Fig. 13.—After firing the casting is lifted out for treatment with dry glaze, which is sprinkled on with a sifter shown on the table. The turntable enables the operator to manipulate the red-hot casting more easily.
White enamel powders free from lead are used entirely by some firms, but the black and coloured enamels on stove grates contain lead. A frit analyzed in the Government Laboratory was found to contain 26·66 per cent. of lead oxide. The fact that all the lead used is in the form of a silicate, even although the silicate is readily soluble in dilute acid, tends, we believe, to cause incidence of poisoning to be less than might have been expected from the amount of dust often present in the air, and attacks, when they occur, to be less severe, as a rule, than they would be were raw carbonate of lead alone used. For the arduous work entailed the men are specially selected. Despite their exposure to lead dust, the majority continue to work for many years without marked signs of lead absorption. The management should provide a suitable room for the men to cool themselves in the intervals of dusting.
Fig. 14.—The cabinet is shown when dry dusting is being done. The casting is worked by tongs through a slot in the side of the cabinet (not seen), while the worker dusts the casting with his arms through the two front holes. He can see his work through the square pane of glass. (Photographs kindly made by Mr. F. W. Hunt, Leeds.)
Manufacture of Electric Accumulators.
[30]—Electric accumulators are secondary batteries which serve for the storage of electricity, in order to allow of a current when desired. A primary battery is one in which the materials become exhausted by chemical action, and, unless a portion or the whole of the materials is renewed, fails to supply electricity. The secondary battery becomes exhausted in the same way, but the chemical contents are of such a nature that it is merely necessary to pass a current of electricity through the battery (charging) in order to recharge them. In the accumulator battery the positive element is peroxide of lead, and the negative element spongy lead. The elements—several positive connected together and several negative—are placed in dilute sulphuric acid contained in vessels of glass.
The form of accumulator in almost universal use now is the pasted plate, but it varies greatly in size, according to the use for which it is required. It may be either large, to act as an equalizer or reservoir of current in electric-lighting installations, or quite small for ignition purposes in motor-cars. The litharge smeared on to one plate becomes converted into the positive element, peroxide of lead, during what is called the “forming process” (passage of the electric current through the dilute sulphuric acid solution in which it is placed), and red lead smeared on to the other becomes spongy lead to form the negative.
The industry gives employment to about 1,200 persons. Plates are first cast in moulds from a bath containing molten lead or of lead with admixture of antimony. Irregularities in the plates so cast are removed by a saw or knife (trimming), and sometimes filed or brushed with a wire brush. The interstices in the plates are next filled in by means of a spatula with paste of litharge or red lead, as the case may be, which has been previously mixed either by hand at the bench or in a special mechanical mixing machine. After drying, the plates are removed to the formation room to be charged. To allow of the passage of the current, positive elements are connected together, and negative also, by means of a soldering iron or, more frequently, of an oxy-hydrogen blowpipe flame. After formation is complete the plates have to be built into batteries, or “assembled.” Tailpieces, technically known as “lugs,” have to be connected with each plate, effected usually by the oxy-hydrogen blowpipe flame. Finally, a connecting bar of lead is cast on or burnt on to the lugs.
Dangers and Prevention.
—In casting, danger is mainly from dust in depositing the skimmings, and from fume also when old accumulator plates are melted down. For these reasons exhaust ventilation over the melting pots should be provided, embracing also (by branch ducts if necessary) the receptacles into which the lead ashes are thrown. In mixing and pasting, the danger is from dust of oxides of lead to be controlled (see [Fig. 6]) by—(1) Exhaust ventilation by branch ducts protecting (a) the barrel from which the material is scooped, (b) the mechanical mixer into which the weighed quantity of oxide is discharged, (c) the bench at which the mixing by hand is done; (2) dampness of benches and floor to prevent raising of dust either by manipulation of the (often) heavy plates or trampling into powder the paste which may fall on the ground.