149 



LEAD MANUFACTURE. 



LEAD MANUFACTURE. 



150 



mines contains the more precious metal in various proportions between 

 these two extremes. The extraction of the silver is always performed 

 when it exists in a proportion sufficient to pay the expense of the pro- 

 . cess, which varies in different localities according to the cost of fuel. 

 The process of extraction, which is called refining, depends upon the 

 well-known circumstance that lead, when heated to reduess, absorbs a 

 large portion of oxygen from the air, and is converted into an oxide ; 

 while silver does not undergo any such change, but retains its metallic 

 form at almost any temperature. A cupel, which is a shallow dish of 

 adequate dimension, is tilled with a mixture of burnt bones and fern 

 ashes pressed down, upon which the lead to be refined is placed in the 

 furnace. As soon as the lead is melted, a blast of air, introduced by 

 the usual means, is made to play forcibly upon the surface. In a short 

 time a crust of yellow oxide is formed, which is driven away as fast as 

 it appears to the opposite side of the furnace, until all or nearly all the 

 lead has been thus converted to an oxide. The silver, which remains 

 behind, is still combined with some portion of lead, and must be sub- 

 jected to a second process similar to that here described, in order to 

 liberate it in sufficient purity. The litharge, into which the lead has 

 been thus converted, is easily restored to its metallic state by again 

 heating it in a furnace in combination with carbonaceous matter, to 

 which it gives up its oxygen. 



This process, although valuable to a certain extent, is defective. There 

 is a considerable waste of material when thus treated, varying according 

 to the quality of the lead. The oxide is very volatile at high tempe- 

 ratures ; and so much of it escapes in a vaporous form during the 

 process of refining, that the difference of weight before and after its 

 completion is on the average about two-fifteenths. Under these cir- 

 cumstances, a very beautiful process was brought forward in 1829 by 

 the late Mr. Hugh Lee Pattinson. As a poor boy employed in a small 

 chemist's shop at Alston, he became familiar with the operations at 

 the lead-works in the vicinity of that town ; and in after years this 

 experience led him to the crystallising method of extracting silver from 

 lead. Having observed that, in a mass of melted lead, crystals were 

 formed as the temperature was diminished below the point of fusion, 

 Mr. Pattinson conceived that these crystals might be more homo- 

 geneous, and would consequently be united with a smaller proportion 

 of silver, than the remaining uncrystallised mass. This idea, proving 

 upon experiment to be correct, has been made practically useful by 

 subjecting the lead to repeated processes of crystallisation by means of 

 a simple apparatus. This consists of a series of hemispherical iron pots, 

 each capable of holding five tons of lead, ranged side by side, and fur- 

 nished with separate fireplaces. The mode of operation is as follows : 

 One of the pots is charged with lead, and when this is melted, the 

 surface is skimmed in order to remove such impurities as are thrown 

 up. The fire is then withdrawn, and the lead is suffered to cool 

 gradually. When the process of crystallisation begins, the crystals are 

 withdrawn by means of ladles with perforations to allow the uncrystal- 

 lised part to run through. These crystals are transferred to the second 

 pot, where they undergo a second melting and crystallisation, and 

 subsequently a third in another pot. This is continued until the 

 crystals collected are found to contain no more than from 10 to 

 15 clwto. of silver per ton, and are consequently melted and cast into 

 pigs for sale as refined lead. The process here described is repeated 

 with the remaining portion of the lead until it is so rich in silver as to 

 contain from JJUu to Sun ounces per ton, after which the silver is 

 extracted by the old process of cupcllation. As the proportion of lead 

 ,h the cupelling process is applied doe* not exceed one-twentieth 

 of the whole quantity of metal, the loss is diminished in a like propor- 

 tion ; by which means the expense of the extraction of silver is so far 

 economised, that it will answer to apply the process to lead which 

 originally contains any proportion greater than three ounces of silver 

 to the ton. 



There is much that is interesting in this process. Not only is there 

 a great saving of lead, but the silver extracted is considerable in amount. 

 Shortly after Mr. Pattinsun's death, in 1858, it was stated that his 

 crystallising process baa been the means of adding 20U,OUU ounces to 

 the silver annually produced or extracted from English ores. 



We shall now notice a few of the principal manufactures connected 

 immediately with this metal such as theet lead, lead pi^es, Uad t/iot, 

 litharge, red lead, and white Uad. 



To make theet Uad, the pigs are brought to a state of fusion in a 

 large pot or cistern, near which is placed the table on which the sheet 

 is to be cast. These pigs are about three feet long by six inches wide, 

 and neigh about 17U pounds each. Two methods of making sheet 

 lead are adopted, cutlimj and rv/ling. In the casting method, the table, 

 which is usually from 18 to 20 feet long and six feet wide, was formerly 

 made of wood ; and indeed wooden tables are still frequently used, but 

 in many works cast-iron has been substituted. The wooden table has 

 its surface protected by a layer of fine sand, which is wetted and spread 

 evenly and tinnly over it before the melted lend is poured on. To 

 prevent the lead from running over the sides a ledge is provided, two 

 or three inches thick, and two inches high, which forms the margin of 

 tin- tnl.lr. An instrument called a ittrike is aUo provided to regulate 



kthe thickness of the sheet, and to spread the melted metal evenly over 

 the table. This strike, which is made wider than the table, rests by 

 It* two ends on the ledges; the tn/." or diunn'tcr of the part within 

 khoe ledges being adjusted according to the intended thickness of the 



sheet, which will be equal to the distance between the lower side of the 

 strike and the layer of sand. In casting the sheet, the fused metal is 

 taken from the cistern with an iron ladle, and put into a triangular 

 shaped iron shovel or peel, placed at the head of the table, which peel 

 being raised so as to pour out the lead upon the table, the strike is 

 brought into use to spread it evenly over the whole surface ; the 

 surplus, if any. falling into a vessel placed for its reception at the foot 

 of the table. Sheet-lead of different thicknesses is described by those 

 who use it as being of so many pounds weight to the superficial square 

 foot. The usual kinds for covering roofs and gutters vary from about 

 ^jth to Jth of an inch in thickness, with a weight varying from about 

 6 pounds to 12 pounds per square foot. In the method of making 

 sheet lead by r<Uing, the lead is first melted in a very large vessel which 

 will contain six tons ; from this, when freed from all impurities, it is 

 allowed to flow through an opened valve into a cast-iron frame called 



Lend Foundry. 



the mould, about six feet square by six inches deep. The lead, thus 

 cast into a thick block, is, when cold, lifted out of the mould by a 

 crane, and placed in the lead mill. This mill consists of two ponderous 



Lead Rollins Mill. 



iron rollers, between which the lead is made to pass by stemn power, 

 and an iron frame with wooden rollers to facilitate the movements of 

 the lead ; the passage is repeatedly made, until the metal is brought 

 down to the required thickness. For some kinds of sheet-lead the 

 successive rollings or millings amount to as many as eight hundred, by 

 which the length is increased from six feet to four hundred feet. 



Lead pipes are sometimes made, when great exactness of shape is 

 not required, by bending a length of sheet-lead of the necessary width 

 over a mandrel, and soldering the edges together. The more usual 

 method of manufacture, however, is by casting and drawing. The 

 casting-box employed is an iron cylinder made in two parts, and put 

 together longitudinally with tlanges ; inside of this cylinder is placed an 

 iron rod or core, which is so fixed as to be concentric with the cylinder 

 without touching it ; a space is thus left, into which the melted lend 

 ix poured. When this is set, the core is removed and the cylinder 

 opened, so as to withdraw the pipe. The pipe being in its present state 



