Applied externally lead salts have practically no action upon the unbroken skin, but applied to sores, ulcers or any exposed mucous membranes they coagulate the albumen in the tissues themselves and contract the small vessels. They are very astrin gent, haemostatic and sedative; the strong solution of the sub acetate is powerfully caustic and is rarely used undiluted. Lead salts are applied as lotions in conditions where a sedative astrin gent effect is desired, as in weeping eczema; in many varieties of chronic ulceration ; and as an injection for various inflammatory discharges from the vagina, ear and urethra, the Liquor Plumbi Subacetatis Dilutum being the one employed. The sedative effect of lead lotion in pruritus is well known. Internally lead has an astringent action on the mucous membranes, causing a sensation of dryness ; the dilute solution of the subacetate forms an effective gargle in tonsilitis. The chief use of the preparations of lead, however, is as an astringent in acute diarrhoea, particularly if ulceration be present, when it is usefully given in combination with opium in the form of the Pilula Plumbi cum Opio. It is useful in haemorrhage from a gastric ulcer or in haemorrhage from the intestine. Lead salts usually produce constipation, and lead is an active ecbolic. Lead is said to enter the blood as an albuminate in which form it is deposited in the tissues. As a rule the soluble salts if taken in sufficient quantities produce acute poisoning, and the insoluble salts chronic plumbism. The symptoms of acute poisoning are pain and diarrhoea, owing to the setting up of an active gastro-enteritis, the faeces being black (due to the formation of a sulphide of lead), thirst, cramps in the legs and muscular twitchings, with torpor, collapse, convulsions and coma. The treatment is the prompt use of emetics, or the stomach should be washed out, and large doses of sodium or magnesium sulphate given in order to form an insoluble sulphate. Stimulants, warmth and opium may be required. For an account of chronic plumbism or saturnine poisoning, see LEAD POISONING.
• Nearly all of the lead production of the world is obtained by smelting galena ore by a reduction process. Sometimes the ore is roasted to remove part of the sulphur before the reduction smelting treatment. But some ore, particularly the extremely high grade mill concentrates, is smelted directly without previous roasting. Three different types of smelting furnace are used to reduce the raw or roasted galena ore to metallic lead. The furnace selected depends to a large extent on the richness of the ore and the kind and amount of other metal impurities which, in the form of different minerals, are associated with the galena. The three types of smelting furnace are the ore hearth or Scotch hearth as it is sometimes called, the blast furnace and the reverberatory furnace.
The ore hearth is used principally for very high grade galena mill concentrates which have been ground rather fine in the mill ing operation and which are fairly free from other metals. The blast furnace is used principally to smelt : lower grade crude ore containing a comparatively large amount of waste rock. The blast
furnace is also used to smelt low grade mill concentrates, con centrates containing large amounts of other metal impurities and many by-products from the ore hearth and from the lead refin eries. Much secondary lead material recovered in the form of waste products from many industries is also treated in the blast furnace. Reverberatory furnaces are used mostly to smelt sec ondary scrap and waste products and are not used to any great extent to smelt primary lead ores or concentrates.
The ore hearth is one of the simplest and oldest furnaces used for the smelting of lead and consists of a shallow hearth, filled with a bath of molten lead on which floats a coke fire maintained by blowing air through pipes or tuyeres at the back of the hearth above the lead bath. The finely ground high grade lead ore is fed on the fire with continual stirring, which results in reduction of a large portion of the galena to metallic lead. The lead passes through to the lead bath underneath leaving a residue of waste materials in the form of slag which contains a considerable amount of lead and which is usually retreated in a blast furnace.
The blast furnace is a shaft furnace filled with a mixture of ore, fluxing materials and coke heated to a high temperature, which is continuously fed at the top and from which molten reduced metallic lead, slag and by-products are removed at the bottom. Combustion of the fuel is maintained by the introduc tion of compressed air through pipes or tuyeres near the bottom. The reverberatory furnace consists of a long brick hearth with low arched roof and has a source of heat at one end and a flue at the opposite end. The material to be smelted is charged on the hearth where smelting with production of metallic lead, slag and by-products takes place.
When lead is reduced to metal in a smelting operation, small amounts of other metals are reduced with the lead. A refining operation is usually necessary to remove the other metal impuri ties and purify the lead for commercial use.
Use of Lead.—The world consumption of lead is about 1,500, 00o tons of which about 750,000 tons or half of the entire con sumption is used in the United States. A large proportion of the total lead consumed is used in the form of metallic lead and its alloys, but the largest and most important single use of lead is in the form of white lead for which purpose about 150,000 tons of metallic lead are consumed annually in the United States.
White lead is the hydrated basic carbonate of lead which, in the form of a finely ground white powder, is a very valuable paint pigment. White lead has been the most important white paint pigmejt for more than 2,000 years and the most durable exterior paints contain a large proportion of white lead. White lead is also used as one of the principal constituents of pottery glazes, for the making of certain chemical colours, and for many other purposes. White lead is made by a chemical process in which, by the aid of acetic acid, carbon dioxide and metallic lead is corroded into hydrated basic lead carbonate.