Page:The American Cyclopædia (1879) Volume X.djvu/269

 LEAD 263 a wire basket and stirred in. The pot is then covered and the lid -luted down and left for some hours, during which time the arsenical compound is decomposed, the greater part of the arsenic combining with the lead, while a portion mixes with the litharge produced by the reaction of the white arsenic on the lead. The mixture is then tested by dropping a por- tion of it through a colander into water. If the particles assume a lenticular form, the arsenic is in excess ; if they are flattened on one side, hollowed in the middle, or elongated, too little arsenic was used. When properly dosed it is run into bars, which are raised to the top of the shot tower, to be there melted and poured through the colanders. These are either hol- low hemispherical iron disks or rectangular flat sheets, each one perforated with a set of holes of uniform size, made perfectly smooth and exact. The lead when poured must be of the proper temperature for the special size to be made, and the workmen are careful to keep a film of the oxide as a lining to the colander, which is thought to have the effect of increas- ing the rotundity of the shot, possibly by ex- pediting its cooling as it passes through. The holes vary from -fa to ^ 7 of an inch, but the shot are of larger diameter than the holes. In falling to the base of the tower the particles of semi-fluid lead, acted upon alike over their whole surface by the current of air, are made to assume the globular form, and by the time they reach the bottom they are sufficiently hardened by cooling to bear the shock of stri- king the surface of the vessel of water placed to receive them. Large-sized shot require a greater height than small-sized, and while 100 ft. is sufficient for the latter, the former will require 150 ft. Dr. Ure notices a shot tower at Villach in Carinthia, 249 ft. high, as the highest erection of this kind. Taken from the cistern of water, the shot are dried, then as- sorted according to their sizes by sifting them in a revolving copper cylinder set slightly in- clined and perforated with holes, which in- crease in size toward the lower end. The smaller sizes thus drop through above and the larger lower down, and each size is received in its own box. The shot receive their superficial finish by being left for some time in a rotating cylinder with some pulverized graphite. Im- perfect shot are separated from the truly spheri- cal by allowing them to roll down an inclined plane, so arranged that the latter run straight down the middle and the former work off to one or the other side. Shot are also made by pour- ing lead upon a revolving table on which is placed a cylinder of perforated sheet brass. The table is revolved with a velocity of 1,000 ft. per minute on the periphery, and the lead is thrown by centrifugal action through the perforations in the sides, forming round bril- liant shot, which strike against a linen screen placed so as to intercept them. A method has been patented in the United States of manu- facturing shot without the high towers, sub- stituting for them a low elevation up which a powerful current of air is blown, thus pro- ducing the effect of a long continued fall. The most important alloys of lead used in the arts are those with antimony and tin. The alloys with antimony are more fusible than lead alone, but are much harder and more readily oxidized. Type metal is of variable composition, but in general may be said to consist of 4 or 5 parts of lead to 1 of antimony ; sometimes tin is added. The alloy of tin and lead is used for soldering. Three varieties, known as fine, common, and coarse solder, are composed respectively of 2 parts tin to 1 of lead, equal parts of tin and lead, and 1 of tin to 2 of lead. Pewter is also composed of lead and tin ; but other metals, as copper, antimony, and zinc, are often added. Common pewters contain 80 parts of lead to 20 of tin, others equal parts of the two metals, while the finer kinds contain but 16 to 20 per cent, of lead. The French government sanctions the use of vessels of 18 per cent, lead and 82 tin as quite harmless for containing wine or vinegar. Al- though so soft in itself, lead has the property of hardening tin. These alloys are distinguish- ed by the facility with which they ignite and burn. The alloy of 4 or 5 parts of lead and 1 of tin burns like charcoal at a red heat, the combustion continuing like that of an inferior peat with the formation of cauliflower excres- cences. This action appears to be due to the affinity which exists between the two oxides, which when fused, either alone or with silica or an alkali, produce a white opaque enamel, used for dial plates and also in earthenware. Bismuth unites readily with lead in all pro- portions, forming alloys which have no appli- cation in the arts, but are used to adulterate mercury. An alloy of 1 part of lead, 1 part of bismuth, and 3 parts of mercury is sufficiently fluid to pass through chamois leather. With bismuth and antimony lead forms an alloy which expands on cooling, and is used in cast- ing stereotype plates. They are composed, according to Mackenzie, of TO per cent, of lead, 15 of antimony, and 15 of bismuth. The triple alloys of lead and bismuth with tin or zinc are remarkable for their low melting point, which lies in many instances below the tem- perature of boiling water. Many metals, as zinc and copper, have but little tendency to form alloys with lead. When mixtures of lead and zinc are kept in a molten condition for some time at a comparatively low temperature, the zinc rises and forms a scum on the surface, leaving the lead nearly free from zinc. When a mixture of lead and copper, which has been quickly solidified, is heated to a point but little above the melting point of lead, a large pro- portion of the lead containing a little copper liquates out, leaving a residue of copper con- taining lead. This behavior of these metals is of practical value in the desilverization of lead and copper. In the first mentioned case, the zinc in separating from the lead carries tho