Page:The American Cyclopædia (1879) Volume X.djvu/268

 262 LEAD of 60 Ibs. or more, while in the Hartz a pres- sure of 15 Ibs. is found to be ample. The oxides of lead and zinc produced by the action of the steam on the zinc crusts contain considerable silver. In the Hartz they are added directly to the rich lead in cupelling ; in Havre they are treated with muriatic acid to dissolve the ox- ide of zinc, and the residue, consisting mostly of chlorides of lead and silver, is melted in an iron pot, and a lead rich in silver obtained. The oxides produced by the dezincification of the poor lead are generally, after removal of shots of metal by washing, used for pigments. It has been found experimentally in the Hartz that when zinc is added in quantity much too small for complete desilverization, as for in- stance when 40 Ibs. of zinc are used for 12 tons of lead, a greater part of the copper and all of the gold is removed in the first skim- mings. In this way small quantities of gold in lead may be concentrated. It is stated that formerly at Rothschild's works in Havre 250 tons of lead were treated every month by Pat- tinson's process, or 10 tons in 24 hours; 50 to 52 men were employed, and coal was used equal in amount to 40 to 50 per cent, of the*weight of the lead. The loss on Cartagena lead was 6 per cent., on pure lead 4 per cent. In 1868, by the zinc process, with two mixing and two receiv- ing pots containing 16 tons each, 20 tons were treated in 24 hours, or 500 tons per week ; 23 men only were employed ; the loss upon pure lead was 1 per cent., and the consumption of coal 10 per cent, of the weight of the lead; the cost was 25 francs per ton, against 55 francs in Pattinson's process. Later results from the Hartz show a loss of lead in the zinc process of about 1-5 per cent, of lead, not in- cluding the lead in the various large products which are either sold as such or worked over. (For a full discussion of the metallurgy of lead, see Percy's work on that subject, London, 1870, from which much of the above has been taken.) Uses and Manufactures of Lead. As metal, lead is principally employed in sheets for sul- phuric acid chambers and concentrating pans, and for linings of tanks, cisterns, &c., in pipes for water and gas supply, and in shot. It fur- ther enters into the composition of many use- ful alloys. The compounds of the metal are mainly used as pigments and in the manufac- ture of flint glass. Sheet lead, formerly made by casting, is now generally made by rolling, or milling, as it is usually termed, the product bearing the name of milled lead. The Chinese prepare their well known tea lead by casting, notwithstanding its thinness. The operation is performed by two men, one of whom pours the molten lead from a crucible upon a large flat slab, when the other quickly places a large stone on the fluid lead, and presses it out to a thin flat plate, which is then removed and trimmed. English milled lead has largely re- placed the tea lead of home manufacture in China. In the process of milling, slabs of lead, formed by casting in open moulds of cast iron, are passed between cylindrical iron rolls until the lead has attained, the desired dimensions. The rolls used are provided with suitable mech- anism by which the distance between them can be regulated, and with reversing gear so that the slabs and sheets can be passed back- ward and forward. When the slab has at- tained unwieldy dimensions by extension, it is divided into pieces of suitable size, which are rolled separately. The same slab may be passed through the rolls 200 to 300 times, and become thereby elongated from 6 or 7 to 400 ft. If in rolling any depressions are observed on the upper surface of the slab, little pieces of sheet lead are placed in front of these depressions so as to force the subjacent lead into them and fill them up ; when this is accomplished the pieces are taken off. The rolling may be conducted immediately after casting, while the slabs are still hot, or they may be allowed to become cold before rolling. Lead pipe was formerly made by casting a short thick cylinder with the required bore, and rolling this cylinder out over a mandrel. Tubes made in this way were lim- ited to 20 or 30 ft. in length. By the improved method a hydrostatic press is employed to force the melted lead through dies of the required sizes. The press is under the floor, through which the piston passes, entering a strong up- right metallic cylinder. This can be filled with lead as required by a spout in the top, and the spout can be then closely shut. The cylinder is kept to the temperature of melting lead by an annular fireplace or receptacle for live coals by which it is surrounded.. Connected with the top of the cylinder is a steel die of the di- ameter required for the outside of the pipe, and through its centre passes from the centre of the piston below the mandrel which deter- mines the diameter of the bore. As the piston is driven upward, the lead in the cylinder is forced through the annular space between the fixed collar or die and the mandrel, and emerg- ing above cools in the form of a finished pipe, and is immediately ' coiled upon a drum sus- pended above the apparatus. Lead shot might with propriety be classed among the alloys of lead, for though sometimes made of simple lead of inferior quality, the metal is very com- monly combined with arsenic, introduced in the form of white arsenic (arsenious acid) or of orpiment (the sulphuret). The effect of the arsenic is to render the hard, brittle qualities of lead, which are contaminated by antimony and iron, softer and more ductile, and of the proper consistency, when melted and subjected to the usual process in shot making, for taking the globular form. The more ductile the lead the less arsenic is required, but hard lead re- quires 1 per cent, or more of arsenic ; usually from 0-3 to 0'8 per cent, is added. When the lead to the amount of two or three tons in a pot is melted, a circle of ashes or powdered charcoal is laid around the edge of the metal, and the arsenical compound wrapped in coarse paper is introduced in the centre by means of