gives up its oxygen.