Lead ores are easily reduced by several methods

Ores free from sulphur, arsenic, antimony, selenium or tellurium, are simply mixed with carbon as a reducing agent (ground coal, coke, charcoal), and a suitable flux to unite with the gangue of the ore, to form a fusible slag, and then smelted down on a hearth or in a reverberatory or a shaft-furnace. The reduction takes place at a red heat, and the metal is collected without difficulty. Ores containing sulphur, etc., must be treated differently. The simplest method is to melt them in contact with scrap-iron. Iron has more affinity for sulphur, etc., than lead has, and therefore forms iron sulphide and sets the lead free. This operation is performed on the hearth of a reverberatory furnace, and is very efficient, but depends for its practicability on the value of scrap-iron at the works. Another method for reducing these ores is to first drive off all the sulphur, etc., by roasting, i.e., a gentle heating to redne in a current of air. This operation converts the ore mostly into lead oxide. which is further treated by any of the methods for reducing oxide ores. Still another proce for reducing the sulphur ores, and the one in largest use, is a partial roasting, followed by a reaction of the oxidized ore upon the remaining non-oxidized ore. On roasting, lead sulphide pa es mostly to oxide, but partly to sulphate, the reactions being PbS+O3-PbO+SO, PbS+O1=PbSO4. If, when part of the ore is thus oxidized, the supply of air is cut off, the oxidized part well mixed with the unoxidized part, and the heat then raised higher than before, the oxygen in the oxidized ore reacts on the sulphur in the unoxidized portion, so as to set all the lead free, according to the reactions, 2PbO+PbS=3Pb+SO2 PbSO4+PbS-2Pb+2SO2. This is called the roasting and reaction proce , and is carried on in reverberatory furnaces. The charge is about a ton of crushed ore at a time: it is roasted with careful stirring for two hours, then the doors are closed and a strong heat employed for about an hour, the lead set free collecting in a depre ion in the hearth, with the slag floating on top IMG:content-1041.jpg:[blocks in formation] of it. The result of these reduction proce es is raw or impure lead, which must be refined. The impurities frequently present are arsenic, antimony, copper, zinc, and bismuth, also any silver or gold which were in the ore. In any method of refining, the precious metals and bismuth remain in the lead, so that their separation involves a different set of proce es, described under the METALLURGY OF SILVER (q.v.). The other metals are usually removed by oxidation in a softening furnace. The impure lead is melted on the hearth of a diately after melting, as the purer lead liquates away from the le fusible alloy of lead with zinc and copper, which, being lighter, floats on the purer lead as a thick. mushy scum, and is removed by perforated ladles. After this the arsenic and antimony oxidize to arsenate and antimonate of lead and flow off as slag. If iron. nickel, or much zinc is present, they are best removed by the action of steam, which may be introduced either directly by a pipe, as superheated steam, or indirectly by stirring the metal with a pole of green wood, an operation called poling. Lead is largely used by itself for roofing, water-pipes, lining tanks for chemical purposes, acid chambers, etc. The most important alloys are Type Metal (see ANTIMONY), Shot Metal (see ARSENIC), Tinman's Solder, containing 30 to 60 per cent of tin, Pewter, and Bearing Metal (see ANTIMONY). Baths of lead, or of alloys of lead and tin, are frequently used for tempering steel instruments. Sheet-iron coated with an alloy of lead (70 to 80 parts) and tin (20 to 30 parts) is called Terne plate, and is in large use for roofing purposes.