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 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 spparatus. 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 The fire is then withdrawn, and the lead is suffered to cool gradually. When the process of crystallization begins, the crystals are withdrawn by means of ladles with perforations to allow the uncrystal lised part to run through.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 theta 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 300 to 500 ounces per ton, after which the silver is extracted by the old process of cupellation. As the proportion of lead to which the cupelling process is applied does not exceed one-twentieth of the whole quantity of metal, the lose is diminished in a like propor tion ; by which means the expense of the extraction of silver is so far ecouomised, that it will answer to apply the process to lead which originally contains any proportion greater than three ounces of silver to the toe.
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. l'attinson's death, in 1858, it was stated that his crystallising process has been the means of adding 200,000 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 sheet lead, lead pipes, lead shot, litharge, red white lead.
To make sheet lead, 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 weigh about 17U pounds each. Two methods of making sheet
lead are adopted, casting and rolling. 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 anbstituted. The wooden table has its surface protected by a layer of fine sand, which is wetted and spread evenly and firmly over it before the melted lead 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 the table. An Instrument called a strike is also provided to regulate the 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 its two ends on the ledges; the size or diameter of the part within those 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 to ith 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 rolling, the lead is first melted in every 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 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 iron rollers, between which the lead is made to pass by steam 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 flanges ; 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 lead is 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 adopted in drawing iron rods. The machinery employed for this pro cess has at different times been much improved in its construction ; so that it is now of rare occurrence to meet with mum imperfect pipe.